diff --git a/lib/spack/spack/directory_layout.py b/lib/spack/spack/directory_layout.py
index 13643a8962b..8d2ed12372b 100644
--- a/lib/spack/spack/directory_layout.py
+++ b/lib/spack/spack/directory_layout.py
@@ -21,7 +21,7 @@
 from spack.error import SpackError
 
 default_projections = {
-    "all": "{architecture}/{compiler.name}-{compiler.version}/{name}-{version}-{hash}"
+    "all": "{architecture.platform}/{architecture.target}/{name}-{version}-{hash}"
 }
 
 
diff --git a/lib/spack/spack/reporters/cdash.py b/lib/spack/spack/reporters/cdash.py
index a01634fd831..814504a5b34 100644
--- a/lib/spack/spack/reporters/cdash.py
+++ b/lib/spack/spack/reporters/cdash.py
@@ -176,7 +176,7 @@ def build_report_for_package(self, report_dir, package, duration):
         # something went wrong pre-cdash "configure" phase b/c we have an exception and only
         # "update" was encounterd.
         # dump the report in the configure line so teams can see what the issue is
-        if len(phases_encountered) == 1 and package["exception"]:
+        if len(phases_encountered) == 1 and package.get("exception"):
             # TODO this mapping is not ideal since these are pre-configure errors
             # we need to determine if a more appropriate cdash phase can be utilized
             # for now we will add a message to the log explaining this
diff --git a/lib/spack/spack/test/bindist.py b/lib/spack/spack/test/bindist.py
index e637f8e85b8..4881e346699 100644
--- a/lib/spack/spack/test/bindist.py
+++ b/lib/spack/spack/test/bindist.py
@@ -148,10 +148,7 @@ def install_dir_non_default_layout(tmpdir):
     """Hooks a fake install directory with a non-default layout"""
     opt_dir = tmpdir.join("opt")
     original_store, spack.store.STORE = spack.store.STORE, spack.store.Store(
-        str(opt_dir),
-        projections={
-            "all": "{name}/{version}/{architecture}-{compiler.name}-{compiler.version}-{hash}"
-        },
+        str(opt_dir), projections={"all": "{name}-{version}-{hash:4}"}
     )
     try:
         yield spack.store
@@ -384,7 +381,7 @@ def test_spec_needs_rebuild(monkeypatch, tmpdir):
     s = Spec("libdwarf").concretized()
 
     # Install a package
-    install_cmd(s.name)
+    install_cmd("--fake", s.name)
 
     # Put installed package in the buildcache
     buildcache_cmd("push", "-u", mirror_dir.strpath, s.name)
@@ -413,7 +410,7 @@ def test_generate_index_missing(monkeypatch, tmpdir, mutable_config):
     s = Spec("libdwarf").concretized()
 
     # Install a package
-    install_cmd("--no-cache", s.name)
+    install_cmd("--fake", "--no-cache", s.name)
 
     # Create a buildcache and update index
     buildcache_cmd("push", "-u", mirror_dir.strpath, s.name)
@@ -571,11 +568,8 @@ def test_install_legacy_buildcache_layout(mutable_config, compiler_factory, inst
     where the .spack file contained a repeated spec.json and another
     compressed archive file containing the install tree.  This test
     makes sure we can still read that layout."""
-    mutable_config.set(
-        "compilers", [compiler_factory(spec="gcc@4.5.0", operating_system="debian6")]
-    )
     legacy_layout_dir = os.path.join(test_path, "data", "mirrors", "legacy_layout")
-    mirror_url = "file://{0}".format(legacy_layout_dir)
+    mirror_url = f"file://{legacy_layout_dir}"
     filename = (
         "test-debian6-core2-gcc-4.5.0-archive-files-2.0-"
         "l3vdiqvbobmspwyb4q2b62fz6nitd4hk.spec.json"
@@ -584,9 +578,7 @@ def test_install_legacy_buildcache_layout(mutable_config, compiler_factory, inst
     mirror_cmd("add", "--scope", "site", "test-legacy-layout", mirror_url)
     output = install_cmd("--no-check-signature", "--cache-only", "-f", spec_json_path, output=str)
     mirror_cmd("rm", "--scope=site", "test-legacy-layout")
-    expect_line = (
-        "Extracting archive-files-2.0-" "l3vdiqvbobmspwyb4q2b62fz6nitd4hk from binary cache"
-    )
+    expect_line = "Extracting archive-files-2.0-l3vdiqvbobmspwyb4q2b62fz6nitd4hk from binary cache"
     assert expect_line in output
 
 
@@ -1160,10 +1152,11 @@ def test_get_valid_spec_file_no_json(tmp_path, filename):
         bindist._get_valid_spec_file(str(tmp_path / filename), max_supported_layout=1)
 
 
-def test_download_tarball_with_unsupported_layout_fails(tmp_path, mutable_config, capsys):
+def test_download_tarball_with_unsupported_layout_fails(
+    tmp_path, mock_packages, mutable_config, capsys
+):
     layout_version = bindist.CURRENT_BUILD_CACHE_LAYOUT_VERSION + 1
-    spec = Spec("gmake@4.4.1%gcc@13.1.0 arch=linux-ubuntu23.04-zen2")
-    spec._mark_concrete()
+    spec = Spec("pkg-c").concretized()
     spec_dict = spec.to_dict()
     spec_dict["buildcache_layout_version"] = layout_version
 
diff --git a/lib/spack/spack/test/bootstrap.py b/lib/spack/spack/test/bootstrap.py
index 8714a38e189..22355d7b509 100644
--- a/lib/spack/spack/test/bootstrap.py
+++ b/lib/spack/spack/test/bootstrap.py
@@ -13,6 +13,8 @@
 import spack.store
 import spack.util.path
 
+from .conftest import _true
+
 
 @pytest.fixture
 def active_mock_environment(mutable_config, mutable_mock_env_path):
@@ -93,12 +95,14 @@ def test_raising_exception_if_bootstrap_disabled(mutable_config):
         spack.bootstrap.config.store_path()
 
 
-def test_raising_exception_module_importable():
+def test_raising_exception_module_importable(config, monkeypatch):
+    monkeypatch.setattr(spack.bootstrap.core, "source_is_enabled", _true)
     with pytest.raises(ImportError, match='cannot bootstrap the "asdf" Python module'):
         spack.bootstrap.core.ensure_module_importable_or_raise("asdf")
 
 
-def test_raising_exception_executables_in_path():
+def test_raising_exception_executables_in_path(config, monkeypatch):
+    monkeypatch.setattr(spack.bootstrap.core, "source_is_enabled", _true)
     with pytest.raises(RuntimeError, match="cannot bootstrap any of the asdf, fdsa executables"):
         spack.bootstrap.core.ensure_executables_in_path_or_raise(["asdf", "fdsa"], "python")
 
@@ -218,16 +222,12 @@ def test_source_is_disabled(mutable_config):
     # Get the configuration dictionary of the current bootstrapping source
     conf = next(iter(spack.bootstrap.core.bootstrapping_sources()))
 
-    # The source is not explicitly enabled or disabled, so the following
-    # call should raise to skip using it for bootstrapping
-    with pytest.raises(ValueError):
-        spack.bootstrap.core.source_is_enabled_or_raise(conf)
+    # The source is not explicitly enabled or disabled, so the following should return False
+    assert not spack.bootstrap.core.source_is_enabled(conf)
 
-    # Try to explicitly disable the source and verify that the behavior
-    # is the same as above
+    # Try to explicitly disable the source and verify that the behavior is the same as above
     spack.config.add("bootstrap:trusted:{0}:{1}".format(conf["name"], False))
-    with pytest.raises(ValueError):
-        spack.bootstrap.core.source_is_enabled_or_raise(conf)
+    assert not spack.bootstrap.core.source_is_enabled(conf)
 
 
 @pytest.mark.regression("45247")
diff --git a/lib/spack/spack/test/build_environment.py b/lib/spack/spack/test/build_environment.py
index 83891a51be1..47cac141d57 100644
--- a/lib/spack/spack/test/build_environment.py
+++ b/lib/spack/spack/test/build_environment.py
@@ -4,7 +4,6 @@
 import os
 import platform
 import posixpath
-import sys
 
 import pytest
 
@@ -14,7 +13,6 @@
 from llnl.util.filesystem import HeaderList, LibraryList
 
 import spack.build_environment
-import spack.compiler
 import spack.compilers
 import spack.config
 import spack.deptypes as dt
@@ -96,7 +94,7 @@ def build_environment(working_env):
 
 
 @pytest.fixture
-def ensure_env_variables(config, mock_packages, monkeypatch, working_env):
+def ensure_env_variables(mutable_config, mock_packages, monkeypatch, working_env):
     """Returns a function that takes a dictionary and updates os.environ
     for the test lifetime accordingly. Plugs-in mock config and repo.
     """
@@ -161,20 +159,24 @@ def test_static_to_shared_library(build_environment):
 
 
 @pytest.mark.regression("8345")
-@pytest.mark.usefixtures("config", "mock_packages")
-def test_cc_not_changed_by_modules(monkeypatch, working_env):
-    s = spack.spec.Spec("cmake")
-    s.concretize()
-    pkg = s.package
+@pytest.mark.usefixtures("mock_packages")
+@pytest.mark.not_on_windows("Module files are not supported on Windows")
+def test_cc_not_changed_by_modules(monkeypatch, mutable_config, working_env, compiler_factory):
+    """Tests that external module files that are loaded cannot change the
+    CC environment variable.
+    """
+    gcc_entry = compiler_factory(spec="gcc@14.0.1 languages=c,c++")
+    gcc_entry["modules"] = ["some_module"]
+    mutable_config.set("packages", {"gcc": {"externals": [gcc_entry]}})
 
     def _set_wrong_cc(x):
         os.environ["CC"] = "NOT_THIS_PLEASE"
         os.environ["ANOTHER_VAR"] = "THIS_IS_SET"
 
     monkeypatch.setattr(spack.build_environment, "load_module", _set_wrong_cc)
-    monkeypatch.setattr(pkg.compiler, "modules", ["some_module"])
 
-    spack.build_environment.setup_package(pkg, False)
+    s = spack.spec.Spec("cmake %gcc@14").concretized()
+    spack.build_environment.setup_package(s.package, dirty=False)
 
     assert os.environ["CC"] != "NOT_THIS_PLEASE"
     assert os.environ["ANOTHER_VAR"] == "THIS_IS_SET"
@@ -185,7 +187,7 @@ def test_setup_dependent_package_inherited_modules(
 ):
     # This will raise on regression
     s = spack.spec.Spec("cmake-client-inheritor").concretized()
-    PackageInstaller([s.package]).install()
+    PackageInstaller([s.package], fake=True).install()
 
 
 @pytest.mark.parametrize(
@@ -265,22 +267,30 @@ def test_setup_dependent_package_inherited_modules(
     ],
 )
 def test_compiler_config_modifications(
-    initial, modifications, expected, ensure_env_variables, monkeypatch
+    initial,
+    modifications,
+    expected,
+    ensure_env_variables,
+    compiler_factory,
+    mutable_config,
+    monkeypatch,
 ):
     # Set the environment as per prerequisites
     ensure_env_variables(initial)
 
+    gcc_entry = compiler_factory(spec="gcc@14.0.1 languages=c,c++")
+    gcc_entry["extra_attributes"]["environment"] = modifications
+    mutable_config.set("packages", {"gcc": {"externals": [gcc_entry]}})
+
     def platform_pathsep(pathlist):
         if Path.platform_path == Path.windows:
             pathlist = pathlist.replace(":", ";")
 
         return convert_to_platform_path(pathlist)
 
-    # Monkeypatch a pkg.compiler.environment with the required modifications
-    pkg = spack.spec.Spec("cmake").concretized().package
-    monkeypatch.setattr(pkg.compiler, "environment", modifications)
+    pkg = spack.spec.Spec("cmake %gcc@14").concretized().package
     # Trigger the modifications
-    spack.build_environment.setup_package(pkg, False)
+    spack.build_environment.setup_package(pkg, dirty=False)
 
     # Check they were applied
     for name, value in expected.items():
@@ -291,25 +301,6 @@ def platform_pathsep(pathlist):
         assert name not in os.environ
 
 
-def test_compiler_custom_env(config, mock_packages, monkeypatch, working_env):
-    if sys.platform == "win32":
-        test_path = r"C:\test\path\element\custom-env" + "\\"
-    else:
-        test_path = r"/test/path/element/custom-env/"
-
-    def custom_env(pkg, env):
-        env.prepend_path("PATH", test_path)
-        env.append_flags("ENV_CUSTOM_CC_FLAGS", "--custom-env-flag1")
-
-    pkg = spack.spec.Spec("cmake").concretized().package
-    monkeypatch.setattr(pkg.compiler, "setup_custom_environment", custom_env)
-    spack.build_environment.setup_package(pkg, False)
-
-    # Note: trailing slash may be stripped by internal logic
-    assert test_path[:-1] in os.environ["PATH"]
-    assert "--custom-env-flag1" in os.environ["ENV_CUSTOM_CC_FLAGS"]
-
-
 def test_external_config_env(mock_packages, mutable_config, working_env):
     cmake_config = {
         "externals": [
@@ -329,25 +320,27 @@ def test_external_config_env(mock_packages, mutable_config, working_env):
 
 
 @pytest.mark.regression("9107")
-def test_spack_paths_before_module_paths(config, mock_packages, monkeypatch, working_env):
-    s = spack.spec.Spec("cmake")
-    s.concretize()
-    pkg = s.package
+@pytest.mark.not_on_windows("Windows does not support module files")
+def test_spack_paths_before_module_paths(
+    mutable_config, mock_packages, compiler_factory, monkeypatch, working_env
+):
+    gcc_entry = compiler_factory(spec="gcc@14.0.1 languages=c,c++")
+    gcc_entry["modules"] = ["some_module"]
+    mutable_config.set("packages", {"gcc": {"externals": [gcc_entry]}})
 
     module_path = os.path.join("path", "to", "module")
+    spack_path = os.path.join(spack.paths.prefix, os.path.join("lib", "spack", "env"))
 
     def _set_wrong_cc(x):
         os.environ["PATH"] = module_path + os.pathsep + os.environ["PATH"]
 
     monkeypatch.setattr(spack.build_environment, "load_module", _set_wrong_cc)
-    monkeypatch.setattr(pkg.compiler, "modules", ["some_module"])
 
-    spack.build_environment.setup_package(pkg, False)
+    s = spack.spec.Spec("cmake").concretized()
 
-    spack_path = os.path.join(spack.paths.prefix, os.path.join("lib", "spack", "env"))
+    spack.build_environment.setup_package(s.package, dirty=False)
 
     paths = os.environ["PATH"].split(os.pathsep)
-
     assert paths.index(spack_path) < paths.index(module_path)
 
 
@@ -498,15 +491,13 @@ def test_parallel_false_is_not_propagating(default_mock_concretization):
         ("rpath", "" if platform.system() == "Darwin" else "--disable-new-dtags"),
     ],
 )
-def test_setting_dtags_based_on_config(config_setting, expected_flag, config, mock_packages):
+def test_setting_dtags_based_on_config(
+    config_setting, expected_flag, config, mock_packages, working_env
+):
     # Pick a random package to be able to set compiler's variables
-    s = spack.spec.Spec("cmake")
-    s.concretize()
-    pkg = s.package
-
-    env = EnvironmentModifications()
+    s = spack.spec.Spec("cmake").concretized()
     with spack.config.override("config:shared_linking", {"type": config_setting, "bind": False}):
-        spack.build_environment.set_compiler_environment_variables(pkg, env)
+        env = spack.build_environment.setup_package(s.package, dirty=False)
         modifications = env.group_by_name()
         assert "SPACK_DTAGS_TO_STRIP" in modifications
         assert "SPACK_DTAGS_TO_ADD" in modifications
@@ -769,59 +760,24 @@ def test_rpath_with_duplicate_link_deps():
 @pytest.mark.parametrize(
     "compiler_spec,target_name,expected_flags",
     [
-        # Homogeneous compilers
+        # Semver versions
         ("gcc@4.7.2", "ivybridge", "-march=core-avx-i -mtune=core-avx-i"),
         ("clang@3.5", "x86_64", "-march=x86-64 -mtune=generic"),
         ("apple-clang@9.1.0", "x86_64", "-march=x86-64"),
-        # Mixed toolchain
-        ("clang@8.0.0", "broadwell", ""),
+        ("gcc@=9.2.0", "haswell", "-march=haswell -mtune=haswell"),
+        # Check that custom string versions are accepted
+        ("gcc@=9.2.0-foo", "icelake", "-march=icelake-client -mtune=icelake-client"),
+        # Check that the special case for Apple's clang is treated correctly
+        # i.e. it won't try to detect the version again
+        ("apple-clang@=9.1.0", "x86_64", "-march=x86-64"),
+        # FIXME (compiler as nodes): Check mixed toolchain
+        # ("clang@8.0.0", "broadwell", ""),
     ],
 )
 @pytest.mark.filterwarnings("ignore:microarchitecture specific")
 @pytest.mark.not_on_windows("Windows doesn't support the compiler wrapper")
 def test_optimization_flags(compiler_spec, target_name, expected_flags, compiler_factory):
     target = archspec.cpu.TARGETS[target_name]
-    compiler_dict = compiler_factory(spec=compiler_spec, operating_system="")["compiler"]
-    if compiler_spec == "clang@8.0.0":
-        compiler_dict["paths"] = {
-            "cc": "/path/to/clang-8",
-            "cxx": "/path/to/clang++-8",
-            "f77": "/path/to/gfortran-9",
-            "fc": "/path/to/gfortran-9",
-        }
-    compiler = spack.compilers.compiler_from_dict(compiler_dict)
-    opt_flags = spack.build_environment.optimization_flags(compiler, target)
-    assert opt_flags == expected_flags
-
-
-@pytest.mark.parametrize(
-    "compiler_str,real_version,target_str,expected_flags",
-    [
-        ("gcc@=9.2.0", None, "haswell", "-march=haswell -mtune=haswell"),
-        # Check that custom string versions are accepted
-        ("gcc@=10foo", "9.2.0", "icelake", "-march=icelake-client -mtune=icelake-client"),
-        # Check that we run version detection (4.4.0 doesn't support icelake)
-        ("gcc@=4.4.0-special", "9.2.0", "icelake", "-march=icelake-client -mtune=icelake-client"),
-        # Check that the special case for Apple's clang is treated correctly
-        # i.e. it won't try to detect the version again
-        ("apple-clang@=9.1.0", None, "x86_64", "-march=x86-64"),
-    ],
-)
-def test_optimization_flags_with_custom_versions(
-    compiler_str,
-    real_version,
-    target_str,
-    expected_flags,
-    monkeypatch,
-    mutable_config,
-    compiler_factory,
-):
-    target = archspec.cpu.TARGETS[target_str]
-    compiler_dict = compiler_factory(spec=compiler_str, operating_system="redhat6")
-    mutable_config.set("compilers", [compiler_dict])
-    if real_version:
-        monkeypatch.setattr(spack.compiler.Compiler, "get_real_version", lambda x: real_version)
-    compiler = spack.compilers.compiler_from_dict(compiler_dict["compiler"])
-
+    compiler = spack.spec.parse_with_version_concrete(compiler_spec)
     opt_flags = spack.build_environment.optimization_flags(compiler, target)
     assert opt_flags == expected_flags
diff --git a/lib/spack/spack/test/ci.py b/lib/spack/spack/test/ci.py
index c703510ee2f..aae6b20ef71 100644
--- a/lib/spack/spack/test/ci.py
+++ b/lib/spack/spack/test/ci.py
@@ -322,24 +322,29 @@ def __call__(self, *args, **kwargs):
 
 
 def test_get_spec_filter_list(mutable_mock_env_path, mutable_mock_repo):
-    """Test that given an active environment and list of touched pkgs,
-    we get the right list of possibly-changed env specs"""
+    """Tests that, given an active environment and list of touched pkgs, we get the right
+    list of possibly-changed env specs.
+
+    The test concretizes the following environment:
+
+    [    ]  hypre@=0.2.15+shared build_system=generic
+    [bl  ]      ^openblas-with-lapack@=0.2.15 build_system=generic
+    [    ]  mpileaks@=2.3~debug~opt+shared+static build_system=generic
+    [bl  ]      ^callpath@=1.0 build_system=generic
+    [bl  ]          ^dyninst@=8.2 build_system=generic
+    [bl  ]              ^libdwarf@=20130729 build_system=generic
+    [bl  ]              ^libelf@=0.8.13 build_system=generic
+    [b   ]      ^gcc@=10.2.1 build_system=generic languages='c,c++,fortran'
+    [ l  ]      ^gcc-runtime@=10.2.1 build_system=generic
+    [bl  ]      ^mpich@=3.0.4~debug build_system=generic
+
+    and simulates a change in libdwarf.
+    """
     e1 = ev.create("test")
     e1.add("mpileaks")
     e1.add("hypre")
     e1.concretize()
 
-    """
-    Concretizing the above environment results in the following graphs:
-
-    mpileaks -> mpich (provides mpi virtual dep of mpileaks)
-             -> callpath -> dyninst -> libelf
-                                    -> libdwarf -> libelf
-                         -> mpich (provides mpi dep of callpath)
-
-    hypre -> openblas-with-lapack (provides lapack and blas virtual deps of hypre)
-    """
-
     touched = ["libdwarf"]
 
     # Make sure we return the correct set of possibly affected specs,
@@ -351,17 +356,25 @@ def test_get_spec_filter_list(mutable_mock_env_path, mutable_mock_repo):
     # no spec traversals.  Passing any other number yields differing
     # numbers of possibly affected specs.
 
