Ensure variant defaults are parsable from CLI. (#18661)

- Add a unit test to check if there are unparsable defaults
- Fix 'rust' and 'nsimd' variants

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

@@ -2,10 +2,6 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-from spack import *
-
-
 class Nsimd(CMakePackage):
     """NSIMD is a vectorization library that abstracts SIMD programming.
     It was designed to exploit the maximum power of processors
@@ -28,11 +24,8 @@ class Nsimd(CMakePackage):
                 'NEON128', 'AARCH64', 'SVE',
             ),
             multi=False)
-    variant('optionals',
-            default=(),
-            description='Optional SIMD features',
-            values=('FMA', 'FP16'),
-            multi=True)
+    variant('optionals', values=any_combination_of('FMA', 'FP16'),
+            description='Optional SIMD features',)
 
     conflicts('simd=none', msg="SIMD instruction set not defined")
 
@@ -56,10 +49,14 @@ def generate_code(self, spec, prefix):
         python(*options)
 
     def cmake_args(self):
+        # Required SIMD argument
         simd = self.spec.variants['simd'].value
-        optionals = ';'.join(self.spec.variants['optionals'].value)
-        cmake_args = [
-            "-DSIMD={0}".format(simd),
-            "-DSIMD_OPTIONALS={0}".format(optionals),
-        ]
+        cmake_args = ["-DSIMD={0}".format(simd)]
+
+        # Optional SIMD instructions to be turned on explicitly
+        optionals_value = self.spec.variants['optionals'].value
+        if optionals_value != 'none':
+            optionals_arg = ';'.join(optionals_value)
+            cmake_args.append("-DSIMD_OPTIONALS={0}".format(optionals_arg))
+
         return cmake_args

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

@@ -2,8 +2,6 @@
 # Spack Project Developers. See the top-level COPYRIGHT file for details.
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
-
-from spack import *
 from six import iteritems
 
 
@@ -56,9 +54,7 @@ class Rust(Package):
         description='Install Rust source files'
     )
     variant(
-        'extra_targets',
-        default=(),
-        multi=True,
+        'extra_targets', default='none', multi=True,
         description='Triples for extra targets to enable. For supported targets, see: https://doc.rust-lang.org/nightly/rustc/platform-support.html'
     )
 
@@ -502,7 +498,10 @@ def configure(self, spec, prefix):
 
         ar = which('ar', required=True)
 
-        extra_targets = list(self.spec.variants['extra_targets'].value)
+        extra_targets = []
+        if self.spec.variants['extra_targets'].value != 'none':
+            extra_targets = list(self.spec.variants['extra_targets'].value)
+
         targets = [self.get_rust_target()] + extra_targets
         target_spec = 'target=[' + \
             ','.join('"{0}"'.format(target) for target in targets) + ']'