rust: rework external find to require both rustc and cargo (#45286)

* rust: rework external find to require both rustc and cargo

* rust: handle unable to parse version

* [@spackbot] updating style on behalf of wdconinc

* rust: not x or not y -> not (x and y)

Co-authored-by: Alec Scott <hi@alecbcs.com>

* rust: pick first rustc found

Co-authored-by: Alec Scott <hi@alecbcs.com>

* rust: list comprehensions

Co-authored-by: Alec Scott <hi@alecbcs.com>

---------

Co-authored-by: wdconinc <wdconinc@users.noreply.github.com>
Co-authored-by: Alec Scott <hi@alecbcs.com>

var/spack/repos/builtin/packages/rust/package.py

@@ -103,10 +103,17 @@ class Rust(Package):
     phases = ["configure", "build", "install"]
 
     @classmethod
-    def determine_version(csl, exe):
-        output = Executable(exe)("--version", output=str, error=str)
+    def determine_spec_details(cls, prefix, exes_in_prefix):
+        rustc_candidates = [x for x in exes_in_prefix if os.path.basename(x) == "rustc"]
+        cargo_candidates = [x for x in exes_in_prefix if os.path.basename(x) == "cargo"]
+        # Both rustc and cargo must be present
+        if not (rustc_candidates and cargo_candidates):
+            return
+        output = Executable(rustc_candidates[0])("--version", output=str, error=str)
         match = re.match(r"rustc (\S+)", output)
-        return match.group(1) if match else None
+        if match:
+            version_str = match.group(1)
+            return Spec.from_detection(f"rust@{version_str}")
 
     def setup_dependent_package(self, module, dependent_spec):
         module.cargo = Executable(os.path.join(self.spec.prefix.bin, "cargo"))