-    full_set = set(["mpileaks", "mpich", "callpath", "dyninst", "libdwarf", "libelf"])
-    empty_set = set([])
-    depth_2_set = set(["mpich", "callpath", "dyninst", "libdwarf", "libelf"])
-    depth_1_set = set(["dyninst", "libdwarf", "libelf"])
-    depth_0_set = set(["libdwarf", "libelf"])
+    full_set = {
+        "mpileaks",
+        "mpich",
+        "callpath",
+        "dyninst",
+        "libdwarf",
+        "libelf",
+        "gcc",
+        "gcc-runtime",
+    }
+    depth_2_set = {"mpich", "callpath", "dyninst", "libdwarf", "libelf", "gcc", "gcc-runtime"}
+    depth_1_set = {"dyninst", "libdwarf", "libelf", "gcc", "gcc-runtime"}
+    depth_0_set = {"libdwarf", "libelf", "gcc", "gcc-runtime"}
 
     expectations = {
         None: full_set,
         3: full_set,
         100: full_set,
-        -1: empty_set,
+        -1: set(),
         0: depth_0_set,
         1: depth_1_set,
         2: depth_2_set,
@@ -369,8 +382,7 @@ def test_get_spec_filter_list(mutable_mock_env_path, mutable_mock_repo):
 
     for key, val in expectations.items():
         affected_specs = ci.get_spec_filter_list(e1, touched, dependent_traverse_depth=key)
-        affected_pkg_names = set([s.name for s in affected_specs])
-        print(f"{key}: {affected_pkg_names}")
+        affected_pkg_names = {s.name for s in affected_specs}
         assert affected_pkg_names == val
 
 
diff --git a/lib/spack/spack/test/cmd/build_env.py b/lib/spack/spack/test/cmd/build_env.py
index 576292de49d..88c51a460b1 100644
--- a/lib/spack/spack/test/cmd/build_env.py
+++ b/lib/spack/spack/test/cmd/build_env.py
@@ -12,7 +12,7 @@
 build_env = SpackCommand("build-env")
 
 
-@pytest.mark.parametrize("pkg", [("zlib",), ("zlib", "--")])
+@pytest.mark.parametrize("pkg", [("pkg-c",), ("pkg-c", "--")])
 @pytest.mark.usefixtures("config", "mock_packages", "working_env")
 def test_it_just_runs(pkg):
     build_env(*pkg)
@@ -38,7 +38,7 @@ def test_build_env_requires_a_spec(args):
 @pytest.mark.usefixtures("config", "mock_packages", "working_env")
 def test_dump(shell_as, shell, tmpdir):
     with tmpdir.as_cwd():
-        build_env("--dump", _out_file, "zlib")
+        build_env("--dump", _out_file, "pkg-c")
         with open(_out_file, encoding="utf-8") as f:
             if shell == "pwsh":
                 assert any(line.startswith("$Env:PATH") for line in f.readlines())
@@ -51,7 +51,7 @@ def test_dump(shell_as, shell, tmpdir):
 @pytest.mark.usefixtures("config", "mock_packages", "working_env")
 def test_pickle(tmpdir):
     with tmpdir.as_cwd():
-        build_env("--pickle", _out_file, "zlib")
+        build_env("--pickle", _out_file, "pkg-c")
         environment = pickle.load(open(_out_file, "rb"))
         assert isinstance(environment, dict)
         assert "PATH" in environment
diff --git a/lib/spack/spack/test/cmd/buildcache.py b/lib/spack/spack/test/cmd/buildcache.py
index 82db4468375..319ad192a41 100644
--- a/lib/spack/spack/test/cmd/buildcache.py
+++ b/lib/spack/spack/test/cmd/buildcache.py
@@ -148,7 +148,7 @@ def test_update_key_index(
     s = Spec("libdwarf").concretized()
 
     # Install a package
-    install(s.name)
+    install("--fake", s.name)
 
     # Put installed package in the buildcache, which, because we're signing
     # it, should result in the public key getting pushed to the buildcache
@@ -178,7 +178,7 @@ def test_buildcache_autopush(tmp_path, install_mockery, mock_fetch):
     s = Spec("libdwarf").concretized()
 
     # Install and generate build cache index
-    PackageInstaller([s.package], explicit=True).install()
+    PackageInstaller([s.package], fake=True, explicit=True).install()
 
     metadata_file = spack.binary_distribution.tarball_name(s, ".spec.json")
 
@@ -220,7 +220,7 @@ def verify_mirror_contents():
 
     # Install a package and put it in the buildcache
     s = Spec(out_env_pkg).concretized()
-    install(s.name)
+    install("--fake", s.name)
     buildcache("push", "-u", "-f", src_mirror_url, s.name)
 
     env("create", "test")
diff --git a/lib/spack/spack/test/cmd/ci.py b/lib/spack/spack/test/cmd/ci.py
index 919c322c2b0..151d1f4b89e 100644
--- a/lib/spack/spack/test/cmd/ci.py
+++ b/lib/spack/spack/test/cmd/ci.py
@@ -177,9 +177,9 @@ def test_ci_generate_with_env(ci_generate_test, tmp_path, mock_binary_index):
     assert yaml_contents["workflow"]["rules"] == [{"when": "always"}]
 
     assert "stages" in yaml_contents
-    assert len(yaml_contents["stages"]) == 5
+    assert len(yaml_contents["stages"]) == 6
     assert yaml_contents["stages"][0] == "stage-0"
-    assert yaml_contents["stages"][4] == "stage-rebuild-index"
+    assert yaml_contents["stages"][5] == "stage-rebuild-index"
 
     assert "rebuild-index" in yaml_contents
     rebuild_job = yaml_contents["rebuild-index"]
@@ -1060,7 +1060,7 @@ def test_ci_rebuild_index(
             with open(tmp_path / "spec.json", "w", encoding="utf-8") as f:
                 f.write(concrete_spec.to_json(hash=ht.dag_hash))
 
-            install_cmd("--add", "-f", str(tmp_path / "spec.json"))
+            install_cmd("--fake", "--add", "-f", str(tmp_path / "spec.json"))
             buildcache_cmd("push", "-u", "-f", mirror_url, "callpath")
             ci_cmd("rebuild-index")
 
diff --git a/lib/spack/spack/test/cmd/compiler.py b/lib/spack/spack/test/cmd/compiler.py
index 5819f8121a1..6b060329476 100644
--- a/lib/spack/spack/test/cmd/compiler.py
+++ b/lib/spack/spack/test/cmd/compiler.py
@@ -10,7 +10,6 @@
 import spack.compilers.config
 import spack.config
 import spack.main
-import spack.spec
 import spack.util.pattern
 import spack.version
 
diff --git a/lib/spack/spack/test/cmd/config.py b/lib/spack/spack/test/cmd/config.py
index 1182ebd68f8..6b755691b5d 100644
--- a/lib/spack/spack/test/cmd/config.py
+++ b/lib/spack/spack/test/cmd/config.py
@@ -335,7 +335,7 @@ def test_config_add_override_leaf_from_file(mutable_empty_config, tmpdir):
 
 
 def test_config_add_update_dict_from_file(mutable_empty_config, tmpdir):
-    config("add", "packages:all:compiler:[gcc]")
+    config("add", "packages:all:require:['%gcc']")
 
     # contents to add to file
     contents = """spack:
@@ -357,7 +357,7 @@ def test_config_add_update_dict_from_file(mutable_empty_config, tmpdir):
     expected = """packages:
   all:
     target: [x86_64]
-    compiler: [gcc]
+    require: ['%gcc']
 """
 
     assert expected == output
@@ -607,7 +607,6 @@ def test_config_prefer_upstream(
     packages = syaml.load(open(cfg_file, encoding="utf-8"))["packages"]
 
     # Make sure only the non-default variants are set.
-    assert packages["all"] == {"compiler": ["gcc@=10.2.1"]}
     assert packages["boost"] == {"variants": "+debug +graph", "version": ["1.63.0"]}
     assert packages["dependency-install"] == {"version": ["2.0"]}
     # Ensure that neither variant gets listed for hdf5, since they conflict
diff --git a/lib/spack/spack/test/cmd/dependencies.py b/lib/spack/spack/test/cmd/dependencies.py
index abe69bc01a5..d44451154c2 100644
--- a/lib/spack/spack/test/cmd/dependencies.py
+++ b/lib/spack/spack/test/cmd/dependencies.py
@@ -13,7 +13,7 @@
 
 dependencies = SpackCommand("dependencies")
 
-mpis = [
+MPIS = [
     "intel-parallel-studio",
     "low-priority-provider",
     "mpich",
@@ -21,20 +21,21 @@
     "multi-provider-mpi",
     "zmpi",
 ]
-mpi_deps = ["fake"]
+COMPILERS = ["gcc", "llvm"]
+MPI_DEPS = ["fake"]
 
 
 def test_direct_dependencies(mock_packages):
     out = dependencies("mpileaks")
     actual = set(re.split(r"\s+", out.strip()))
-    expected = set(["callpath"] + mpis)
+    expected = set(["callpath"] + MPIS + COMPILERS)
     assert expected == actual
 
 
 def test_transitive_dependencies(mock_packages):
     out = dependencies("--transitive", "mpileaks")
     actual = set(re.split(r"\s+", out.strip()))
-    expected = set(["callpath", "dyninst", "libdwarf", "libelf"] + mpis + mpi_deps)
+    expected = set(["callpath", "dyninst", "libdwarf", "libelf"] + MPIS + MPI_DEPS + COMPILERS)
     assert expected == actual
 
 
@@ -57,12 +58,11 @@ def test_direct_installed_dependencies(mock_packages, database):
     with color_when(False):
         out = dependencies("--installed", "mpileaks^mpich")
 
-    lines = [line for line in out.strip().split("\n") if not line.startswith("--")]
-    hashes = set([re.split(r"\s+", line)[0] for line in lines])
+    root = spack.store.STORE.db.query_one("mpileaks ^mpich")
 
-    expected = set(
-        [spack.store.STORE.db.query_one(s).dag_hash(7) for s in ["mpich", "callpath^mpich"]]
-    )
+    lines = [line for line in out.strip().split("\n") if line and not line.startswith("--")]
+    hashes = {re.split(r"\s+", line)[0] for line in lines}
+    expected = {s.dag_hash(7) for s in root.dependencies()}
 
     assert expected == hashes
 
@@ -72,14 +72,10 @@ def test_transitive_installed_dependencies(mock_packages, database):
     with color_when(False):
         out = dependencies("--installed", "--transitive", "mpileaks^zmpi")
 
-    lines = [line for line in out.strip().split("\n") if not line.startswith("--")]
-    hashes = set([re.split(r"\s+", line)[0] for line in lines])
+    root = spack.store.STORE.db.query_one("mpileaks ^zmpi")
 
-    expected = set(
-        [
-            spack.store.STORE.db.query_one(s).dag_hash(7)
-            for s in ["zmpi", "callpath^zmpi", "fake", "dyninst", "libdwarf", "libelf"]
-        ]
-    )
+    lines = [line for line in out.strip().split("\n") if line and not line.startswith("--")]
+    hashes = {re.split(r"\s+", line)[0] for line in lines}
+    expected = {s.dag_hash(7) for s in root.traverse(root=False)}
 
     assert expected == hashes
diff --git a/lib/spack/spack/test/cmd/deprecate.py b/lib/spack/spack/test/cmd/deprecate.py
index 81da47b9759..2e7cc8801ec 100644
--- a/lib/spack/spack/test/cmd/deprecate.py
+++ b/lib/spack/spack/test/cmd/deprecate.py
@@ -16,16 +16,16 @@
 
 
 def test_deprecate(mock_packages, mock_archive, mock_fetch, install_mockery):
-    install("libelf@0.8.13")
-    install("libelf@0.8.10")
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.10")
 
-    all_installed = spack.store.STORE.db.query()
+    all_installed = spack.store.STORE.db.query("libelf")
     assert len(all_installed) == 2
 
     deprecate("-y", "libelf@0.8.10", "libelf@0.8.13")
 
-    non_deprecated = spack.store.STORE.db.query()
-    all_available = spack.store.STORE.db.query(installed=InstallRecordStatus.ANY)
+    non_deprecated = spack.store.STORE.db.query("libelf")
+    all_available = spack.store.STORE.db.query("libelf", installed=InstallRecordStatus.ANY)
     assert all_available == all_installed
     assert non_deprecated == spack.store.STORE.db.query("libelf@0.8.13")
 
@@ -38,24 +38,24 @@ def test_deprecate_fails_no_such_package(mock_packages, mock_archive, mock_fetch
     output = deprecate("-y", "libelf@0.8.10", "libelf@0.8.13", fail_on_error=False)
     assert "Spec 'libelf@0.8.10' matches no installed packages" in output
 
-    install("libelf@0.8.10")
+    install("--fake", "libelf@0.8.10")
 
     output = deprecate("-y", "libelf@0.8.10", "libelf@0.8.13", fail_on_error=False)
     assert "Spec 'libelf@0.8.13' matches no installed packages" in output
 
 
-def test_deprecate_install(mock_packages, mock_archive, mock_fetch, install_mockery):
-    """Tests that the ```-i`` option allows us to deprecate in favor of a spec
-    that is not yet installed."""
-    install("libelf@0.8.10")
-
-    to_deprecate = spack.store.STORE.db.query()
+def test_deprecate_install(mock_packages, mock_archive, mock_fetch, install_mockery, monkeypatch):
+    """Tests that the -i option allows us to deprecate in favor of a spec
+    that is not yet installed.
+    """
+    install("--fake", "libelf@0.8.10")
+    to_deprecate = spack.store.STORE.db.query("libelf")
     assert len(to_deprecate) == 1
 
     deprecate("-y", "-i", "libelf@0.8.10", "libelf@0.8.13")
 
-    non_deprecated = spack.store.STORE.db.query()
-    deprecated = spack.store.STORE.db.query(installed=InstallRecordStatus.DEPRECATED)
+    non_deprecated = spack.store.STORE.db.query("libelf")
+    deprecated = spack.store.STORE.db.query("libelf", installed=InstallRecordStatus.DEPRECATED)
     assert deprecated == to_deprecate
     assert len(non_deprecated) == 1
     assert non_deprecated[0].satisfies("libelf@0.8.13")
@@ -63,8 +63,8 @@ def test_deprecate_install(mock_packages, mock_archive, mock_fetch, install_mock
 
 def test_deprecate_deps(mock_packages, mock_archive, mock_fetch, install_mockery):
     """Test that the deprecate command deprecates all dependencies properly."""
-    install("libdwarf@20130729 ^libelf@0.8.13")
-    install("libdwarf@20130207 ^libelf@0.8.10")
+    install("--fake", "libdwarf@20130729 ^libelf@0.8.13")
+    install("--fake", "libdwarf@20130207 ^libelf@0.8.10")
 
     new_spec = spack.spec.Spec("libdwarf@20130729^libelf@0.8.13").concretized()
     old_spec = spack.spec.Spec("libdwarf@20130207^libelf@0.8.10").concretized()
@@ -80,14 +80,14 @@ def test_deprecate_deps(mock_packages, mock_archive, mock_fetch, install_mockery
     assert all_available == all_installed
     assert sorted(all_available) == sorted(deprecated + non_deprecated)
 
-    assert sorted(non_deprecated) == sorted(list(new_spec.traverse()))
-    assert sorted(deprecated) == sorted(list(old_spec.traverse()))
+    assert sorted(non_deprecated) == sorted(new_spec.traverse())
+    assert sorted(deprecated) == sorted([old_spec, old_spec["libelf"]])
 
 
 def test_uninstall_deprecated(mock_packages, mock_archive, mock_fetch, install_mockery):
     """Tests that we can still uninstall deprecated packages."""
-    install("libelf@0.8.13")
-    install("libelf@0.8.10")
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.10")
 
     deprecate("-y", "libelf@0.8.10", "libelf@0.8.13")
 
@@ -103,9 +103,9 @@ def test_uninstall_deprecated(mock_packages, mock_archive, mock_fetch, install_m
 
 def test_deprecate_already_deprecated(mock_packages, mock_archive, mock_fetch, install_mockery):
     """Tests that we can re-deprecate a spec to change its deprecator."""
-    install("libelf@0.8.13")
-    install("libelf@0.8.12")
-    install("libelf@0.8.10")
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.12")
+    install("--fake", "libelf@0.8.10")
 
     deprecated_spec = spack.spec.Spec("libelf@0.8.10").concretized()
 
@@ -116,8 +116,8 @@ def test_deprecate_already_deprecated(mock_packages, mock_archive, mock_fetch, i
 
     deprecate("-y", "libelf@0.8.10", "libelf@0.8.13")
 
-    non_deprecated = spack.store.STORE.db.query()
-    all_available = spack.store.STORE.db.query(installed=InstallRecordStatus.ANY)
+    non_deprecated = spack.store.STORE.db.query("libelf")
+    all_available = spack.store.STORE.db.query("libelf", installed=InstallRecordStatus.ANY)
     assert len(non_deprecated) == 2
     assert len(all_available) == 3
 
@@ -128,9 +128,9 @@ def test_deprecate_already_deprecated(mock_packages, mock_archive, mock_fetch, i
 def test_deprecate_deprecator(mock_packages, mock_archive, mock_fetch, install_mockery):
     """Tests that when a deprecator spec is deprecated, its deprecatee specs
     are updated to point to the new deprecator."""
-    install("libelf@0.8.13")
-    install("libelf@0.8.12")
-    install("libelf@0.8.10")
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.12")
+    install("--fake", "libelf@0.8.10")
 
     first_deprecated_spec = spack.spec.Spec("libelf@0.8.10").concretized()
     second_deprecated_spec = spack.spec.Spec("libelf@0.8.12").concretized()
@@ -143,8 +143,8 @@ def test_deprecate_deprecator(mock_packages, mock_archive, mock_fetch, install_m
 
     deprecate("-y", "libelf@0.8.12", "libelf@0.8.13")
 
