Fix regression in spec format string for indiviual variants (#46206)

Fix a regression in {variants.X} and {variants.X.value} spec format strings.

lib/spack/spack/spec.py

@@ -4420,9 +4420,12 @@ def format_attribute(match_object: Match) -> str:
                 if part.startswith("_"):
                     raise SpecFormatStringError("Attempted to format private attribute")
                 else:
-                    if part == "variants" and isinstance(current, vt.VariantMap):
+                    if isinstance(current, vt.VariantMap):
                         # subscript instead of getattr for variant names
+                        try:
                             current = current[part]
+                        except KeyError:
+                            raise SpecFormatStringError(f"Variant '{part}' does not exist")
                     else:
                         # aliases
                         if part == "arch":

lib/spack/spack/test/spec_semantics.py

@@ -672,6 +672,13 @@ def test_spec_formatting(self, default_mock_concretization):
             ("{/hash}", "/", lambda s: "/" + s.dag_hash()),
         ]
 
+        variants_segments = [
+            ("{variants.debug}", spec, "debug"),
+            ("{variants.foo}", spec, "foo"),
+            ("{^pkg-a.variants.bvv}", spec["pkg-a"], "bvv"),
+            ("{^pkg-a.variants.foo}", spec["pkg-a"], "foo"),
+        ]
+
         other_segments = [
             ("{spack_root}", spack.paths.spack_root),
             ("{spack_install}", spack.store.STORE.layout.root),
@@ -699,6 +706,12 @@ def check_prop(check_spec, fmt_str, prop, getter):
             callpath, fmt_str = depify("callpath", named_str, sigil)
             assert spec.format(fmt_str) == getter(callpath)
 
+        for named_str, test_spec, variant_name in variants_segments:
+            assert test_spec.format(named_str) == str(test_spec.variants[variant_name])
+            assert test_spec.format(named_str[:-1] + ".value}") == str(
+                test_spec.variants[variant_name].value
+            )
+
         for named_str, expected in other_segments:
             actual = spec.format(named_str)
             assert expected == actual
@@ -731,6 +744,7 @@ def test_spec_formatting_sigil_mismatches(self, default_mock_concretization, fmt
             r"{dag_hash}",
             r"{foo}",
             r"{+variants.debug}",
+            r"{variants.this_variant_does_not_exist}",
         ],
     )
     def test_spec_formatting_bad_formats(self, default_mock_concretization, fmt_str):