Concrete multi-valued variants (#49756)

Similar to the range-or-specific-version ambiguity of `@1.2` in the past,
which was solved with `@1.2` vs `@=1.2` we still have the ambiguity of
`name=a,b,c` in multi-valued variants. Do they mean "at least a,b,c" or
"exactly a,b,c"?

This issue comes up in for example `gcc languages=c,cxx`; there's no
way to exclude `fortran`.

The ambiguity is resolved with syntax `:=` to distinguish concrete from
abstract.

The following strings parse as **concrete** variants:

* `name:=a,b,c` => values exactly {a, b, c}
* `name:=a` => values exactly {a}
* `+name` => values exactly {True}
* `~name` => values exactly {False}

The following strings parse as **abstract** variants:

* `name=a,b,c` values at least {a, b, c}
* `name=*` special case for testing existence of a variant; values are at
  least the empty set {}

As a reminder

* `satisfies(lhs, rhs)` means `concretizations(lhs)` ⊆ `concretizations(rhs)`
* `intersects(lhs, rhs)` means `concretizations(lhs)` ∩ `concretizations(rhs)` ≠ ∅

where `concretizations(...)` is the set of sets of variant values in this case.

The satisfies semantics are:

* rhs abstract: rhs values is a subset of lhs values (whether lhs is abstract or concrete)
* lhs concrete, rhs concrete: set equality
* lhs abstract, rhs concrete: false

and intersects should mean

* lhs and rhs abstract: true (the union is a valid concretization under both)
* lhs or rhs abstract: true iff the abstract variant's values are a subset of the concrete one
* lhs concrete, rhs concrete: set equality

Concrete specs with single-valued variants are printed `+foo`, `~foo` and `foo=bar`;
only multi-valued variants are printed with `foo:=bar,baz` to reduce the visual noise.

lib/spack/docs/basic_usage.rst

@@ -1291,55 +1291,61 @@ based on site policies.
 Variants
 ^^^^^^^^
 
-Variants are named options associated with a particular package. They are
-optional, as each package must provide default values for each variant it
-makes available. Variants can be specified using
-a flexible parameter syntax ``name=<value>``. For example,
-``spack install mercury debug=True`` will install mercury built with debug
-flags. The names of particular variants available for a package depend on
+Variants are named options associated with a particular package and are
+typically used to enable or disable certain features at build time. They
+are optional, as each package must provide default values for each variant
+it makes available.
+
+The names of variants available for a particular package depend on
 what was provided by the package author. ``spack info <package>`` will
 provide information on what build variants are available.
 
-For compatibility with earlier versions, variants which happen to be
-boolean in nature can be specified by a syntax that represents turning
-options on and off. For example, in the previous spec we could have
-supplied ``mercury +debug`` with the same effect of enabling the debug
-compile time option for the libelf package.
+There are different types of variants:
 
-Depending on the package a variant may have any default value.  For
-``mercury`` here, ``debug`` is ``False`` by default, and we turned it on
-with ``debug=True`` or ``+debug``.  If a variant is ``True`` by default
-you can turn it off by either adding ``-name`` or ``~name`` to the spec.
+1. Boolean variants. Typically used to enable or disable a feature at
+   compile time. For example, a package might have a ``debug`` variant that
+   can be explicitly enabled with ``+debug`` and disabled with ``~debug``.
+2. Single-valued variants. Often used to set defaults. For example, a package
+   might have a ``compression`` variant that determines the default
+   compression algorithm, which users could set to ``compression=gzip`` or
+   ``compression=zstd``.
+3. Multi-valued variants. A package might have a ``fabrics`` variant that
+   determines which network fabrics to support. Users could set this to
+   ``fabrics=verbs,ofi`` to enable both InfiniBand verbs and OpenFabrics
+   interfaces. The values are separated by commas.
 
-There are two syntaxes here because, depending on context, ``~`` and
-``-`` may mean different things.  In most shells, the following will
-result in the shell performing home directory substitution:
+   The meaning of ``fabrics=verbs,ofi`` is to enable *at least* the specified
+   fabrics, but other fabrics may be enabled as well. If the intent is to
+   enable *only* the specified fabrics, then the ``fabrics:=verbs,ofi``
+   syntax should be used with the ``:=`` operator.
 
-.. code-block:: sh
+.. note::
 
-   mpileaks ~debug   # shell may try to substitute this!
-   mpileaks~debug    # use this instead
+   In certain shells, the the ``~`` character is expanded to the home
+   directory. To avoid these issues, avoid whitespace between the package
+   name and the variant:
 
-If there is a user called ``debug``, the ``~`` will be incorrectly
-expanded.  In this situation, you would want to write ``libelf
--debug``.  However, ``-`` can be ambiguous when included after a
-package name without spaces:
+   .. code-block:: sh
 
-.. code-block:: sh
+      mpileaks ~debug   # shell may try to substitute this!
+      mpileaks~debug    # use this instead
 
-   mpileaks-debug     # wrong!
-   mpileaks -debug    # right
+   Alternatively, you can use the ``-`` character to disable a variant,
+   but be aware that this requires a space between the package name and
+   the variant:
 
-Spack allows the ``-`` character to be part of package names, so the
-above will be interpreted as a request for the ``mpileaks-debug``
-package, not a request for ``mpileaks`` built without ``debug``
-options.  In this scenario, you should write ``mpileaks~debug`` to
-avoid ambiguity.
+   .. code-block:: sh
 
-When spack normalizes specs, it prints them out with no spaces boolean
-variants using the backwards compatibility syntax and uses only ``~``
-for disabled boolean variants.  The ``-`` and spaces on the command
-line are provided for convenience and legibility.
+      mpileaks-debug     # wrong: refers to a package named "mpileaks-debug"
+      mpileaks -debug    # right: refers to a package named mpileaks with debug disabled
+
+   As a last resort, ``debug=False`` can also be used to disable a boolean variant.
+
+
+
+"""""""""""""""""""""""""""""""""""
+Variant propagation to dependencies
+"""""""""""""""""""""""""""""""""""
 
 Spack allows variants to propagate their value to the package's
 dependency by using ``++``, ``--``, and ``~~`` for boolean variants.