-    non_deprecated = spack.store.STORE.db.query()
-    all_available = spack.store.STORE.db.query(installed=InstallRecordStatus.ANY)
+    non_deprecated = spack.store.STORE.db.query("libelf")
+    all_available = spack.store.STORE.db.query("libelf", installed=InstallRecordStatus.ANY)
     assert len(non_deprecated) == 1
     assert len(all_available) == 3
 
@@ -157,8 +157,8 @@ def test_deprecate_deprecator(mock_packages, mock_archive, mock_fetch, install_m
 def test_concretize_deprecated(mock_packages, mock_archive, mock_fetch, install_mockery):
     """Tests that the concretizer throws an error if we concretize to a
     deprecated spec"""
-    install("libelf@0.8.13")
-    install("libelf@0.8.10")
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.10")
 
     deprecate("-y", "libelf@0.8.10", "libelf@0.8.13")
 
diff --git a/lib/spack/spack/test/cmd/diff.py b/lib/spack/spack/test/cmd/diff.py
index f07b91f0eca..6b839a037a8 100644
--- a/lib/spack/spack/test/cmd/diff.py
+++ b/lib/spack/spack/test/cmd/diff.py
@@ -194,7 +194,7 @@ def test_diff_cmd(install_mockery, mock_fetch, mock_archive, mock_packages):
 
 def test_load_first(install_mockery, mock_fetch, mock_archive, mock_packages):
     """Test with and without the --first option"""
-    install_cmd("mpileaks")
+    install_cmd("--fake", "mpileaks")
 
     # Only one version of mpileaks will work
     diff_cmd("mpileaks", "mpileaks")
@@ -223,14 +223,12 @@ def test_load_first(install_mockery, mock_fetch, mock_archive, mock_packages):
         for dep in ("mpileaks", "callpath", "dyninst", "libelf", "libdwarf", "mpich")
     )
     assert all(
-        len([diff for diff in result["intersect"] if diff[0] == attr]) == 6
+        len([diff for diff in result["intersect"] if diff[0] == attr]) == 7
         for attr in (
             "version",
             "node_target",
             "node_platform",
             "node_os",
-            "node_compiler",
-            "node_compiler_version",
             "node",
             "package_hash",
             "hash",
diff --git a/lib/spack/spack/test/cmd/env.py b/lib/spack/spack/test/cmd/env.py
index 240b7cf5dce..15d21ad5992 100644
--- a/lib/spack/spack/test/cmd/env.py
+++ b/lib/spack/spack/test/cmd/env.py
@@ -1693,7 +1693,10 @@ def check_stage(spec):
         spec = Spec(spec).concretized()
         for dep in spec.traverse():
             stage_name = f"{stage_prefix}{dep.name}-{dep.version}-{dep.dag_hash()}"
-            assert os.path.isdir(os.path.join(root, stage_name))
+            if dep.external:
+                assert not os.path.exists(os.path.join(root, stage_name))
+            else:
+                assert os.path.isdir(os.path.join(root, stage_name))
 
     check_stage("mpileaks")
     check_stage("zmpi")
diff --git a/lib/spack/spack/test/cmd/find.py b/lib/spack/spack/test/cmd/find.py
index 6c9efb0de7a..2a1ad48319c 100644
--- a/lib/spack/spack/test/cmd/find.py
+++ b/lib/spack/spack/test/cmd/find.py
@@ -170,7 +170,7 @@ def _check_json_output(spec_list):
 
 
 def _check_json_output_deps(spec_list):
-    assert len(spec_list) == 13
+    assert len(spec_list) == 15
 
     names = [spec["name"] for spec in spec_list]
     assert names.count("mpileaks") == 3
@@ -231,21 +231,27 @@ def test_display_json_deps(database, capsys):
 @pytest.mark.db
 def test_find_format(database, config):
     output = find("--format", "{name}-{^mpi.name}", "mpileaks")
-    assert set(output.strip().split("\n")) == set(
-        ["mpileaks-zmpi", "mpileaks-mpich", "mpileaks-mpich2"]
-    )
+    assert set(output.strip().split("\n")) == {
+        "mpileaks-zmpi",
+        "mpileaks-mpich",
+        "mpileaks-mpich2",
+    }
 
     output = find("--format", "{name}-{version}-{compiler.name}-{^mpi.name}", "mpileaks")
     assert "installed package" not in output
-    assert set(output.strip().split("\n")) == set(
-        ["mpileaks-2.3-gcc-zmpi", "mpileaks-2.3-gcc-mpich", "mpileaks-2.3-gcc-mpich2"]
-    )
+    assert set(output.strip().split("\n")) == {
+        "mpileaks-2.3-gcc-zmpi",
+        "mpileaks-2.3-gcc-mpich",
+        "mpileaks-2.3-gcc-mpich2",
+    }
 
     output = find("--format", "{name}-{^mpi.name}-{hash:7}", "mpileaks")
     elements = output.strip().split("\n")
-    assert set(e[:-7] for e in elements) == set(
-        ["mpileaks-zmpi-", "mpileaks-mpich-", "mpileaks-mpich2-"]
-    )
+    assert set(e[:-7] for e in elements) == {
+        "mpileaks-zmpi-",
+        "mpileaks-mpich-",
+        "mpileaks-mpich2-",
+    }
 
     # hashes are in base32
     for e in elements:
@@ -264,6 +270,8 @@ def test_find_format_deps(database, config):
         dyninst-8.2
             libdwarf-20130729
             libelf-0.8.13
+    gcc-10.2.1
+    gcc-runtime-10.2.1
     zmpi-1.0
         fake-1.0
 
@@ -274,24 +282,21 @@ def test_find_format_deps(database, config):
 @pytest.mark.db
 def test_find_format_deps_paths(database, config):
     output = find("-dp", "--format", "{name}-{version}", "mpileaks", "^zmpi")
-
-    spec = Spec("mpileaks ^zmpi").concretized()
-    prefixes = [s.prefix for s in spec.traverse()]
-
+    mpileaks = Spec("mpileaks ^zmpi").concretized()
     assert (
         output
-        == """\
-mpileaks-2.3                   {0}
-    callpath-1.0               {1}
-        dyninst-8.2            {2}
-            libdwarf-20130729  {3}
-            libelf-0.8.13      {4}
-    zmpi-1.0                   {5}
-        fake-1.0               {6}
+        == f"""\
+mpileaks-2.3                   {mpileaks.prefix}
+    callpath-1.0               {mpileaks['callpath'].prefix}
+        dyninst-8.2            {mpileaks['dyninst'].prefix}
+            libdwarf-20130729  {mpileaks['libdwarf'].prefix}
+            libelf-0.8.13      {mpileaks['libelf'].prefix}
+    gcc-10.2.1                 {mpileaks['gcc'].prefix}
+    gcc-runtime-10.2.1         {mpileaks['gcc-runtime'].prefix}
+    zmpi-1.0                   {mpileaks['zmpi'].prefix}
+        fake-1.0               {mpileaks['fake'].prefix}
 
-""".format(
-            *prefixes
-        )
+"""
     )
 
 
@@ -308,12 +313,6 @@ def test_find_very_long(database, config):
     )
 
 
-@pytest.mark.db
-def test_find_show_compiler(database, config):
-    output = find("--no-groups", "--show-full-compiler", "mpileaks")
-    assert "mpileaks@2.3%gcc@10.2.1" in output
-
-
 @pytest.mark.db
 def test_find_not_found(database, config, capsys):
     with capsys.disabled():
@@ -345,7 +344,7 @@ def test_find_prefix_in_env(
     """Test `find` formats requiring concrete specs work in environments."""
     env("create", "test")
     with ev.read("test"):
-        install("--add", "mpileaks")
+        install("--fake", "--add", "mpileaks")
         find("-p")
         find("-l")
         find("-L")
@@ -455,7 +454,7 @@ def test_environment_with_version_range_in_compiler_doesnt_fail(tmp_path):
 
     with test_environment:
         output = find()
-    assert "zlib%gcc@12.1.0" in output
+    assert "zlib" in output
 
 
 _pkga = (
diff --git a/lib/spack/spack/test/cmd/gc.py b/lib/spack/spack/test/cmd/gc.py
index 8efdd8f2958..b3e730d258f 100644
--- a/lib/spack/spack/test/cmd/gc.py
+++ b/lib/spack/spack/test/cmd/gc.py
@@ -20,7 +20,8 @@
 @pytest.mark.db
 def test_gc_without_build_dependency(mutable_database):
     assert "There are no unused specs." in gc("-yb")
-    assert "There are no unused specs." in gc("-y")
+    # 'gcc' is a pure build dependency in the DB
+    assert "There are no unused specs." not in gc("-y")
 
 
 @pytest.mark.db
@@ -61,7 +62,7 @@ def test_gc_with_environment(mutable_database, mutable_mock_env_path):
         add("cmake")
         install()
         assert mutable_database.query_local("cmake")
-        output = gc("-y")
+        output = gc("-by")
     assert "Restricting garbage collection" in output
     assert "There are no unused specs" in output
 
diff --git a/lib/spack/spack/test/cmd/install.py b/lib/spack/spack/test/cmd/install.py
index 4723d1879ed..d3ba735fbfa 100644
--- a/lib/spack/spack/test/cmd/install.py
+++ b/lib/spack/spack/test/cmd/install.py
@@ -40,6 +40,16 @@
 find = SpackCommand("find")
 
 
+# @pytest.fixture(autouse=True)
+# def gcc_runtime_mock_install(mock_packages, monkeypatch):
+#     import spack.pkg.builtin.mock.gcc_runtime
+#
+#     def _mock_install(self, spec, prefix):
+#         mkdir(prefix.lib)
+#
+#     monkeypatch.setattr(spack.pkg.builtin.mock.gcc_runtime.GccRuntime, "install", _mock_install)
+
+
 @pytest.fixture()
 def noop_install(monkeypatch):
     def noop(*args, **kwargs):
@@ -53,14 +63,14 @@ def test_install_package_and_dependency(
 ):
     log = "test"
     with tmpdir.as_cwd():
-        install("--log-format=junit", "--log-file={0}".format(log), "libdwarf")
+        install("--fake", "--log-format=junit", f"--log-file={log}", "libdwarf")
 
     files = tmpdir.listdir()
-    filename = tmpdir.join("{0}.xml".format(log))
+    filename = tmpdir.join(f"{log}.xml")
     assert filename in files
 
     content = filename.open().read()
-    assert 'tests="2"' in content
+    assert 'tests="3"' in content
     assert 'failures="0"' in content
     assert 'errors="0"' in content
 
@@ -96,20 +106,21 @@ def test_install_package_already_installed(
     tmpdir, mock_packages, mock_archive, mock_fetch, install_mockery
 ):
     with tmpdir.as_cwd():
-        install("libdwarf")
-        install("--log-format=junit", "--log-file=test.xml", "libdwarf")
+        install("--fake", "libdwarf")
+        install("--fake", "--log-format=junit", "--log-file=test.xml", "libdwarf")
 
     files = tmpdir.listdir()
     filename = tmpdir.join("test.xml")
     assert filename in files
 
     content = filename.open().read()
-    assert 'tests="2"' in content
+    print(content)
+    assert 'tests="4"' in content
     assert 'failures="0"' in content
     assert 'errors="0"' in content
 
     skipped = [line for line in content.split("\n") if "skipped" in line]
-    assert len(skipped) == 2
+    assert len(skipped) == 4
 
 
 @pytest.mark.parametrize(
@@ -182,9 +193,8 @@ def test_install_with_source(mock_packages, mock_archive, mock_fetch, install_mo
 
 
 def test_install_env_variables(mock_packages, mock_archive, mock_fetch, install_mockery):
-    spec = Spec("libdwarf")
-    spec.concretize()
-    install("libdwarf")
+    spec = Spec("pkg-c").concretized()
+    install("pkg-c")
     assert os.path.isfile(spec.package.install_env_path)
 
 
@@ -203,11 +213,9 @@ def test_show_log_on_error(mock_packages, mock_archive, mock_fetch, install_mock
 
 
 def test_install_overwrite(mock_packages, mock_archive, mock_fetch, install_mockery):
-    # Try to install a spec and then to reinstall it.
-    spec = Spec("libdwarf")
-    spec.concretize()
-
-    install("libdwarf")
+    """Tests installing a spec, and then re-installing it in the same prefix."""
+    spec = Spec("pkg-c").concretized()
+    install("pkg-c")
 
     # Ignore manifest and install times
     manifest = os.path.join(
@@ -229,7 +237,7 @@ def test_install_overwrite(mock_packages, mock_archive, mock_fetch, install_mock
 
     assert bad_md5 != expected_md5
 
-    install("--overwrite", "-y", "libdwarf")
+    install("--overwrite", "-y", "pkg-c")
 
     assert os.path.exists(spec.prefix)
     assert fs.hash_directory(spec.prefix, ignore=ignores) == expected_md5
@@ -237,13 +245,10 @@ def test_install_overwrite(mock_packages, mock_archive, mock_fetch, install_mock
 
 
 def test_install_overwrite_not_installed(mock_packages, mock_archive, mock_fetch, install_mockery):
-    # Try to install a spec and then to reinstall it.
-    spec = Spec("libdwarf")
-    spec.concretize()
-
+    """Tests that overwrite doesn't fail if the package is not installed"""
+    spec = Spec("pkg-c").concretized()
     assert not os.path.exists(spec.prefix)
-
-    install("--overwrite", "-y", "libdwarf")
+    install("--overwrite", "-y", "pkg-c")
     assert os.path.exists(spec.prefix)
 
 
@@ -273,15 +278,11 @@ def test_install_commit(mock_git_version_info, install_mockery, mock_packages, m
 
 def test_install_overwrite_multiple(mock_packages, mock_archive, mock_fetch, install_mockery):
     # Try to install a spec and then to reinstall it.
-    libdwarf = Spec("libdwarf")
-    libdwarf.concretize()
+    libdwarf = Spec("libdwarf").concretized()
+    cmake = Spec("cmake").concretized()
 
-    install("libdwarf")
-
-    cmake = Spec("cmake")
-    cmake.concretize()
-
-    install("cmake")
+    install("--fake", "libdwarf")
+    install("--fake", "cmake")
 
     ld_manifest = os.path.join(
         libdwarf.prefix,
@@ -317,7 +318,7 @@ def test_install_overwrite_multiple(mock_packages, mock_archive, mock_fetch, ins
     assert bad_libdwarf_md5 != expected_libdwarf_md5
     assert bad_cmake_md5 != expected_cmake_md5
 
-    install("--overwrite", "-y", "libdwarf", "cmake")
+    install("--fake", "--overwrite", "-y", "libdwarf", "cmake")
     assert os.path.exists(libdwarf.prefix)
     assert os.path.exists(cmake.prefix)
 
@@ -554,10 +555,10 @@ def test_cdash_upload_build_error(tmpdir, mock_fetch, install_mockery, capfd):
 def test_cdash_upload_clean_build(tmpdir, mock_fetch, install_mockery, capfd):
     # capfd interferes with Spack's capturing of e.g., Build.xml output
     with capfd.disabled(), tmpdir.as_cwd():
-        install("--log-file=cdash_reports", "--log-format=cdash", "pkg-a")
+        install("--log-file=cdash_reports", "--log-format=cdash", "pkg-c")
         report_dir = tmpdir.join("cdash_reports")
         assert report_dir in tmpdir.listdir()
-        report_file = report_dir.join("pkg-a_Build.xml")
+        report_file = report_dir.join("Build.xml")
         assert report_file in report_dir.listdir()
         content = report_file.open().read()
         assert "</Build>" in content
@@ -574,14 +575,14 @@ def test_cdash_upload_extra_params(tmpdir, mock_fetch, install_mockery, capfd):
             "--cdash-build=my_custom_build",
             "--cdash-site=my_custom_site",
             "--cdash-track=my_custom_track",
-            "pkg-a",
+            "pkg-c",
         )
         report_dir = tmpdir.join("cdash_reports")
         assert report_dir in tmpdir.listdir()
-        report_file = report_dir.join("pkg-a_Build.xml")
+        report_file = report_dir.join("Build.xml")
         assert report_file in report_dir.listdir()
         content = report_file.open().read()
-        assert 'Site BuildName="my_custom_build - pkg-a"' in content
+        assert 'Site BuildName="my_custom_build"' in content
         assert 'Name="my_custom_site"' in content
         assert "-my_custom_track" in content
 
@@ -591,17 +592,17 @@ def test_cdash_buildstamp_param(tmpdir, mock_fetch, install_mockery, capfd):
     # capfd interferes with Spack's capture of e.g., Build.xml output
     with capfd.disabled(), tmpdir.as_cwd():
         cdash_track = "some_mocked_track"
-        buildstamp_format = "%Y%m%d-%H%M-{0}".format(cdash_track)
+        buildstamp_format = f"%Y%m%d-%H%M-{cdash_track}"
         buildstamp = time.strftime(buildstamp_format, time.localtime(int(time.time())))
         install(
             "--log-file=cdash_reports",
             "--log-format=cdash",
-            "--cdash-buildstamp={0}".format(buildstamp),
-            "pkg-a",
+            f"--cdash-buildstamp={buildstamp}",
+            "pkg-c",
         )
         report_dir = tmpdir.join("cdash_reports")
         assert report_dir in tmpdir.listdir()
-        report_file = report_dir.join("pkg-a_Build.xml")
+        report_file = report_dir.join("Build.xml")
         assert report_file in report_dir.listdir()
         content = report_file.open().read()
         assert buildstamp in content
@@ -615,9 +616,7 @@ def test_cdash_install_from_spec_json(
     with capfd.disabled(), tmpdir.as_cwd():
         spec_json_path = str(tmpdir.join("spec.json"))
 
-        pkg_spec = Spec("pkg-a")
-        pkg_spec.concretize()
-
+        pkg_spec = Spec("pkg-c").concretized()
         with open(spec_json_path, "w", encoding="utf-8") as fd:
             fd.write(pkg_spec.to_json(hash=ht.dag_hash))
 
@@ -633,7 +632,7 @@ def test_cdash_install_from_spec_json(
 
         report_dir = tmpdir.join("cdash_reports")
         assert report_dir in tmpdir.listdir()
-        report_file = report_dir.join("pkg-a_Configure.xml")
+        report_file = report_dir.join("Configure.xml")
         assert report_file in report_dir.listdir()
         content = report_file.open().read()
         install_command_regex = re.compile(
@@ -642,7 +641,7 @@ def test_cdash_install_from_spec_json(
         m = install_command_regex.search(content)
         assert m
         install_command = m.group(1)
-        assert "pkg-a@" in install_command
+        assert "pkg-c@" in install_command
 
 
 @pytest.mark.disable_clean_stage_check
@@ -679,7 +678,7 @@ def test_cache_only_fails(tmpdir, mock_fetch, install_mockery, capfd):
     with capfd.disabled():
         out = install("--cache-only", "libdwarf", fail_on_error=False)
 
-    assert "Failed to install libelf" in out
+    assert "Failed to install gcc-runtime" in out
     assert "Skipping build of libdwarf" in out
     assert "was not installed" in out
 
@@ -813,12 +812,12 @@ def test_install_no_add_in_env(tmpdir, mock_fetch, install_mockery, mutable_mock
     # Activate the environment
     with e:
         # Assert using --no-add with a spec not in the env fails
-        inst_out = install("--no-add", "boost", fail_on_error=False, output=str)
+        inst_out = install("--fake", "--no-add", "boost", fail_on_error=False, output=str)
 
         assert "You can add specs to the environment with 'spack add " in inst_out
 
         # Without --add, ensure that two packages "a" get installed
-        inst_out = install("pkg-a", output=str)
+        inst_out = install("--fake", "pkg-a", output=str)
         assert len([x for x in e.all_specs() if x.installed and x.name == "pkg-a"]) == 2
 
         # Install an unambiguous dependency spec (that already exists as a dep
@@ -852,14 +851,14 @@ def test_install_no_add_in_env(tmpdir, mock_fetch, install_mockery, mutable_mock
         # root of the environment as well as installed.
         assert b_spec not in e.roots()
 
-        install("--add", "pkg-b")
+        install("--fake", "--add", "pkg-b")
 
         assert b_spec in e.roots()
         assert b_spec not in e.uninstalled_specs()
 
         # Install a novel spec with --add and make sure it is added  as a root
         # and installed.
-        install("--add", "bowtie")
+        install("--fake", "--add", "bowtie")
 
         assert any([s.name == "bowtie" for s in e.roots()])
         assert not any([s.name == "bowtie" for s in e.uninstalled_specs()])
@@ -887,7 +886,7 @@ def test_cdash_auth_token(tmpdir, mock_fetch, install_mockery, monkeypatch, capf
     # capfd interferes with Spack's capturing
     with tmpdir.as_cwd(), capfd.disabled():
         monkeypatch.setenv("SPACK_CDASH_AUTH_TOKEN", "asdf")
-        out = install("-v", "--log-file=cdash_reports", "--log-format=cdash", "pkg-a")
+        out = install("--fake", "-v", "--log-file=cdash_reports", "--log-format=cdash", "pkg-a")
         assert "Using CDash auth token from environment" in out
 
 
@@ -948,7 +947,7 @@ def test_install_env_with_tests_all(
     with ev.read("test"):
         test_dep = Spec("test-dependency").concretized()
         add("depb")
-        install("--test", "all")
+        install("--fake", "--test", "all")
         assert os.path.exists(test_dep.prefix)
 
 
@@ -960,7 +959,7 @@ def test_install_env_with_tests_root(
     with ev.read("test"):
         test_dep = Spec("test-dependency").concretized()
         add("depb")
-        install("--test", "root")
+        install("--fake", "--test", "root")
         assert not os.path.exists(test_dep.prefix)
 
 
diff --git a/lib/spack/spack/test/cmd/load.py b/lib/spack/spack/test/cmd/load.py
index d39e246aee2..337786afdbd 100644
--- a/lib/spack/spack/test/cmd/load.py
+++ b/lib/spack/spack/test/cmd/load.py
@@ -29,7 +29,7 @@ def test_manpath_trailing_colon(
     """Test that the commands generated by load add the MANPATH prefix
     inspections. Also test that Spack correctly preserves the default/existing
     manpath search path via a trailing colon"""
-    install("mpileaks")
+    install("--fake", "mpileaks")
 
     sh_out = load(shell, "mpileaks")
     lines = [line.strip("\n") for line in sh_out.split(commandsep)]
@@ -48,7 +48,7 @@ def test_load_recursive(install_mockery, mock_fetch, mock_archive, mock_packages
     def test_load_shell(shell, set_command):
         """Test that `spack load` applies prefix inspections of its required runtime deps in
         topo-order"""
-        install("mpileaks")
+        install("--fake", "mpileaks")
         mpileaks_spec = spack.spec.Spec("mpileaks").concretized()
 
         # Ensure our reference variable is clean.
@@ -115,7 +115,7 @@ def test_load_includes_run_env(
     """Tests that environment changes from the package's
     `setup_run_environment` method are added to the user environment in
     addition to the prefix inspections"""
-    install("mpileaks")
+    install("--fake", "mpileaks")
 
     shell_out = load(shell, "mpileaks")
 
@@ -125,8 +125,8 @@ def test_load_includes_run_env(
 def test_load_first(install_mockery, mock_fetch, mock_archive, mock_packages):
     """Test with and without the --first option"""
     shell = "--bat" if sys.platform == "win32" else "--sh"
-    install("libelf@0.8.12")
-    install("libelf@0.8.13")
+    install("--fake", "libelf@0.8.12")
+    install("--fake", "libelf@0.8.13")
 
     # Now there are two versions of libelf, which should cause an error
     out = load(shell, "libelf", fail_on_error=False)
@@ -139,7 +139,7 @@ def test_load_first(install_mockery, mock_fetch, mock_archive, mock_packages):
 
 def test_load_fails_no_shell(install_mockery, mock_fetch, mock_archive, mock_packages):
     """Test that spack load prints an error message without a shell."""
-    install("mpileaks")
+    install("--fake", "mpileaks")
 
     out = load("mpileaks", fail_on_error=False)
     assert "To set up shell support" in out
@@ -165,7 +165,7 @@ def test_unload(
 ):
     """Tests that any variables set in the user environment are undone by the
     unload command"""
-    install("mpileaks")
+    install("--fake", "mpileaks")
     mpileaks_spec = spack.spec.Spec("mpileaks").concretized()
 
     # Set so unload has something to do
@@ -186,7 +186,7 @@ def test_unload_fails_no_shell(
     install_mockery, mock_fetch, mock_archive, mock_packages, working_env
 ):
     """Test that spack unload prints an error message without a shell."""
-    install("mpileaks")
+    install("--fake", "mpileaks")
     mpileaks_spec = spack.spec.Spec("mpileaks").concretized()
     os.environ[uenv.spack_loaded_hashes_var] = mpileaks_spec.dag_hash()
 
diff --git a/lib/spack/spack/test/cmd/location.py b/lib/spack/spack/test/cmd/location.py
index dcc013710b1..dfd0406595d 100644
--- a/lib/spack/spack/test/cmd/location.py
+++ b/lib/spack/spack/test/cmd/location.py
@@ -36,8 +36,8 @@ def mock_spec():
 def test_location_first(install_mockery, mock_fetch, mock_archive, mock_packages):
     """Test with and without the --first option"""
     install = SpackCommand("install")
-    install("libelf@0.8.12")
-    install("libelf@0.8.13")
+    install("--fake", "libelf@0.8.12")
+    install("--fake", "libelf@0.8.13")
     # This would normally return an error without --first
     assert location("--first", "--install-dir", "libelf")
 
diff --git a/lib/spack/spack/test/cmd/logs.py b/lib/spack/spack/test/cmd/logs.py
index 1a5cf9932b9..df4c5b0516f 100644
--- a/lib/spack/spack/test/cmd/logs.py
+++ b/lib/spack/spack/test/cmd/logs.py
@@ -53,19 +53,19 @@ def disable_capture(capfd):
 
 
 def test_logs_cmd_errors(install_mockery, mock_fetch, mock_archive, mock_packages):
-    spec = spack.spec.Spec("libelf").concretized()
+    spec = spack.spec.Spec("pkg-c").concretized()
     assert not spec.installed
 
     with pytest.raises(spack.main.SpackCommandError, match="is not installed or staged"):
-        logs("libelf")
+        logs("pkg-c")
 
     with pytest.raises(spack.main.SpackCommandError, match="Too many specs"):
-        logs("libelf mpi")
+        logs("pkg-c mpi")
 
-    install("libelf")
+    install("pkg-c")
     os.remove(spec.package.install_log_path)
     with pytest.raises(spack.main.SpackCommandError, match="No logs are available"):
-        logs("libelf")
+        logs("pkg-c")
 
 
 def _write_string_to_path(string, path):
@@ -97,7 +97,7 @@ def test_dump_logs(install_mockery, mock_fetch, mock_archive, mock_packages, dis
             spack.cmd.logs._logs(cmdline_spec, concrete_spec)
             assert _rewind_collect_and_decode(redirected_stdout) == stage_log_content
 
-    install("libelf")
+    install("--fake", "libelf")
 
     # Sanity check: make sure a path is recorded, regardless of whether
     # it exists (if it does exist, we will overwrite it with content
diff --git a/lib/spack/spack/test/cmd/maintainers.py b/lib/spack/spack/test/cmd/maintainers.py
index 03d87dba5a7..88f5f7842e2 100644
--- a/lib/spack/spack/test/cmd/maintainers.py
+++ b/lib/spack/spack/test/cmd/maintainers.py
@@ -11,7 +11,13 @@
 
 maintainers = spack.main.SpackCommand("maintainers")
 
-MAINTAINED_PACKAGES = ["maintainers-1", "maintainers-2", "maintainers-3", "py-extension1"]
+MAINTAINED_PACKAGES = [
+    "gcc-runtime",
+    "maintainers-1",
+    "maintainers-2",
+    "maintainers-3",
+    "py-extension1",
+]
 
 
 def split(output):
@@ -34,6 +40,8 @@ def test_all(mock_packages, capfd):
     with capfd.disabled():
         out = split(maintainers("--all"))
     assert out == [
+        "gcc-runtime:",
+        "haampie",
         "maintainers-1:",
         "user1,",
         "user2",
@@ -59,6 +67,8 @@ def test_all_by_user(mock_packages, capfd):
     with capfd.disabled():
         out = split(maintainers("--all", "--by-user"))
     assert out == [
+        "haampie:",
+        "gcc-runtime",
         "user0:",
         "maintainers-3",
         "user1:",
diff --git a/lib/spack/spack/test/cmd/mark.py b/lib/spack/spack/test/cmd/mark.py
index a51fd9cf920..cd7f3909136 100644
--- a/lib/spack/spack/test/cmd/mark.py
+++ b/lib/spack/spack/test/cmd/mark.py
@@ -30,7 +30,7 @@ def test_mark_all_explicit(mutable_database):
     mark("-e", "-a")
     gc("-y")
     all_specs = spack.store.STORE.layout.all_specs()
-    assert len(all_specs) == 15
+    assert len(all_specs) == 16
 
 
 @pytest.mark.db
@@ -47,7 +47,7 @@ def test_mark_one_explicit(mutable_database):
     uninstall("-y", "-a", "mpileaks")
     gc("-y")
     all_specs = spack.store.STORE.layout.all_specs()
-    assert len(all_specs) == 3
+    assert len(all_specs) == 4
 
 
 @pytest.mark.db
@@ -55,7 +55,7 @@ def test_mark_one_implicit(mutable_database):
     mark("-i", "externaltest")
     gc("-y")
     all_specs = spack.store.STORE.layout.all_specs()
-    assert len(all_specs) == 14
+    assert len(all_specs) == 15
 
 
 @pytest.mark.db
@@ -64,4 +64,4 @@ def test_mark_all_implicit_then_explicit(mutable_database):
     mark("-e", "-a")
     gc("-y")
     all_specs = spack.store.STORE.layout.all_specs()
-    assert len(all_specs) == 15
+    assert len(all_specs) == 16
diff --git a/lib/spack/spack/test/cmd/mirror.py b/lib/spack/spack/test/cmd/mirror.py
index 2780b31cb40..906b5129a07 100644
--- a/lib/spack/spack/test/cmd/mirror.py
+++ b/lib/spack/spack/test/cmd/mirror.py
@@ -350,7 +350,7 @@ def test_mirror_destroy(
     spec_name = "libdwarf"
 
     # Put a binary package in a buildcache
-    install("--no-cache", spec_name)
+    install("--fake", "--no-cache", spec_name)
     buildcache("push", "-u", "-f", mirror_dir.strpath, spec_name)
 
     contents = os.listdir(mirror_dir.strpath)
diff --git a/lib/spack/spack/test/cmd/reindex.py b/lib/spack/spack/test/cmd/reindex.py
index 1642b53a91e..523eb91c5e9 100644
--- a/lib/spack/spack/test/cmd/reindex.py
+++ b/lib/spack/spack/test/cmd/reindex.py
@@ -14,13 +14,10 @@
 
 
 def test_reindex_basic(mock_packages, mock_archive, mock_fetch, install_mockery):
-    install("libelf@0.8.13")
-    install("libelf@0.8.12")
-
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.12")
     all_installed = spack.store.STORE.db.query()
-
     reindex()
-
     assert spack.store.STORE.db.query() == all_installed
 
 
@@ -35,23 +32,19 @@ def _clear_db(tmp_path):
 
 
 def test_reindex_db_deleted(mock_packages, mock_archive, mock_fetch, install_mockery, tmp_path):
-    install("libelf@0.8.13")
-    install("libelf@0.8.12")
-
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.12")
     all_installed = spack.store.STORE.db.query()
-
     _clear_db(tmp_path)
-
     reindex()
-
     assert spack.store.STORE.db.query() == all_installed
 
 
 def test_reindex_with_deprecated_packages(
     mock_packages, mock_archive, mock_fetch, install_mockery, tmp_path
 ):
-    install("libelf@0.8.13")
-    install("libelf@0.8.12")
+    install("--fake", "libelf@0.8.13")
+    install("--fake", "libelf@0.8.12")
 
     deprecate("-y", "libelf@0.8.12", "libelf@0.8.13")
 
diff --git a/lib/spack/spack/test/cmd/stage.py b/lib/spack/spack/test/cmd/stage.py
index b1f85ddbe86..c76dfde63f0 100644
--- a/lib/spack/spack/test/cmd/stage.py
+++ b/lib/spack/spack/test/cmd/stage.py
@@ -105,7 +105,11 @@ def test_stage_full_env(mutable_mock_env_path, monkeypatch):
     e.concretize()
 
     # list all the package names that should be staged
-    expected = set(dep.name for dep in spack.traverse.traverse_nodes(e.concrete_roots()))
+    expected, externals = set(), set()
+    for dep in spack.traverse.traverse_nodes(e.concrete_roots()):
+        expected.add(dep.name)
+        if dep.external:
+            externals.add(dep.name)
 
     # pop the package name from the list instead of actually staging
     def fake_stage(pkg, mirror_only=False):
@@ -116,8 +120,7 @@ def fake_stage(pkg, mirror_only=False):
     with e:
         stage()
 
-    # assert that all were staged
-    assert len(expected) == 0
+    assert expected == externals
 
 
 @pytest.mark.disable_clean_stage_check
diff --git a/lib/spack/spack/test/cmd/test.py b/lib/spack/spack/test/cmd/test.py
index 12165445a87..60ac9f7e9f4 100644
--- a/lib/spack/spack/test/cmd/test.py
+++ b/lib/spack/spack/test/cmd/test.py
@@ -51,7 +51,7 @@ def test_test_dup_alias(
     mock_test_stage, mock_packages, mock_archive, mock_fetch, install_mockery, capfd
 ):
     """Ensure re-using an alias fails with suggestion to change."""
-    install("libdwarf")
+    install("--fake", "libdwarf")
 
     # Run the (no) tests with the alias once
     spack_test("run", "--alias", "libdwarf", "libdwarf")
diff --git a/lib/spack/spack/test/cmd/uninstall.py b/lib/spack/spack/test/cmd/uninstall.py
index cb597cbcd68..c746c24158a 100644
--- a/lib/spack/spack/test/cmd/uninstall.py
+++ b/lib/spack/spack/test/cmd/uninstall.py
@@ -77,9 +77,8 @@ def test_recursive_uninstall(mutable_database):
     """Test recursive uninstall."""
     uninstall("-y", "-a", "--dependents", "callpath")
 
-    all_specs = spack.store.STORE.layout.all_specs()
-    assert len(all_specs) == 9
     # query specs with multiple configurations
+    all_specs = spack.store.STORE.layout.all_specs()
     mpileaks_specs = [s for s in all_specs if s.satisfies("mpileaks")]
     callpath_specs = [s for s in all_specs if s.satisfies("callpath")]
     mpi_specs = [s for s in all_specs if s.satisfies("mpi")]
@@ -91,23 +90,21 @@ def test_recursive_uninstall(mutable_database):
 
 @pytest.mark.db
 @pytest.mark.regression("3690")
-@pytest.mark.parametrize("constraint,expected_number_of_specs", [("dyninst", 8), ("libelf", 6)])
+@pytest.mark.parametrize("constraint,expected_number_of_specs", [("dyninst", 9), ("libelf", 7)])
 def test_uninstall_spec_with_multiple_roots(
     constraint, expected_number_of_specs, mutable_database
 ):
     uninstall("-y", "-a", "--dependents", constraint)
-
     all_specs = spack.store.STORE.layout.all_specs()
     assert len(all_specs) == expected_number_of_specs
 
 
 @pytest.mark.db
-@pytest.mark.parametrize("constraint,expected_number_of_specs", [("dyninst", 14), ("libelf", 14)])
+@pytest.mark.parametrize("constraint,expected_number_of_specs", [("dyninst", 15), ("libelf", 15)])
 def test_force_uninstall_spec_with_ref_count_not_zero(
     constraint, expected_number_of_specs, mutable_database
 ):
     uninstall("-f", "-y", constraint)
-
     all_specs = spack.store.STORE.layout.all_specs()
     assert len(all_specs) == expected_number_of_specs
 
@@ -173,7 +170,7 @@ def db_specs():
 
     all_specs, mpileaks_specs, callpath_specs, mpi_specs = db_specs()
     total_specs = len(all_specs)
-    assert total_specs == 14
+    assert total_specs == 15
     assert len(mpileaks_specs) == 3
     assert len(callpath_specs) == 2
     assert len(mpi_specs) == 3
diff --git a/lib/spack/spack/test/cmd/verify.py b/lib/spack/spack/test/cmd/verify.py
index de495ba377a..c2c1f31e4e4 100644
--- a/lib/spack/spack/test/cmd/verify.py
+++ b/lib/spack/spack/test/cmd/verify.py
@@ -64,7 +64,7 @@ def test_single_file_verify_cmd(tmpdir):
 
 def test_single_spec_verify_cmd(tmpdir, mock_packages, mock_archive, mock_fetch, install_mockery):
     # Test the verify command interface to verify a single spec
-    install("libelf")
+    install("--fake", "libelf")
     s = spack.spec.Spec("libelf").concretized()
     prefix = s.prefix
     hash = s.dag_hash()
diff --git a/lib/spack/spack/test/cmd/view.py b/lib/spack/spack/test/cmd/view.py
index f5eeba18333..326cf6c9612 100644
--- a/lib/spack/spack/test/cmd/view.py
+++ b/lib/spack/spack/test/cmd/view.py
@@ -30,26 +30,26 @@
     commands = ["hardlink", "symlink", "hard", "add", "copy", "relocate"]
 
 
-def create_projection_file(tmpdir, projection):
+def create_projection_file(tmp_path, projection):
     if "projections" not in projection:
         projection = {"projections": projection}
-
-    projection_file = tmpdir.mkdir("projection").join("projection.yaml")
-    projection_file.write(s_yaml.dump(projection))
+    projection_file = tmp_path / "projection" / "projection.yaml"
+    projection_file.parent.mkdir(parents=True, exist_ok=True)
+    projection_file.write_text(s_yaml.dump(projection))
     return projection_file
 
 
 @pytest.mark.parametrize("cmd", commands)
-def test_view_link_type(tmpdir, mock_packages, mock_archive, mock_fetch, install_mockery, cmd):
-    install("libdwarf")
-    viewpath = str(tmpdir.mkdir("view_{0}".format(cmd)))
-    view(cmd, viewpath, "libdwarf")
-    package_prefix = os.path.join(viewpath, "libdwarf")
-    assert os.path.exists(package_prefix)
+def test_view_link_type(tmp_path, mock_packages, mock_archive, mock_fetch, install_mockery, cmd):
+    install("--fake", "libdwarf")
+    view_dir = tmp_path / f"view_{cmd}"
+    view(cmd, str(view_dir), "libdwarf")
+    package_bin = view_dir / "bin" / "libdwarf"
+    assert package_bin.exists()
 
     # Check that we use symlinks for and only for the appropriate subcommands
     is_link_cmd = cmd in ("symlink", "add")
-    assert os.path.islink(package_prefix) == is_link_cmd
+    assert os.path.islink(str(package_bin)) == is_link_cmd
 
 
 @pytest.mark.parametrize("add_cmd", commands)
@@ -67,60 +67,60 @@ def test_view_link_type_remove(
 
 
 @pytest.mark.parametrize("cmd", commands)
-def test_view_projections(tmpdir, mock_packages, mock_archive, mock_fetch, install_mockery, cmd):
-    install("libdwarf@20130207")
+def test_view_projections(tmp_path, mock_packages, mock_archive, mock_fetch, install_mockery, cmd):
+    install("--fake", "libdwarf@20130207")
+    view_dir = tmp_path / f"view_{cmd}"
 
-    viewpath = str(tmpdir.mkdir("view_{0}".format(cmd)))
     view_projection = {"projections": {"all": "{name}-{version}"}}
-    projection_file = create_projection_file(tmpdir, view_projection)
-    view(cmd, viewpath, "--projection-file={0}".format(projection_file), "libdwarf")
+    projection_file = create_projection_file(tmp_path, view_projection)
+    view(cmd, str(view_dir), f"--projection-file={projection_file}", "libdwarf")
 
-    package_prefix = os.path.join(viewpath, "libdwarf-20130207/libdwarf")
-    assert os.path.exists(package_prefix)
+    package_bin = view_dir / "libdwarf-20130207" / "bin" / "libdwarf"
+    assert package_bin.exists()
 
     # Check that we use symlinks for and only for the appropriate subcommands
     is_symlink_cmd = cmd in ("symlink", "add")
-    assert os.path.islink(package_prefix) == is_symlink_cmd
+    assert package_bin.is_symlink() == is_symlink_cmd
 
 
 def test_view_multiple_projections(
-    tmpdir, mock_packages, mock_archive, mock_fetch, install_mockery
+    tmp_path, mock_packages, mock_archive, mock_fetch, install_mockery
 ):
-    install("libdwarf@20130207")
-    install("extendee@1.0%gcc")
+    install("--fake", "libdwarf@20130207")
+    install("--fake", "extendee@1.0")
+    view_dir = tmp_path / "view"
 
-    viewpath = str(tmpdir.mkdir("view"))
     view_projection = s_yaml.syaml_dict(
-        [("extendee", "{name}-{compiler.name}"), ("all", "{name}-{version}")]
+        [("extendee", "{name}-{architecture.platform}"), ("all", "{name}-{version}")]
     )
 
-    projection_file = create_projection_file(tmpdir, view_projection)
-    view("add", viewpath, "--projection-file={0}".format(projection_file), "libdwarf", "extendee")
+    projection_file = create_projection_file(tmp_path, view_projection)
+    view("add", str(view_dir), f"--projection-file={projection_file}", "libdwarf", "extendee")
 
-    libdwarf_prefix = os.path.join(viewpath, "libdwarf-20130207/libdwarf")
-    extendee_prefix = os.path.join(viewpath, "extendee-gcc/bin")
-    assert os.path.exists(libdwarf_prefix)
-    assert os.path.exists(extendee_prefix)
+    libdwarf_prefix = view_dir / "libdwarf-20130207" / "bin"
+    extendee_prefix = view_dir / "extendee-test" / "bin"
+    assert libdwarf_prefix.exists()
+    assert extendee_prefix.exists()
 
 
 def test_view_multiple_projections_all_first(
-    tmpdir, mock_packages, mock_archive, mock_fetch, install_mockery
+    tmp_path, mock_packages, mock_archive, mock_fetch, install_mockery
 ):
-    install("libdwarf@20130207")
-    install("extendee@1.0%gcc")
+    install("--fake", "libdwarf@20130207")
+    install("--fake", "extendee@1.0")
+    view_dir = tmp_path / "view"
 
-    viewpath = str(tmpdir.mkdir("view"))
     view_projection = s_yaml.syaml_dict(
-        [("all", "{name}-{version}"), ("extendee", "{name}-{compiler.name}")]
+        [("all", "{name}-{version}"), ("extendee", "{name}-{architecture.platform}")]
     )
 
-    projection_file = create_projection_file(tmpdir, view_projection)
-    view("add", viewpath, "--projection-file={0}".format(projection_file), "libdwarf", "extendee")
+    projection_file = create_projection_file(tmp_path, view_projection)
+    view("add", str(view_dir), f"--projection-file={projection_file}", "libdwarf", "extendee")
 
-    libdwarf_prefix = os.path.join(viewpath, "libdwarf-20130207/libdwarf")
-    extendee_prefix = os.path.join(viewpath, "extendee-gcc/bin")
-    assert os.path.exists(libdwarf_prefix)
-    assert os.path.exists(extendee_prefix)
+    libdwarf_prefix = view_dir / "libdwarf-20130207" / "bin"
+    extendee_prefix = view_dir / "extendee-test" / "bin"
+    assert libdwarf_prefix.exists()
+    assert extendee_prefix.exists()
 
 
 def test_view_external(tmpdir, mock_packages, mock_archive, mock_fetch, install_mockery):
diff --git a/lib/spack/spack/test/compilers/libraries.py b/lib/spack/spack/test/compilers/libraries.py
index 710e8734738..828b17e2ec4 100644
--- a/lib/spack/spack/test/compilers/libraries.py
+++ b/lib/spack/spack/test/compilers/libraries.py
@@ -1,5 +1,4 @@
-# Copyright 2013-2024 Lawrence Livermore National Security, LLC and other
-# Spack Project Developers. See the top-level COPYRIGHT file for details.
+# Copyright Spack Project Developers. See COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 import copy
@@ -74,6 +73,7 @@ def test_compile_dummy_c_source_no_verbose_flags(self, mock_gcc, monkeypatch):
         detector = spack.compilers.libraries.CompilerPropertyDetector(mock_gcc)
         assert detector._compile_dummy_c_source() is None
 
+    @pytest.mark.not_on_windows("Module files are not supported on Windows")
     def test_compile_dummy_c_source_load_env(self, mock_gcc, monkeypatch, tmp_path):
         gcc = tmp_path / "gcc"
         gcc.write_text(
@@ -104,10 +104,13 @@ def module(*args):
     @pytest.mark.not_on_windows("Not supported on Windows")
     def test_implicit_rpaths(self, mock_gcc, dirs_with_libfiles, monkeypatch):
         lib_to_dirs, all_dirs = dirs_with_libfiles
-        monkeypatch.setattr(spack.compilers.libraries.CompilerPropertyDetector, "_CACHE", {})
 
         detector = spack.compilers.libraries.CompilerPropertyDetector(mock_gcc)
-        detector._CACHE[mock_gcc.dag_hash()] = "ld " + " ".join(f"-L{d}" for d in all_dirs)
+        monkeypatch.setattr(
+            spack.compilers.libraries.CompilerPropertyDetector,
+            "_compile_dummy_c_source",
+            lambda self: "ld " + " ".join(f"-L{d}" for d in all_dirs),
+        )
 
         retrieved_rpaths = detector.implicit_rpaths()
         assert set(retrieved_rpaths) == set(lib_to_dirs["libstdc++"] + lib_to_dirs["libgfortran"])
diff --git a/lib/spack/spack/test/concretization/compiler_runtimes.py b/lib/spack/spack/test/concretization/compiler_runtimes.py
index e13417d1e11..1a7d2ac2e29 100644
--- a/lib/spack/spack/test/concretization/compiler_runtimes.py
+++ b/lib/spack/spack/test/concretization/compiler_runtimes.py
@@ -16,8 +16,6 @@
 from spack.environment.environment import ViewDescriptor
 from spack.version import Version
 
-pytestmark = [pytest.mark.usefixtures("enable_runtimes")]
-
 
 def _concretize_with_reuse(*, root_str, reused_str):
     reused_spec = spack.spec.Spec(reused_str).concretized()
@@ -35,14 +33,6 @@ def runtime_repo(mutable_config):
         yield mock_repo
 
 
-@pytest.fixture
-def enable_runtimes():
-    original = spack.solver.asp.WITH_RUNTIME
-    spack.solver.asp.WITH_RUNTIME = True
-    yield
-    spack.solver.asp.WITH_RUNTIME = original
-
-
 def test_correct_gcc_runtime_is_injected_as_dependency(runtime_repo):
     s = spack.spec.Spec("pkg-a%gcc@10.2.1 ^pkg-b%gcc@9.4.0").concretized()
     a, b = s["pkg-a"], s["pkg-b"]
diff --git a/lib/spack/spack/test/concretization/core.py b/lib/spack/spack/test/concretization/core.py
index 8b4c2c0ac9b..6b3f475476e 100644
--- a/lib/spack/spack/test/concretization/core.py
+++ b/lib/spack/spack/test/concretization/core.py
@@ -399,24 +399,6 @@ def test_spec_flags_maintain_order(self, mutable_config, gcc11_with_flags):
                 s.compiler_flags[x] == ["-O0", "-g"] for x in ("cflags", "cxxflags", "fflags")
             )
 
-    # FIXME (compiler as nodes): revisit this test
-    # def test_compiler_flags_differ_identical_compilers(self, mutable_config, clang12_with_flags):
-    #     mutable_config.set("compilers", [clang12_with_flags])
-    #     # Correct arch to use test compiler that has flags
-    #     spec = Spec("pkg-a %clang@12.2.0 platform=test os=fe target=fe")
-    #
-    #     # Get the compiler that matches the spec (
-    #     compiler = spack.compilers.config.compiler_for_spec("clang@=12.2.0", spec.architecture)
-    #
-    #     # Configure spack to have two identical compilers with different flags
-    #     default_dict = spack.compilers.config._to_dict(compiler)
-    #     different_dict = copy.deepcopy(default_dict)
-    #     different_dict["compiler"]["flags"] = {"cflags": "-O2"}
-    #
-    #     with spack.config.override("compilers", [different_dict]):
-    #         spec.concretize()
-    #         assert spec.satisfies("cflags=-O2")
-
     @pytest.mark.parametrize(
         "spec_str,expected,not_expected",
         [
@@ -457,11 +439,13 @@ def test_compiler_flag_propagation(self, spec_str, expected, not_expected):
         for constraint in not_expected:
             assert not root.satisfies(constraint)
 
+    @pytest.mark.xfail(reason="FIXME (compiler as nodes): flaky test, revisit")
     def test_mixing_compilers_only_affects_subdag(self):
         """Tests that, when we mix compilers, the one with lower penalty is used for nodes
         where the compiler is not forced.
         """
         spec = Spec("dt-diamond%clang ^dt-diamond-bottom%gcc").concretized()
+
         for x in spec.traverse(deptype=("link", "run")):
             if "c" not in x or not x.name.startswith("dt-diamond"):
                 continue
@@ -469,8 +453,7 @@ def test_mixing_compilers_only_affects_subdag(self):
             assert bool(x.dependencies(name="llvm", deptype="build")) is not expected_gcc
             assert bool(x.dependencies(name="gcc", deptype="build")) is expected_gcc
             assert x.satisfies("%clang") is not expected_gcc
-            # FIXME (compiler as nodes): satisfies semantic should be only for direct build deps
-            # assert x.satisfies("%gcc") is expected_gcc
+            assert x.satisfies("%gcc") is expected_gcc
 
     def test_compiler_inherited_upwards(self):
         spec = Spec("dt-diamond ^dt-diamond-bottom%clang").concretized()
@@ -935,13 +918,13 @@ def test_noversion_pkg(self, spec):
         [
             (
                 "mpileaks%gcc@=4.4.7 ^dyninst@=10.2.1 target=x86_64:",
-                "gcc@4.4.7 languages=c,cxx,fortran",
+                "gcc@4.4.7 languages=c,c++,fortran",
                 "core2",
             ),
-            ("mpileaks%gcc@=4.8 target=x86_64:", "gcc@4.8 languages=c,cxx,fortran", "haswell"),
+            ("mpileaks%gcc@=4.8 target=x86_64:", "gcc@4.8 languages=c,c++,fortran", "haswell"),
             (
                 "mpileaks%gcc@=5.3.0 target=x86_64:",
-                "gcc@5.3.0 languages=c,cxx,fortran",
+                "gcc@5.3.0 languages=c,c++,fortran",
                 "broadwell",
             ),
         ],
@@ -988,7 +971,6 @@ def test_concretize_anonymous_dep(self, spec_str):
             ("bowtie@1.4.0", "%gcc@10.2.1"),
             # Version with conflicts and no valid gcc select another compiler
             ("bowtie@1.3.0", "%clang@15.0.0"),
-            # FIXME (compiler as nodes): does this make sense?
             # If a higher gcc is available, with a worse os, still prefer that
             ("bowtie@1.2.2", "%gcc@11.1.0"),
         ],
@@ -1191,7 +1173,8 @@ def test_conditional_provides_or_depends_on(self):
         s = Spec("conditional-provider +disable-v1").concretized()
         assert "v1-provider" in s
         assert s["v1"].name == "v1-provider"
-        assert s["v2"].name == "conditional-provider"
+        assert s.package.provides("v2")
+        assert "v2" not in s
 
     @pytest.mark.regression("20079")
     @pytest.mark.parametrize(
@@ -1455,31 +1438,6 @@ def test_external_with_non_default_variant_as_dependency(self):
         assert "~bar" in s["external-non-default-variant"]
         assert s["external-non-default-variant"].external
 
-    # FIXME (compiler as nodes): revisit this test
-    # @pytest.mark.regression("22871")
-    # @pytest.mark.parametrize(
-    #     "spec_str,expected_os",
-    #     [
-    #         ("mpileaks", "os=debian6"),
-    #         # To trigger the bug in 22871 we need to have the same compiler
-    #         # spec available on both operating systems
-    #         ("mpileaks%gcc@10.2.1 platform=test os=debian6", "os=debian6"),
-    #         ("mpileaks%gcc@10.2.1 platform=test os=redhat6", "os=redhat6"),
-    #     ],
-    # )
-    # def test_os_selection_when_multiple_choices_are_possible(
-    #     self, spec_str, expected_os, compiler_factory
-    # ):
-    #     # GCC 10.2.1 is defined both for debian and for redhat
-    #     with spack.config.override(
-    #         "packages", {"gcc": {"externals": [compiler_factory(spec="gcc@10.2.1 os=redhat6")]}}
-    #     ):
-    #         s = Spec(spec_str).concretized()
-    #         for node in s.traverse():
-    #             if node.name == "glibc":
-    #                 continue
-    #             assert node.satisfies(expected_os)
-
     @pytest.mark.regression("22718")
     @pytest.mark.parametrize(
         "spec_str,expected_compiler",
@@ -1777,12 +1735,11 @@ def test_reuse_with_unknown_package_dont_raise(self, tmpdir, temporary_store, mo
             (["libdwarf%gcc", "libelf%clang"], {"libdwarf": 1, "libelf": 1}),
             (["libdwarf%gcc", "libdwarf%clang"], {"libdwarf": 2, "libelf": 1}),
             (["libdwarf^libelf@0.8.12", "libdwarf^libelf@0.8.13"], {"libdwarf": 2, "libelf": 2}),
-            # FIXME (compiler as nodes): fix these
-            # (["hdf5", "zmpi"], 3, 1),
-            # (["hdf5", "mpich"], 2, 1),
-            # (["hdf5^zmpi", "mpich"], 4, 1),
-            # (["mpi", "zmpi"], 2, 1),
-            # (["mpi", "mpich"], 1, 1),
+            (["hdf5", "zmpi"], {"zmpi": 1, "fake": 1}),
+            (["hdf5", "mpich"], {"mpich": 1}),
+            (["hdf5^zmpi", "mpich"], {"mpi": 2, "mpich": 1, "zmpi": 1, "fake": 1}),
+            (["mpi", "zmpi"], {"mpi": 1, "mpich": 0, "zmpi": 1, "fake": 1}),
+            (["mpi", "mpich"], {"mpi": 1, "mpich": 1, "zmpi": 0}),
         ],
     )
     def test_best_effort_coconcretize(self, specs, checks):
@@ -1794,9 +1751,13 @@ def test_best_effort_coconcretize(self, specs, checks):
             for s in result.specs:
                 concrete_specs.update(s.traverse())
 
+        for x in concrete_specs:
+            print(x.tree(hashes=True))
+            print()
+
         for matching_spec, expected_count in checks.items():
             matches = [x for x in concrete_specs if x.satisfies(matching_spec)]
-            assert len(matches) == expected_count, matching_spec
+            assert len(matches) == expected_count
 
     @pytest.mark.parametrize(
         "specs,expected_spec,occurances",
@@ -2210,10 +2171,19 @@ def test_unsolved_specs_raises_error(self, monkeypatch, mock_packages):
             solver.driver.solve(setup, specs, reuse=[])
 
     @pytest.mark.regression("43141")
-    def test_clear_error_when_unknown_compiler_requested(self, mock_packages):
+    @pytest.mark.parametrize(
+        "spec_str,expected_match",
+        [
+            # A package does not exist
+            ("pkg-a ^foo", "since 'foo' does not exist"),
+            # Request a compiler for a package that doesn't need it
+            ("pkg-c %gcc", "according to its recipe"),
+        ],
+    )
+    def test_errors_on_statically_checked_preconditions(self, spec_str, expected_match):
         """Tests that the solver can report a case where the compiler cannot be set"""
-        with pytest.raises(spack.error.UnsatisfiableSpecError, match="since 'foo' does not exist"):
-            Spec("pkg-a %foo").concretized()
+        with pytest.raises(spack.error.UnsatisfiableSpecError, match=expected_match):
+            Spec(spec_str).concretized()
 
     @pytest.mark.regression("36339")
     @pytest.mark.parametrize(
diff --git a/lib/spack/spack/test/concretization/preferences.py b/lib/spack/spack/test/concretization/preferences.py
index 4d31d4f3a5b..43c55fca63c 100644
--- a/lib/spack/spack/test/concretization/preferences.py
+++ b/lib/spack/spack/test/concretization/preferences.py
@@ -256,7 +256,7 @@ def test_external_mpi(self):
         # make sure this doesn't give us an external first.
         spec = Spec("mpi")
         spec.concretize()
-        assert not spec["mpi"].external
+        assert not spec.external and spec.package.provides("mpi")
 
         # load config
         conf = syaml.load_config(
@@ -292,7 +292,7 @@ def mock_module(cmd, module):
 
         spec = Spec("mpi")
         spec.concretize()
-        assert not spec["mpi"].external
+        assert not spec.external and spec.package.provides("mpi")
 
         # load config
         conf = syaml.load_config(
diff --git a/lib/spack/spack/test/concretization/requirements.py b/lib/spack/spack/test/concretization/requirements.py
index b4ec7c36ab3..2d3b0253341 100644
--- a/lib/spack/spack/test/concretization/requirements.py
+++ b/lib/spack/spack/test/concretization/requirements.py
@@ -493,7 +493,7 @@ def test_default_requirements_with_all(spec_str, requirement_str, concretize_sco
     spec = Spec(spec_str).concretized()
     assert "c" in spec
     for s in spec.traverse():
-        if "c" in s:
+        if "c" in s and s.name not in ("gcc", "llvm"):
             assert s.satisfies(requirement_str)
 
 
diff --git a/lib/spack/spack/test/config.py b/lib/spack/spack/test/config.py
index 11829ed92bc..57c51ec57b1 100644
--- a/lib/spack/spack/test/config.py
+++ b/lib/spack/spack/test/config.py
@@ -273,10 +273,10 @@ def test_add_config_path(mutable_config):
     assert set_value == "/path/to/config.yaml"
 
     # Now a package:all setting
-    path = "packages:all:compiler:[gcc]"
+    path = "packages:all:target:[x86_64]"
     spack.config.add(path)
-    compilers = spack.config.get("packages")["all"]["compiler"]
-    assert "gcc" in compilers
+    targets = spack.config.get("packages")["all"]["target"]
+    assert "x86_64" in targets
 
     # Try quotes to escape brackets
     path = (
@@ -923,7 +923,6 @@ def test_single_file_scope(config, env_yaml):
         # from the single-file config
         assert spack.config.get("config:verify_ssl") is False
         assert spack.config.get("config:dirty") is False
-        assert spack.config.get("packages:all:compiler") == ["gcc@4.5.3", "gcc", "clang"]
 
         # from the lower config scopes
         assert spack.config.get("config:checksum") is True
@@ -947,7 +946,7 @@ def test_single_file_scope_section_override(tmpdir, config):
         verify_ssl: False
     packages::
         all:
-            compiler: [ 'gcc@4.5.3' ]
+            target: [ x86_64 ]
     repos:
         - /x/y/z
 """
@@ -960,7 +959,7 @@ def test_single_file_scope_section_override(tmpdir, config):
     with spack.config.override(scope):
         # from the single-file config
         assert spack.config.get("config:verify_ssl") is False
-        assert spack.config.get("packages:all:compiler") == ["gcc@4.5.3"]
+        assert spack.config.get("packages:all:target") == ["x86_64"]
 
         # from the lower config scopes
         assert spack.config.get("config:checksum") is True
@@ -1225,10 +1224,10 @@ def test_user_config_path_is_default_when_env_var_is_empty(working_env):
 
 
 def test_default_install_tree(monkeypatch, default_config):
-    s = spack.spec.Spec("nonexistent@x.y.z %none@a.b.c arch=foo-bar-baz")
+    s = spack.spec.Spec("nonexistent@x.y.z arch=foo-bar-baz")
     monkeypatch.setattr(s, "dag_hash", lambda length: "abc123")
     _, _, projections = spack.store.parse_install_tree(spack.config.get("config"))
-    assert s.format(projections["all"]) == "foo-bar-baz/none-a.b.c/nonexistent-x.y.z-abc123"
+    assert s.format(projections["all"]) == "foo/baz/nonexistent-x.y.z-abc123"
 
 
 def test_local_config_can_be_disabled(working_env):
@@ -1299,7 +1298,7 @@ def test_config_collect_urls(mutable_empty_config, mock_spider_configs, url, isf
         (github_url.format("tree"), False, False),
         (gitlab_url, False, False),
         ("{0}/README.md".format(github_url.format("blob")), True, True),
-        ("{0}/compilers.yaml".format(gitlab_url), True, False),
+        ("{0}/packages.yaml".format(gitlab_url), True, False),
         (None, False, True),
     ],
 )
diff --git a/lib/spack/spack/test/database.py b/lib/spack/spack/test/database.py
index 0beb1e44a17..7757bc82fc5 100644
--- a/lib/spack/spack/test/database.py
+++ b/lib/spack/spack/test/database.py
@@ -64,11 +64,11 @@ def upstream_and_downstream_db(tmpdir, gen_mock_layout):
 @pytest.mark.parametrize(
     "install_tree,result",
     [
-        ("all", ["pkg-b", "pkg-c"]),
+        ("all", ["pkg-b", "pkg-c", "gcc-runtime", "gcc"]),
         ("upstream", ["pkg-c"]),
-        ("local", ["pkg-b"]),
+        ("local", ["pkg-b", "gcc-runtime", "gcc"]),
         ("{u}", ["pkg-c"]),
-        ("{d}", ["pkg-b"]),
+        ("{d}", ["pkg-b", "gcc-runtime", "gcc"]),
     ],
 )
 def test_query_by_install_tree(
@@ -84,7 +84,7 @@ def test_query_by_install_tree(
     down_db.add(b)
 
     specs = down_db.query(install_tree=install_tree.format(u=up_db.root, d=down_db.root))
-    assert [s.name for s in specs] == result
+    assert {s.name for s in specs} == set(result)
 
 
 def test_spec_installed_upstream(
@@ -453,7 +453,7 @@ def test_005_db_exists(database):
 def test_010_all_install_sanity(database):
     """Ensure that the install layout reflects what we think it does."""
     all_specs = spack.store.STORE.layout.all_specs()
-    assert len(all_specs) == 15
+    assert len(all_specs) == 16
 
     # Query specs with multiple configurations
     mpileaks_specs = [s for s in all_specs if s.satisfies("mpileaks")]
@@ -570,7 +570,7 @@ def test_050_basic_query(database):
     """Ensure querying database is consistent with what is installed."""
     # query everything
     total_specs = len(spack.store.STORE.db.query())
-    assert total_specs == 17
+    assert total_specs == 19
 
     # query specs with multiple configurations
     mpileaks_specs = database.query("mpileaks")
@@ -792,11 +792,11 @@ def check_unused(roots, deptype, expected):
         assert set(u.name for u in unused) == set(expected)
 
     default_dt = dt.LINK | dt.RUN
-    check_unused(None, default_dt, ["cmake"])
+    check_unused(None, default_dt, ["cmake", "gcc"])
     check_unused(
         [si, ml_mpich, ml_mpich2, ml_zmpi, externaltest],
         default_dt,
-        ["trivial-smoke-test", "cmake"],
+        ["trivial-smoke-test", "cmake", "gcc"],
     )
     check_unused(
         [si, ml_mpich, ml_mpich2, ml_zmpi, externaltest],
@@ -811,7 +811,7 @@ def check_unused(roots, deptype, expected):
     check_unused(
         [si, ml_mpich, ml_mpich2, ml_zmpi],
         default_dt,
-        ["trivial-smoke-test", "cmake", "externaltest", "externaltool", "externalvirtual"],
+        ["trivial-smoke-test", "cmake", "externaltest", "externaltool", "externalvirtual", "gcc"],
     )
 
 
@@ -1046,7 +1046,7 @@ def test_check_parents(spec_str, parent_name, expected_nparents, database):
 def test_db_all_hashes(database):
     # ensure we get the right number of hashes without a read transaction
     hashes = database.all_hashes()
-    assert len(hashes) == 17
+    assert len(hashes) == 19
 
     # and make sure the hashes match
     with database.read_transaction():
diff --git a/lib/spack/spack/test/directory_layout.py b/lib/spack/spack/test/directory_layout.py
index 81aafc297bb..60636794a51 100644
--- a/lib/spack/spack/test/directory_layout.py
+++ b/lib/spack/spack/test/directory_layout.py
@@ -34,11 +34,7 @@ def test_yaml_directory_layout_parameters(tmpdir, default_mock_concretization):
     layout_default = DirectoryLayout(str(tmpdir))
     path_default = layout_default.relative_path_for_spec(spec)
     assert path_default == str(
-        Path(
-            spec.format(
-                "{architecture}/" "{compiler.name}-{compiler.version}/" "{name}-{version}-{hash}"
-            )
-        )
+        Path(spec.format("{architecture.platform}/{architecture.target}/{name}-{version}-{hash}"))
     )
 
     # Test hash_length parameter works correctly
diff --git a/lib/spack/spack/test/graph.py b/lib/spack/spack/test/graph.py
index 2708c320b91..d7dcfa38e27 100644
--- a/lib/spack/spack/test/graph.py
+++ b/lib/spack/spack/test/graph.py
@@ -46,44 +46,63 @@ def test_ascii_graph_mpileaks(config, mock_packages, monkeypatch):
     graph_str = stream.getvalue()
     graph_str = "\n".join([line.rstrip() for line in graph_str.split("\n")])
 
+    print(graph_str)
+
     assert (
         graph_str
         == r"""o mpileaks
 |\
-| o callpath
-|/|
-o | mpich
- /
-o dyninst
-|\
-| o libdwarf
-|/
-o libelf
-"""
-        or graph_str
-        == r"""o mpileaks
-|\
-o | callpath
-|\|
-| o mpich
-|
-o dyninst
-|\
-o | libdwarf
-|/
-o libelf
-"""
-        or graph_str
-        == r"""o mpileaks
-|\
-| o callpath
-|/|
-| o dyninst
 | |\
-o | | mpich
- / /
-| o libdwarf
+| | |\
+| | | o callpath
+| |_|/|
+|/| |/|
+| |/|/|
+o | | | mpich
+|\| | |
+| |/ /
+|/| |
+| | o dyninst
+| |/|
+|/|/|
+| | |\
+| | | o libdwarf
+| |_|/|
+|/| |/|
+| |/|/
+| | o libelf
+| |/|
+|/|/
+| o gcc-runtime
 |/
-o libelf
+o gcc
+"""
+        or graph_str
+        == r"""o mpileaks
+|\
+| |\
+| | |\
+| | | o callpath
+| |_|/|
+|/| |/|
+| |/|/|
+| | | o dyninst
+| | |/|
+| |/|/|
+| | | |\
+o | | | | mpich
+|\| | | |
+| |/ / /
+|/| | |
+| | | o libdwarf
+| |_|/|
+|/| |/|
+| |/|/
+| | o libelf
+| |/|
+|/|/
+| o gcc-runtime
+|/
+o gcc
 """
     )
diff --git a/lib/spack/spack/test/installer.py b/lib/spack/spack/test/installer.py
index af9ba26fef6..7db5a8685c4 100644
--- a/lib/spack/spack/test/installer.py
+++ b/lib/spack/spack/test/installer.py
@@ -1024,6 +1024,7 @@ def test_install_fail_multi(install_mockery, mock_fetch, monkeypatch):
 
 def test_install_fail_fast_on_detect(install_mockery, monkeypatch, capsys):
     """Test fail_fast install when an install failure is detected."""
+    # Note: this test depends on the order of the installations
     b, c = spack.spec.Spec("pkg-b").concretized(), spack.spec.Spec("pkg-c").concretized()
     b_id, c_id = inst.package_id(b), inst.package_id(c)
 
@@ -1036,9 +1037,9 @@ def test_install_fail_fast_on_detect(install_mockery, monkeypatch, capsys):
     with pytest.raises(spack.error.InstallError, match="after first install failure"):
         installer.install()
 
-    assert b_id in installer.failed, "Expected b to be marked as failed"
-    assert c_id not in installer.failed, "Expected no attempt to install pkg-c"
-    assert f"{b_id} failed to install" in capsys.readouterr().err
+    assert c_id in installer.failed, "Expected b to be marked as failed"
+    assert b_id not in installer.failed, "Expected no attempt to install pkg-c"
+    assert f"{c_id} failed to install" in capsys.readouterr().err
 
 
 def _test_install_fail_fast_on_except_patch(installer, **kwargs):
@@ -1071,10 +1072,11 @@ def test_install_fail_fast_on_except(install_mockery, monkeypatch, capsys):
 def test_install_lock_failures(install_mockery, monkeypatch, capfd):
     """Cover basic install lock failure handling in a single pass."""
 
+    # Note: this test relies on installing a package with no dependencies
     def _requeued(installer, task, install_status):
         tty.msg("requeued {0}".format(task.pkg.spec.name))
 
-    installer = create_installer(["pkg-b"], {})
+    installer = create_installer(["pkg-c"], {})
 
     # Ensure never acquire a lock
     monkeypatch.setattr(inst.PackageInstaller, "_ensure_locked", _not_locked)
@@ -1093,13 +1095,14 @@ def _requeued(installer, task, install_status):
 
 def test_install_lock_installed_requeue(install_mockery, monkeypatch, capfd):
     """Cover basic install handling for installed package."""
-    b = spack.spec.Spec("pkg-b").concretized()
-    b_pkg_id = inst.package_id(b)
-    installer = create_installer([b])
+    # Note: this test relies on installing a package with no dependencies
+    concrete_spec = spack.spec.Spec("pkg-c").concretized()
+    pkg_id = inst.package_id(concrete_spec)
+    installer = create_installer([concrete_spec])
 
     def _prep(installer, task):
-        installer.installed.add(b_pkg_id)
-        tty.msg(f"{b_pkg_id} is installed")
+        installer.installed.add(pkg_id)
+        tty.msg(f"{pkg_id} is installed")
 
         # also do not allow the package to be locked again
         monkeypatch.setattr(inst.PackageInstaller, "_ensure_locked", _not_locked)
@@ -1116,7 +1119,7 @@ def _requeued(installer, task, install_status):
     with pytest.raises(spack.error.InstallError, match="request failed"):
         installer.install()
 
-    assert b_pkg_id not in installer.installed
+    assert pkg_id not in installer.installed
 
     expected = ["is installed", "read locked", "requeued"]
     for exp, ln in zip(expected, capfd.readouterr().out.splitlines()):
@@ -1125,6 +1128,7 @@ def _requeued(installer, task, install_status):
 
 def test_install_read_locked_requeue(install_mockery, monkeypatch, capfd):
     """Cover basic read lock handling for uninstalled package with requeue."""
+    # Note: this test relies on installing a package with no dependencies
     orig_fn = inst.PackageInstaller._ensure_locked
 
     def _read(installer, lock_type, pkg):
@@ -1147,7 +1151,7 @@ def _requeued(installer, task, install_status):
     # Ensure don't continually requeue the task
     monkeypatch.setattr(inst.PackageInstaller, "_requeue_task", _requeued)
 
-    installer = create_installer(["pkg-b"], {})
+    installer = create_installer(["pkg-c"], {})
 
     with pytest.raises(spack.error.InstallError, match="request failed"):
         installer.install()
@@ -1162,7 +1166,8 @@ def _requeued(installer, task, install_status):
 
 def test_install_skip_patch(install_mockery, mock_fetch):
     """Test the path skip_patch install path."""
-    installer = create_installer(["pkg-b"], {"fake": False, "skip_patch": True})
+    # Note: this test relies on installing a package with no dependencies
+    installer = create_installer(["pkg-c"], {"fake": False, "skip_patch": True})
     installer.install()
     assert inst.package_id(installer.build_requests[0].pkg.spec) in installer.installed
 
@@ -1182,8 +1187,9 @@ def test_overwrite_install_backup_success(temporary_store, config, mock_packages
     When doing an overwrite install that fails, Spack should restore the backup
     of the original prefix, and leave the original spec marked installed.
     """
+    # Note: this test relies on installing a package with no dependencies
     # Get a build task. TODO: refactor this to avoid calling internal methods
-    installer = create_installer(["pkg-b"])
+    installer = create_installer(["pkg-c"])
     installer._init_queue()
     task = installer._pop_task()
 
@@ -1224,6 +1230,7 @@ def test_overwrite_install_backup_failure(temporary_store, config, mock_packages
     original prefix. If that fails, the spec is lost, and it should be removed
     from the database.
     """
+    # Note: this test relies on installing a package with no dependencies
 
     class InstallerThatAccidentallyDeletesTheBackupDir:
         def _install_task(self, task, install_status):
@@ -1243,7 +1250,7 @@ def remove(self, spec):
             self.called = True
 
     # Get a build task. TODO: refactor this to avoid calling internal methods
-    installer = create_installer(["pkg-b"])
+    installer = create_installer(["pkg-c"])
     installer._init_queue()
     task = installer._pop_task()
 
diff --git a/lib/spack/spack/test/link_paths.py b/lib/spack/spack/test/link_paths.py
index f6a88f331ed..5c07c8e9877 100644
--- a/lib/spack/spack/test/link_paths.py
+++ b/lib/spack/spack/test/link_paths.py
@@ -7,6 +7,7 @@
 
 import pytest
 
+import spack.compilers.libraries
 import spack.paths
 from spack.compilers.libraries import parse_non_system_link_dirs
 
diff --git a/lib/spack/spack/test/llnl/util/lang.py b/lib/spack/spack/test/llnl/util/lang.py
index 1701cbc290b..8e17d18ce12 100644
--- a/lib/spack/spack/test/llnl/util/lang.py
+++ b/lib/spack/spack/test/llnl/util/lang.py
@@ -6,7 +6,6 @@
 import re
 import sys
 from datetime import datetime, timedelta
-from textwrap import dedent
 
 import pytest
 
@@ -290,13 +289,6 @@ def inner():
     with h.forward("top-level"):
         raise TypeError("ok")
 
-    assert h.grouped_message(with_tracebacks=False) == dedent(
-        """\
-    due to the following failures:
-    inner method raised ValueError: wow!
-    top-level raised TypeError: ok"""
-    )
-
 
 def test_grouped_exception_base_type():
     h = llnl.util.lang.GroupedExceptionHandler()
diff --git a/lib/spack/spack/test/modules/lmod.py b/lib/spack/spack/test/modules/lmod.py
index c90884573bf..3e045990e6f 100644
--- a/lib/spack/spack/test/modules/lmod.py
+++ b/lib/spack/spack/test/modules/lmod.py
@@ -111,7 +111,7 @@ def test_compilers_provided_different_name(
         self, factory, module_configuration, compiler_factory
     ):
         with spack.config.override(
-            "compilers", [compiler_factory(spec="clang@3.3", operating_system="debian6")]
+            "packages", {"llvm": {"externals": [compiler_factory(spec="llvm@3.3")]}}
         ):
             module_configuration("complex_hierarchy")
             module, spec = factory("intel-oneapi-compilers%clang@3.3")
@@ -119,7 +119,7 @@ def test_compilers_provided_different_name(
             provides = module.conf.provides
 
             assert "compiler" in provides
-            assert provides["compiler"] == spack.spec.CompilerSpec("oneapi@=3.0")
+            assert provides["compiler"] == spack.spec.Spec("intel-oneapi-compilers@=3.0")
 
     def test_simple_case(self, modulefile_content, module_configuration):
         """Tests the generation of a simple Lua module file."""
@@ -138,7 +138,7 @@ def test_autoload_direct(self, modulefile_content, module_configuration):
         module_configuration("autoload_direct")
         content = modulefile_content(mpileaks_spec_string)
 
-        assert len([x for x in content if "depends_on(" in x]) == 2
+        assert len([x for x in content if "depends_on(" in x]) == 3
 
     def test_autoload_all(self, modulefile_content, module_configuration):
         """Tests the automatic loading of all dependencies."""
@@ -146,7 +146,7 @@ def test_autoload_all(self, modulefile_content, module_configuration):
         module_configuration("autoload_all")
         content = modulefile_content(mpileaks_spec_string)
 
-        assert len([x for x in content if "depends_on(" in x]) == 5
+        assert len([x for x in content if "depends_on(" in x]) == 6
 
     def test_alter_environment(self, modulefile_content, module_configuration):
         """Tests modifications to run-time environment."""
@@ -264,7 +264,7 @@ def test_exclude(self, modulefile_content, module_configuration):
         module_configuration("exclude")
         content = modulefile_content(mpileaks_spec_string)
 
-        assert len([x for x in content if "depends_on(" in x]) == 1
+        assert len([x for x in content if "depends_on(" in x]) == 2
 
     def test_no_hash(self, factory, module_configuration):
         """Makes sure that virtual providers (in the hierarchy) always
@@ -371,7 +371,7 @@ def test_guess_core_compilers(self, factory, module_configuration, monkeypatch):
         module_configuration("missing_core_compilers")
 
         # Our mock paths must be detected as system paths
-        monkeypatch.setattr(spack.util.environment, "SYSTEM_DIRS", ["/path/to"])
+        monkeypatch.setattr(spack.util.environment, "SYSTEM_DIRS", ["/path/bin"])
 
         # We don't want to really write into user configuration
         # when running tests
@@ -433,7 +433,7 @@ def test_modules_relative_to_view(
     ):
         with ev.create_in_dir(str(tmpdir), with_view=True) as e:
             module_configuration("with_view")
-            install("--add", "cmake")
+            install("--fake", "--add", "cmake")
 
             spec = spack.spec.Spec("cmake").concretized()
 
diff --git a/lib/spack/spack/test/modules/tcl.py b/lib/spack/spack/test/modules/tcl.py
index 8cc6b25493b..ccef66fb4e7 100644
--- a/lib/spack/spack/test/modules/tcl.py
+++ b/lib/spack/spack/test/modules/tcl.py
@@ -45,8 +45,8 @@ def test_autoload_direct(self, modulefile_content, module_configuration):
             len([x for x in content if "if {![info exists ::env(LMOD_VERSION_MAJOR)]} {" in x])
             == 1
         )
-        assert len([x for x in content if "depends-on " in x]) == 2
-        assert len([x for x in content if "module load " in x]) == 2
+        assert len([x for x in content if "depends-on " in x]) == 3
+        assert len([x for x in content if "module load " in x]) == 3
 
         # dtbuild1 has
         # - 1 ('run',) dependency
@@ -76,8 +76,8 @@ def test_autoload_all(self, modulefile_content, module_configuration):
             len([x for x in content if "if {![info exists ::env(LMOD_VERSION_MAJOR)]} {" in x])
             == 1
         )
-        assert len([x for x in content if "depends-on " in x]) == 5
-        assert len([x for x in content if "module load " in x]) == 5
+        assert len([x for x in content if "depends-on " in x]) == 6
+        assert len([x for x in content if "module load " in x]) == 6
 
         # dtbuild1 has
         # - 1 ('run',) dependency
@@ -101,7 +101,7 @@ def test_prerequisites_direct(
         module_configuration("prerequisites_direct")
         content = modulefile_content(f"mpileaks target={host_architecture_str}")
 
-        assert len([x for x in content if "prereq" in x]) == 2
+        assert len([x for x in content if "prereq" in x]) == 3
 
     def test_prerequisites_all(
         self, modulefile_content, module_configuration, host_architecture_str
@@ -111,7 +111,7 @@ def test_prerequisites_all(
         module_configuration("prerequisites_all")
         content = modulefile_content(f"mpileaks target={host_architecture_str}")
 
-        assert len([x for x in content if "prereq" in x]) == 5
+        assert len([x for x in content if "prereq" in x]) == 6
 
     def test_alter_environment(self, modulefile_content, module_configuration):
         """Tests modifications to run-time environment."""
@@ -236,7 +236,7 @@ def test_exclude(self, modulefile_content, module_configuration, host_architectu
         module_configuration("exclude")
         content = modulefile_content("mpileaks ^zmpi")
 
-        assert len([x for x in content if "module load " in x]) == 1
+        assert len([x for x in content if "module load " in x]) == 2
 
         # Catch "Exception" to avoid using FileNotFoundError on Python 3
         # and IOError on Python 2 or common bases like EnvironmentError
@@ -246,7 +246,7 @@ def test_exclude(self, modulefile_content, module_configuration, host_architectu
 
         content = modulefile_content(f"zmpi target={host_architecture_str}")
 
-        assert len([x for x in content if "module load " in x]) == 1
+        assert len([x for x in content if "module load " in x]) == 2
 
     def test_naming_scheme_compat(self, factory, module_configuration):
         """Tests backwards compatibility for naming_scheme key"""
@@ -487,8 +487,8 @@ def test_autoload_with_constraints(self, modulefile_content, module_configuratio
 
         # Test the mpileaks that should have the autoloaded dependencies
         content = modulefile_content("mpileaks ^mpich2")
-        assert len([x for x in content if "depends-on " in x]) == 2
-        assert len([x for x in content if "module load " in x]) == 2
+        assert len([x for x in content if "depends-on " in x]) == 3
+        assert len([x for x in content if "module load " in x]) == 3
 
         # Test the mpileaks that should NOT have the autoloaded dependencies
         content = modulefile_content("mpileaks ^mpich")
diff --git a/lib/spack/spack/test/multimethod.py b/lib/spack/spack/test/multimethod.py
index 24408ad9bc2..cf5e5d7bf41 100644
--- a/lib/spack/spack/test/multimethod.py
+++ b/lib/spack/spack/test/multimethod.py
@@ -6,7 +6,6 @@
 
 import pytest
 
-import spack.config
 import spack.platforms
 import spack.spec
 from spack.multimethod import NoSuchMethodError
@@ -53,7 +52,7 @@ def test_no_version_match(pkg_name):
         # Constraints on compilers with a default
         ("%gcc", "has_a_default", "gcc"),
         ("%clang", "has_a_default", "clang"),
-        ("%apple-clang os=elcapitan", "has_a_default", "default"),
+        ("%gcc@9", "has_a_default", "default"),
         # Constraints on dependencies
         ("^zmpi", "different_by_dep", "zmpi"),
         ("^mpich", "different_by_dep", "mpich"),
@@ -68,13 +67,9 @@ def test_no_version_match(pkg_name):
     ],
 )
 def test_multimethod_calls(
-    pkg_name, constraint_str, method_name, expected_result, compiler_factory
+    pkg_name, constraint_str, method_name, expected_result, default_mock_concretization
 ):
-    # Add apple-clang, as it is required by one of the tests
-    with spack.config.override(
-        "compilers", [compiler_factory(spec="apple-clang@9.1.0", operating_system="elcapitan")]
-    ):
-        s = spack.spec.Spec(pkg_name + constraint_str).concretized()
+    s = default_mock_concretization(f"{pkg_name}{constraint_str}")
     msg = f"Method {method_name} from {s} is giving a wrong result"
     assert getattr(s.package, method_name)() == expected_result, msg
 
diff --git a/lib/spack/spack/test/package_class.py b/lib/spack/spack/test/package_class.py
index c3446a3592b..93bd6b4d0bf 100644
--- a/lib/spack/spack/test/package_class.py
+++ b/lib/spack/spack/test/package_class.py
@@ -16,8 +16,6 @@
 
 import llnl.util.filesystem as fs
 
-import spack.compilers.config
-import spack.config
 import spack.deptypes as dt
 import spack.error
 import spack.install_test
@@ -34,21 +32,28 @@ def mpi_names(mock_repo_path):
     return [spec.name for spec in mock_repo_path.providers_for("mpi")]
 
 
+@pytest.fixture(scope="module")
+def compiler_names(mock_repo_path):
+    return [spec.name for spec in mock_repo_path.providers_for("c")]
+
+
 @pytest.fixture()
-def mpileaks_possible_deps(mock_packages, mpi_names):
+def mpileaks_possible_deps(mock_packages, mpi_names, compiler_names):
     possible = {
-        "callpath": set(["dyninst"] + mpi_names),
+        "callpath": set(["dyninst"] + mpi_names + compiler_names),
         "low-priority-provider": set(),
-        "dyninst": set(["libdwarf", "libelf"]),
+        "dyninst": set(["libdwarf", "libelf"] + compiler_names),
         "fake": set(),
+        "gcc": set(),
         "intel-parallel-studio": set(),
-        "libdwarf": set(["libelf"]),
-        "libelf": set(),
-        "mpich": set(),
-        "mpich2": set(),
-        "mpileaks": set(["callpath"] + mpi_names),
+        "libdwarf": set(["libelf"] + compiler_names),
+        "libelf": set(compiler_names),
+        "llvm": set(),
+        "mpich": set(compiler_names),
+        "mpich2": set(compiler_names),
+        "mpileaks": set(["callpath"] + mpi_names + compiler_names),
         "multi-provider-mpi": set(),
-        "zmpi": set(["fake"]),
+        "zmpi": set(["fake"] + compiler_names),
     }
     return possible
 
@@ -58,32 +63,39 @@ def test_possible_dependencies(mock_packages, mpileaks_possible_deps):
     expanded_possible_deps = pkg_cls.possible_dependencies(expand_virtuals=True)
     assert mpileaks_possible_deps == expanded_possible_deps
     assert {
-        "callpath": {"dyninst", "mpi"},
-        "dyninst": {"libdwarf", "libelf"},
-        "libdwarf": {"libelf"},
-        "libelf": set(),
+        "c": set(),
+        "callpath": {"dyninst", "mpi", "c"},
+        "cxx": set(),
+        "dyninst": {"libdwarf", "libelf", "c"},
+        "libdwarf": {"libelf", "c", "cxx"},
+        "libelf": {"c"},
         "mpi": set(),
-        "mpileaks": {"callpath", "mpi"},
+        "mpileaks": {"c", "callpath", "mpi"},
     } == pkg_cls.possible_dependencies(expand_virtuals=False)
 
 
 def test_possible_direct_dependencies(mock_packages, mpileaks_possible_deps):
     pkg_cls = spack.repo.PATH.get_pkg_class("mpileaks")
     deps = pkg_cls.possible_dependencies(transitive=False, expand_virtuals=False)
-    assert {"callpath": set(), "mpi": set(), "mpileaks": {"callpath", "mpi"}} == deps
+    assert {
+        "c": set(),
+        "callpath": set(),
+        "mpi": set(),
+        "mpileaks": {"callpath", "mpi", "c"},
+    } == deps
 
 
-def test_possible_dependencies_virtual(mock_packages, mpi_names):
-    expected = dict(
-        (name, set(dep for dep in spack.repo.PATH.get_pkg_class(name).dependencies_by_name()))
-        for name in mpi_names
+def test_possible_dependencies_virtual(mock_packages, mpi_names, compiler_names):
+    expected = {
+        name: set(spack.repo.PATH.get_pkg_class(name).dependencies_by_name()) for name in mpi_names
+    }
+
+    # One mock MPI has a dependency, and they depend on C, C++, and Fortran
+    expected.update({"fake": set(), "c": set(), "cxx": set(), "fortran": set()})
+    assert expected == spack.package_base.possible_dependencies(
+        "mpi", expand_virtuals=False, transitive=False
     )
 
-    # only one mock MPI has a dependency
-    expected["fake"] = set()
-
-    assert expected == spack.package_base.possible_dependencies("mpi", transitive=False)
-
 
 def test_possible_dependencies_missing(mock_packages):
     pkg_cls = spack.repo.PATH.get_pkg_class("missing-dependency")
@@ -117,10 +129,10 @@ def test_possible_dependencies_with_multiple_classes(mock_packages, mpileaks_pos
     expected = mpileaks_possible_deps.copy()
     expected.update(
         {
-            "dt-diamond": set(["dt-diamond-left", "dt-diamond-right"]),
-            "dt-diamond-left": set(["dt-diamond-bottom"]),
-            "dt-diamond-right": set(["dt-diamond-bottom"]),
-            "dt-diamond-bottom": set(),
+            "dt-diamond": {"dt-diamond-left", "dt-diamond-right", "gcc", "llvm"},
+            "dt-diamond-left": {"dt-diamond-bottom", "gcc", "llvm"},
+            "dt-diamond-right": {"dt-diamond-bottom", "gcc", "llvm"},
+            "dt-diamond-bottom": {"gcc", "llvm"},
         }
     )
 
@@ -274,11 +286,9 @@ def test_package_fetcher_fails():
 
 
 def test_package_test_no_compilers(mock_packages, monkeypatch, capfd):
-    def compilers(compiler, arch_spec):
-        return None
-
-    monkeypatch.setattr(spack.compilers.config, "compilers_for_spec", compilers)
-
+    """Ensures that a test which needs the compiler, and build dependencies, to run, is skipped
+    if no compiler is available.
+    """
     s = spack.spec.Spec("pkg-a")
     pkg = BaseTestPackage(s)
     pkg.test_requires_compiler = True
diff --git a/lib/spack/spack/test/packages.py b/lib/spack/spack/test/packages.py
index 302dbe8df81..368055aebb9 100644
--- a/lib/spack/spack/test/packages.py
+++ b/lib/spack/spack/test/packages.py
@@ -68,7 +68,7 @@ def test_inheritance_of_directives(self):
 
         # Check dictionaries that should have been filled by directives
         dependencies = pkg_cls.dependencies_by_name()
-        assert len(dependencies) == 3
+        assert len(dependencies) == 4
         assert "cmake" in dependencies
         assert "openblas" in dependencies
         assert "mpi" in dependencies
diff --git a/lib/spack/spack/test/spec_dag.py b/lib/spack/spack/test/spec_dag.py
index 5de2bd87f6e..4efa08aeb18 100644
--- a/lib/spack/spack/test/spec_dag.py
+++ b/lib/spack/spack/test/spec_dag.py
@@ -8,6 +8,7 @@
 
 import spack.deptypes as dt
 import spack.error
+import spack.installer
 import spack.repo
 import spack.util.hash as hashutil
 import spack.version
@@ -79,8 +80,7 @@ def test_test_deptype(tmpdir):
         assert "z" not in spec
 
 
-@pytest.mark.usefixtures("config")
-def test_installed_deps(monkeypatch, mock_packages):
+def test_installed_deps(monkeypatch, install_mockery):
     """Ensure that concrete specs and their build deps don't constrain solves.
 
     Preinstall a package ``c`` that has a constrained build dependency on ``d``, then
@@ -100,28 +100,21 @@ def test_installed_deps(monkeypatch, mock_packages):
     #   |/ \|   c --> d build
     #   d   e   c --> e build/link
     #
-    a, b, c, d, e = ["installed-deps-%s" % s for s in "abcde"]
+    a, b, c, d, e = [f"installed-deps-{s}" for s in "abcde"]
 
     # install C, which will force d's version to be 2
     # BUT d is only a build dependency of C, so it won't constrain
     # link/run dependents of C when C is depended on as an existing
     # (concrete) installation.
-    c_spec = Spec(c)
-    c_spec.concretize()
+    c_spec = Spec(c).concretized()
     assert c_spec[d].version == spack.version.Version("2")
 
-    installed_names = [s.name for s in c_spec.traverse()]
-
-    def _mock_installed(self):
-        return self.name in installed_names
-
-    monkeypatch.setattr(Spec, "installed", _mock_installed)
+    spack.installer.PackageInstaller([c_spec.package], fake=True, explicit=True).install()
 
     # install A, which depends on B, C, D, and E, and force A to
     # use the installed C.  It should *not* force A to use the installed D
     # *if* we're doing a fresh installation.
-    a_spec = Spec(a)
-    a_spec._add_dependency(c_spec, depflag=dt.BUILD | dt.LINK, virtuals=())
+    a_spec = Spec(f"{a} ^/{c_spec.dag_hash()}")
     a_spec.concretize()
     assert spack.version.Version("2") == a_spec[c][d].version
     assert spack.version.Version("2") == a_spec[e].version
@@ -183,120 +176,231 @@ def test_conflicting_package_constraints(self, set_dependency):
         with pytest.raises(spack.error.UnsatisfiableSpecError):
             spec.concretize()
 
-    def test_preorder_node_traversal(self):
-        dag = Spec("mpileaks ^zmpi").concretized()
+    @pytest.mark.parametrize(
+        "pairs,traverse_kwargs",
+        [
+            # Preorder node traversal
+            (
+                [
+                    (0, "mpileaks"),
+                    (1, "callpath"),
+                    (2, "dyninst"),
+                    (3, "gcc"),
+                    (3, "gcc-runtime"),
+                    (3, "libdwarf"),
+                    (4, "libelf"),
+                    (2, "zmpi"),
+                    (3, "fake"),
+                ],
+                {},
+            ),
+            # Preorder edge traversal
+            (
+                [
+                    (0, "mpileaks"),
+                    (1, "callpath"),
+                    (2, "dyninst"),
+                    (3, "gcc"),
+                    (3, "gcc-runtime"),
+                    (4, "gcc"),
+                    (3, "libdwarf"),
+                    (4, "gcc"),
+                    (4, "gcc-runtime"),
+                    (4, "libelf"),
+                    (5, "gcc"),
+                    (5, "gcc-runtime"),
+                    (3, "libelf"),
+                    (2, "gcc"),
+                    (2, "gcc-runtime"),
+                    (2, "zmpi"),
+                    (3, "fake"),
+                    (3, "gcc"),
+                    (3, "gcc-runtime"),
+                    (1, "gcc"),
+                    (1, "gcc-runtime"),
+                    (1, "zmpi"),
+                ],
+                {"cover": "edges"},
+            ),
+            # Preorder path traversal
+            (
+                [
+                    (0, "mpileaks"),
+                    (1, "callpath"),
+                    (2, "dyninst"),
+                    (3, "gcc"),
+                    (3, "gcc-runtime"),
+                    (4, "gcc"),
+                    (3, "libdwarf"),
+                    (4, "gcc"),
+                    (4, "gcc-runtime"),
+                    (5, "gcc"),
+                    (4, "libelf"),
+                    (5, "gcc"),
+                    (5, "gcc-runtime"),
+                    (6, "gcc"),
+                    (3, "libelf"),
+                    (4, "gcc"),
+                    (4, "gcc-runtime"),
+                    (5, "gcc"),
+                    (2, "gcc"),
+                    (2, "gcc-runtime"),
+                    (3, "gcc"),
+                    (2, "zmpi"),
+                    (3, "fake"),
+                    (3, "gcc"),
+                    (3, "gcc-runtime"),
+                    (4, "gcc"),
+                    (1, "gcc"),
+                    (1, "gcc-runtime"),
+                    (2, "gcc"),
+                    (1, "zmpi"),
+                    (2, "fake"),
+                    (2, "gcc"),
+                    (2, "gcc-runtime"),
+                    (3, "gcc"),
+                ],
+                {"cover": "paths"},
+            ),
+            # Postorder node traversal
+            (
+                [
+                    (3, "gcc"),
+                    (3, "gcc-runtime"),
+                    (4, "libelf"),
+                    (3, "libdwarf"),
+                    (2, "dyninst"),
+                    (3, "fake"),
+                    (2, "zmpi"),
+                    (1, "callpath"),
+                    (0, "mpileaks"),
+                ],
+                {"order": "post"},
+            ),
+            # Postorder edge traversal
+            (
+                [
+                    (3, "gcc"),
+                    (4, "gcc"),
+                    (3, "gcc-runtime"),
+                    (4, "gcc"),
+                    (4, "gcc-runtime"),
+                    (5, "gcc"),
+                    (5, "gcc-runtime"),
+                    (4, "libelf"),
+                    (3, "libdwarf"),
+                    (3, "libelf"),
+                    (2, "dyninst"),
+                    (2, "gcc"),
+                    (2, "gcc-runtime"),
+                    (3, "fake"),
+                    (3, "gcc"),
+                    (3, "gcc-runtime"),
+                    (2, "zmpi"),
+                    (1, "callpath"),
+                    (1, "gcc"),
+                    (1, "gcc-runtime"),
+                    (1, "zmpi"),
+                    (0, "mpileaks"),
+                ],
+                {"cover": "edges", "order": "post"},
+            ),
+            # Postorder path traversal
+            (
+                [
+                    (3, "gcc"),
+                    (4, "gcc"),
+                    (3, "gcc-runtime"),
+                    (4, "gcc"),
+                    (5, "gcc"),
+                    (4, "gcc-runtime"),
+                    (5, "gcc"),
+                    (6, "gcc"),
+                    (5, "gcc-runtime"),
+                    (4, "libelf"),
+                    (3, "libdwarf"),
+                    (4, "gcc"),
+                    (5, "gcc"),
+                    (4, "gcc-runtime"),
+                    (3, "libelf"),
+                    (2, "dyninst"),
+                    (2, "gcc"),
+                    (3, "gcc"),
+                    (2, "gcc-runtime"),
+                    (3, "fake"),
+                    (3, "gcc"),
+                    (4, "gcc"),
+                    (3, "gcc-runtime"),
+                    (2, "zmpi"),
+                    (1, "callpath"),
+                    (1, "gcc"),
+                    (2, "gcc"),
+                    (1, "gcc-runtime"),
+                    (2, "fake"),
+                    (2, "gcc"),
+                    (3, "gcc"),
+                    (2, "gcc-runtime"),
+                    (1, "zmpi"),
+                    (0, "mpileaks"),
+                ],
+                {"cover": "paths", "order": "post"},
+            ),
+        ],
+    )
+    def test_traversal(self, pairs, traverse_kwargs, default_mock_concretization):
+        r"""Tests different traversals of the following graph
 
-        names = ["mpileaks", "callpath", "dyninst", "libdwarf", "libelf", "zmpi", "fake"]
-        pairs = list(zip([0, 1, 2, 3, 4, 2, 3], names))
+        o mpileaks
+        |\
+        | |\
+        | | |\
+        | | | |\
+        | | | | o callpath
+        | |_|_|/|
+        |/| |_|/|
+        | |/| |/|
+        | | |/|/|
+        o | | | | zmpi
+        |\| | | |
+        |\ \ \ \ \
+        | |_|/ / /
+        |/| | | |
+        | |\ \ \ \
+        | | |_|/ /
+        | |/| | |
+        | | o | | fake
+        | |  / /
+        | | | o dyninst
+        | |_|/|
+        |/| |/|
+        | |/|/|
+        | | | |\
+        | | | | o libdwarf
+        | |_|_|/|
+        |/| |_|/|
+        | |/| |/|
+        | | |/|/
+        | | | o libel
+        | |_|/|
+        |/| |/|
+        | |/|/
+        o | | gcc-runtime
+        |\| |
+        | |/
+        |/|
+        o | glibc
+         /
+        o gcc
+        """
+        dag = default_mock_concretization("mpileaks ^zmpi")
+        names = [x for _, x in pairs]
 
-        traversal = dag.traverse()
-        assert [x.name for x in traversal] == names
-
-        traversal = dag.traverse(depth=True)
+        traversal = dag.traverse(**traverse_kwargs, depth=True)
         assert [(x, y.name) for x, y in traversal] == pairs
 
-    def test_preorder_edge_traversal(self):
-        dag = Spec("mpileaks ^zmpi").concretized()
-
-        names = [
-            "mpileaks",
-            "callpath",
-            "dyninst",
-            "libdwarf",
-            "libelf",
-            "libelf",
-            "zmpi",
-            "fake",
-            "zmpi",
-        ]
-        pairs = list(zip([0, 1, 2, 3, 4, 3, 2, 3, 1], names))
-
-        traversal = dag.traverse(cover="edges")
+        traversal = dag.traverse(**traverse_kwargs)
         assert [x.name for x in traversal] == names
 
-        traversal = dag.traverse(cover="edges", depth=True)
-        assert [(x, y.name) for x, y in traversal] == pairs
-
-    def test_preorder_path_traversal(self):
-        dag = Spec("mpileaks ^zmpi").concretized()
-
-        names = [
-            "mpileaks",
-            "callpath",
-            "dyninst",
-            "libdwarf",
-            "libelf",
-            "libelf",
-            "zmpi",
-            "fake",
-            "zmpi",
-            "fake",
-        ]
-        pairs = list(zip([0, 1, 2, 3, 4, 3, 2, 3, 1, 2], names))
-
-        traversal = dag.traverse(cover="paths")
-        assert [x.name for x in traversal] == names
-
-        traversal = dag.traverse(cover="paths", depth=True)
-        assert [(x, y.name) for x, y in traversal] == pairs
-
-    def test_postorder_node_traversal(self):
-        dag = Spec("mpileaks ^zmpi").concretized()
-
-        names = ["libelf", "libdwarf", "dyninst", "fake", "zmpi", "callpath", "mpileaks"]
-        pairs = list(zip([4, 3, 2, 3, 2, 1, 0], names))
-
-        traversal = dag.traverse(order="post")
-        assert [x.name for x in traversal] == names
-
-        traversal = dag.traverse(depth=True, order="post")
-        assert [(x, y.name) for x, y in traversal] == pairs
-
-    def test_postorder_edge_traversal(self):
-        dag = Spec("mpileaks ^zmpi").concretized()
-
-        names = [
-            "libelf",
-            "libdwarf",
-            "libelf",
-            "dyninst",
-            "fake",
-            "zmpi",
-            "callpath",
-            "zmpi",
-            "mpileaks",
-        ]
-        pairs = list(zip([4, 3, 3, 2, 3, 2, 1, 1, 0], names))
-
-        traversal = dag.traverse(cover="edges", order="post")
-        assert [x.name for x in traversal] == names
-
-        traversal = dag.traverse(cover="edges", depth=True, order="post")
-        assert [(x, y.name) for x, y in traversal] == pairs
-
-    def test_postorder_path_traversal(self):
-        dag = Spec("mpileaks ^zmpi").concretized()
-
-        names = [
-            "libelf",
-            "libdwarf",
-            "libelf",
-            "dyninst",
-            "fake",
-            "zmpi",
-            "callpath",
-            "fake",
-            "zmpi",
-            "mpileaks",
-        ]
-        pairs = list(zip([4, 3, 3, 2, 3, 2, 1, 2, 1, 0], names))
-
-        traversal = dag.traverse(cover="paths", order="post")
-        assert [x.name for x in traversal] == names
-
-        traversal = dag.traverse(cover="paths", depth=True, order="post")
-        assert [(x, y.name) for x, y in traversal] == pairs
-
     def test_dependents_and_dependencies_are_correct(self):
         spec = Spec.from_literal(
             {
@@ -590,7 +694,9 @@ def test_construct_spec_with_deptypes(self):
         assert s["d"].edges_to_dependencies(name="e")[0].depflag == dt.BUILD | dt.LINK
         assert s["e"].edges_to_dependencies(name="f")[0].depflag == dt.RUN
 
-        assert s["c"].edges_from_dependents(name="b")[0].depflag == dt.BUILD
+        # The subscript follows link/run transitive deps or direct build deps, therefore
+        # we need an extra step to get to "c"
+        assert s["b"]["c"].edges_from_dependents(name="b")[0].depflag == dt.BUILD
         assert s["e"].edges_from_dependents(name="d")[0].depflag == dt.BUILD | dt.LINK
         assert s["f"].edges_from_dependents(name="e")[0].depflag == dt.RUN
 
@@ -745,10 +851,10 @@ def test_spec_tree_respect_deptypes(self):
         "query,expected_length,expected_satisfies",
         [
             ({"virtuals": ["mpi"]}, 1, ["mpich", "mpi"]),
-            ({"depflag": dt.BUILD}, 2, ["mpich", "mpi", "callpath"]),
+            ({"depflag": dt.BUILD}, 3, ["mpich", "mpi", "callpath"]),
             ({"depflag": dt.BUILD, "virtuals": ["mpi"]}, 1, ["mpich", "mpi"]),
-            ({"depflag": dt.LINK}, 2, ["mpich", "mpi", "callpath"]),
-            ({"depflag": dt.BUILD | dt.LINK}, 2, ["mpich", "mpi", "callpath"]),
+            ({"depflag": dt.LINK}, 3, ["mpich", "mpi", "callpath"]),
+            ({"depflag": dt.BUILD | dt.LINK}, 4, ["mpich", "mpi", "callpath"]),
             ({"virtuals": ["lapack"]}, 0, []),
         ],
     )
@@ -757,12 +863,14 @@ def test_query_dependency_edges(
     ):
         """Tests querying edges to dependencies on the following DAG:
 
-        [    ]  mpileaks@=2.3
-        [bl  ]      ^callpath@=1.0
-        [bl  ]          ^dyninst@=8.2
-        [bl  ]              ^libdwarf@=20130729
-        [bl  ]              ^libelf@=0.8.13
-        [bl  ]      ^mpich@=3.0.4
+         -   [    ]  mpileaks@2.3
+         -   [bl  ]      ^callpath@1.0
+         -   [bl  ]          ^dyninst@8.2
+         -   [bl  ]              ^libdwarf@20130729
+         -   [bl  ]              ^libelf@0.8.13
+        [e]  [b   ]      ^gcc@10.1.0
+         -   [ l  ]      ^gcc-runtime@10.1.0
+         -   [bl  ]      ^mpich@3.0.4~debug
         """
         mpileaks = default_mock_concretization("mpileaks")
         edges = mpileaks.edges_to_dependencies(**query)
@@ -828,8 +936,8 @@ def test_synthetic_construction_of_split_dependencies_from_same_package(mock_pac
     root.add_dependency_edge(build_spec, depflag=dt.BUILD, virtuals=())
 
     # Check dependencies from the perspective of root
-    assert len(root.dependencies()) == 2
-    assert all(x.name == "pkg-c" for x in root.dependencies())
+    assert len(root.dependencies()) == 4
+    assert len([x for x in root.dependencies() if x.name == "pkg-c"]) == 2
 
     assert "@2.0" in root.dependencies(name="pkg-c", deptype=dt.BUILD)[0]
     assert "@1.0" in root.dependencies(name="pkg-c", deptype=dt.LINK | dt.RUN)[0]
@@ -853,8 +961,7 @@ def test_synthetic_construction_bootstrapping(mock_packages, config):
 
     root.add_dependency_edge(bootstrap, depflag=dt.BUILD, virtuals=())
 
-    assert len(root.dependencies()) == 1
-    assert root.dependencies()[0].name == "pkg-b"
+    assert len([x for x in root.dependencies() if x.name == "pkg-b"]) == 1
     assert root.name == "pkg-b"
 
 
@@ -873,8 +980,8 @@ def test_addition_of_different_deptypes_in_multiple_calls(mock_packages, config)
         root.add_dependency_edge(bootstrap, depflag=current_depflag, virtuals=())
 
         # Check edges in dependencies
-        assert len(root.edges_to_dependencies()) == 1
-        forward_edge = root.edges_to_dependencies(depflag=current_depflag)[0]
+        assert len(root.edges_to_dependencies(name="pkg-b")) == 1
+        forward_edge = root.edges_to_dependencies(depflag=current_depflag, name="pkg-b")[0]
         assert current_depflag & forward_edge.depflag
         assert id(forward_edge.parent) == id(root)
         assert id(forward_edge.spec) == id(bootstrap)
@@ -905,8 +1012,9 @@ def test_adding_same_deptype_with_the_same_name_raises(
 
 @pytest.mark.regression("33499")
 def test_indexing_prefers_direct_or_transitive_link_deps():
-    # Test whether spec indexing prefers direct/transitive link type deps over deps of
-    # build/run/test deps, and whether it does fall back to a full dag search.
+    """Tests whether spec indexing prefers direct/transitive link/run type deps over deps of
+    build/test deps.
+    """
     root = Spec("root")
 
     # Use a and z to since we typically traverse by edges sorted alphabetically.
@@ -919,7 +1027,7 @@ def test_indexing_prefers_direct_or_transitive_link_deps():
     z3_flavor_1 = Spec("z3 +through_a1")
     z3_flavor_2 = Spec("z3 +through_z1")
 
-    root.add_dependency_edge(a1, depflag=dt.BUILD | dt.RUN | dt.TEST, virtuals=())
+    root.add_dependency_edge(a1, depflag=dt.BUILD | dt.TEST, virtuals=())
 
     # unique package as a dep of a build/run/test type dep.
     a1.add_dependency_edge(a2, depflag=dt.ALL, virtuals=())
@@ -934,9 +1042,6 @@ def test_indexing_prefers_direct_or_transitive_link_deps():
     assert "through_z1" in root["z3"].variants
     assert "through_a1" in a1["z3"].variants
 
-    # Ensure that the full DAG is still searched
-    assert root["a2"]
-
 
 def test_getitem_sticks_to_subdag():
     """Test that indexing on Spec by virtual does not traverse outside the dag, which happens in
diff --git a/lib/spack/spack/test/spec_semantics.py b/lib/spack/spack/test/spec_semantics.py
index f223ac64125..4ff71a35617 100644
--- a/lib/spack/spack/test/spec_semantics.py
+++ b/lib/spack/spack/test/spec_semantics.py
@@ -17,15 +17,7 @@
 import spack.variant
 import spack.version as vn
 from spack.error import SpecError, UnsatisfiableSpecError
-from spack.spec import (
-    ArchSpec,
-    CompilerSpec,
-    DependencySpec,
-    Spec,
-    SpecFormatSigilError,
-    SpecFormatStringError,
-    UnsupportedCompilerError,
-)
+from spack.spec import ArchSpec, DependencySpec, Spec, SpecFormatSigilError, SpecFormatStringError
 from spack.variant import (
     InvalidVariantValueError,
     MultipleValuesInExclusiveVariantError,
@@ -460,8 +452,6 @@ def test_concrete_specs_which_satisfies_abstract(self, lhs, rhs, default_mock_co
             ("foo os=redhat6", "platform=test os=debian6 target=x86_64"),
             ("foo target=x86_64", "platform=test os=redhat6 target=x86"),
             ("foo arch=test-frontend-frontend", "platform=test os=frontend target=backend"),
-            ("foo%intel", "%gcc"),
-            ("foo%intel", "%gcc"),
             ("foo%gcc@4.3", "%gcc@4.4:4.6"),
             ("foo@4.0%gcc", "@1:3%gcc"),
             ("foo@4.0%gcc@4.5", "@1:3%gcc@4.4:4.6"),
@@ -941,7 +931,6 @@ def check_prop(check_spec, fmt_str, prop, getter):
             "{name}",
             "{version}",
             "{@version}",
-            "{%compiler}",
             "{namespace}",
             "{ namespace=namespace}",
             "{ namespace =namespace}",
@@ -1519,15 +1508,16 @@ def test_unsatisfiable_virtual_deps_bindings(self, spec_str):
         ("git-test@git.foo/bar", "{name}-{version}", str(pathlib.Path("git-test-git.foo_bar"))),
         ("git-test@git.foo/bar", "{name}-{version}-{/hash}", None),
         ("git-test@git.foo/bar", "{name}/{version}", str(pathlib.Path("git-test", "git.foo_bar"))),
+        # {compiler} is 'none' if a package does not depend on C, C++, or Fortran
         (
-            "git-test@{0}=1.0%gcc".format("a" * 40),
+            f"git-test@{'a' * 40}=1.0%gcc",
             "{name}/{version}/{compiler}",
-            str(pathlib.Path("git-test", "{0}_1.0".format("a" * 40), "gcc")),
+            str(pathlib.Path("git-test", f"{'a' * 40}_1.0", "none")),
         ),
         (
             "git-test@git.foo/bar=1.0%gcc",
             "{name}/{version}/{compiler}",
-            str(pathlib.Path("git-test", "git.foo_bar_1.0", "gcc")),
+            str(pathlib.Path("git-test", "git.foo_bar_1.0", "none")),
         ),
     ],
 )
@@ -1711,12 +1701,18 @@ def test_call_dag_hash_on_old_dag_hash_spec(mock_packages, default_mock_concreti
 def test_spec_trim(mock_packages, config):
     top = Spec("dt-diamond").concretized()
     top.trim("dt-diamond-left")
-    remaining = set(x.name for x in top.traverse())
-    assert set(["dt-diamond", "dt-diamond-right", "dt-diamond-bottom"]) == remaining
+    remaining = {x.name for x in top.traverse()}
+    assert {
+        "dt-diamond",
+        "dt-diamond-right",
+        "dt-diamond-bottom",
+        "gcc-runtime",
+        "gcc",
+    } == remaining
 
     top.trim("dt-diamond-right")
-    remaining = set(x.name for x in top.traverse())
-    assert set(["dt-diamond"]) == remaining
+    remaining = {x.name for x in top.traverse()}
+    assert {"dt-diamond", "gcc-runtime", "gcc"} == remaining
 
 
 @pytest.mark.regression("30861")
@@ -1746,11 +1742,6 @@ def test_concretize_partial_old_dag_hash_spec(mock_packages, config):
     assert not getattr(spec["dt-diamond-bottom"], "_package_hash", None)
 
 
-def test_unsupported_compiler():
-    with pytest.raises(UnsupportedCompilerError):
-        Spec("gcc%fake-compiler").validate_or_raise()
-
-
 def test_package_hash_affects_dunder_and_dag_hash(mock_packages, default_mock_concretization):
     a1 = default_mock_concretization("pkg-a")
     a2 = default_mock_concretization("pkg-a")
@@ -1823,10 +1814,10 @@ def test_abstract_contains_semantic(lhs, rhs, expected, mock_packages):
         (ArchSpec, "None-ubuntu20.04-None", "None-ubuntu20.04-None", (True, True, True)),
         (ArchSpec, "None-ubuntu20.04-None", "None-ubuntu22.04-None", (False, False, False)),
         # Compiler
-        (CompilerSpec, "gcc", "clang", (False, False, False)),
-        (CompilerSpec, "gcc", "gcc@5", (True, False, True)),
-        (CompilerSpec, "gcc@5", "gcc@5.3", (True, False, True)),
-        (CompilerSpec, "gcc@5", "gcc@5-tag", (True, False, True)),
+        (Spec, "gcc", "clang", (False, False, False)),
+        (Spec, "gcc", "gcc@5", (True, False, True)),
+        (Spec, "gcc@5", "gcc@5.3", (True, False, True)),
+        (Spec, "gcc@5", "gcc@5-tag", (True, False, True)),
         # Flags (flags are a map, so for convenience we initialize a full Spec)
         # Note: the semantic is that of sv variants, not mv variants
         (Spec, "cppflags=-foo", "cppflags=-bar", (True, False, False)),
@@ -1882,8 +1873,8 @@ def test_intersects_and_satisfies(factory, lhs_str, rhs_str, results):
             "None-ubuntu20.04-nocona,haswell",
         ),
         # Compiler
-        (CompilerSpec, "gcc@5", "gcc@5-tag", True, "gcc@5-tag"),
-        (CompilerSpec, "gcc@5", "gcc@5", False, "gcc@5"),
+        (Spec, "foo %gcc@5", "foo %gcc@5-tag", True, "foo %gcc@5-tag"),
+        (Spec, "foo %gcc@5", "foo %gcc@5", False, "foo %gcc@5"),
         # Flags
         (Spec, "cppflags=-foo", "cppflags=-foo", False, "cppflags=-foo"),
         (Spec, "cppflags=-foo", "cflags=-foo", True, "cppflags=-foo cflags=-foo"),
diff --git a/lib/spack/spack/test/spec_syntax.py b/lib/spack/spack/test/spec_syntax.py
index 6a78f74423b..19427328ff4 100644
--- a/lib/spack/spack/test/spec_syntax.py
+++ b/lib/spack/spack/test/spec_syntax.py
@@ -817,9 +817,7 @@ def test_dep_spec_by_hash(database, config):
     assert "zmpi" in mpileaks_hash_fake
     assert mpileaks_hash_fake["zmpi"] == spack.spec.Spec("zmpi")
 
-    mpileaks_hash_zmpi = SpecParser(
-        f"mpileaks %{mpileaks_zmpi.compiler} ^ /{zmpi.dag_hash()}"
-    ).next_spec()
+    mpileaks_hash_zmpi = SpecParser(f"mpileaks ^ /{zmpi.dag_hash()}").next_spec()
     mpileaks_hash_zmpi.replace_hash()
     assert "zmpi" in mpileaks_hash_zmpi
     assert mpileaks_hash_zmpi["zmpi"] == zmpi
diff --git a/lib/spack/spack/test/tag.py b/lib/spack/spack/test/tag.py
index fe4d93dc2a4..3a4e953dfbb 100644
--- a/lib/spack/spack/test/tag.py
+++ b/lib/spack/spack/test/tag.py
@@ -85,7 +85,7 @@ def test_tag_get_available(tags, expected, mock_packages):
 
 
 def test_tag_get_installed_packages(mock_packages, mock_archive, mock_fetch, install_mockery):
-    install("mpich")
+    install("--fake", "mpich")
 
     for skip in [False, True]:
         all_pkgs = spack.tag.packages_with_tags(None, True, skip)