dependencies: Make MissingCompiler an implementation detail

Since type checkers now view MisingCompiler as a Compiler, and it
provides a method to determine that it is missing (language ==
'missing'), we don't need to expose it anywhere, it's just an
implementation detail of the dependencies.base module.

mesonbuild/cmake/tracetargets.py

@@ -13,7 +13,6 @@
     from .traceparser import CMakeTraceParser
     from ..environment import Environment
     from ..compilers import Compiler
-    from ..dependencies import MissingCompiler
 
 class ResolvedTarget:
     def __init__(self) -> None:
@@ -26,7 +25,7 @@ def resolve_cmake_trace_targets(target_name: str,
                                 trace: 'CMakeTraceParser',
                                 env: 'Environment',
                                 *,
-                                clib_compiler: T.Union['MissingCompiler', 'Compiler'] = None,
+                                clib_compiler: T.Optional[Compiler] = None,
                                 not_found_warning: T.Callable[[str], None] = lambda x: None) -> ResolvedTarget:
     res = ResolvedTarget()
     targets = [target_name]

mesonbuild/dependencies/__init__.py

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-from .base import Dependency, InternalDependency, ExternalDependency, NotFoundDependency, MissingCompiler
+from .base import Dependency, InternalDependency, ExternalDependency, NotFoundDependency
 from .base import (
         ExternalLibrary, DependencyException, DependencyMethods,
         BuiltinDependency, SystemDependency, get_leaf_external_dependencies)
@@ -28,7 +28,6 @@
     'ExternalLibrary',
     'DependencyException',
     'DependencyMethods',
-    'MissingCompiler',
 
     'find_external_dependency',
     'get_dep_identifier',

mesonbuild/dependencies/base.py

@@ -374,7 +374,7 @@ def __init__(self, type_name: DependencyTypeName, environment: 'Environment', kw
         # Is this dependency to be run on the build platform?
         self.clib_compiler = detect_compiler(self.name, environment, self.for_machine, self.language)
 
-    def get_compiler(self) -> T.Union['MissingCompiler', 'Compiler']:
+    def get_compiler(self) -> Compiler:
         return self.clib_compiler
 
     def get_partial_dependency(self, *, compile_args: bool = False,
@@ -582,7 +582,7 @@ def process_method_kw(possible: T.Iterable[DependencyMethods], kwargs: Dependenc
     return methods
 
 def detect_compiler(name: str, env: 'Environment', for_machine: MachineChoice,
-                    language: T.Optional[str]) -> T.Union['MissingCompiler', 'Compiler']:
+                    language: T.Optional[str]) -> Compiler:
     """Given a language and environment find the compiler used."""
     compilers = env.coredata.compilers[for_machine]
 

mesonbuild/dependencies/qt.py

@@ -38,7 +38,6 @@
     from ..compilers import Compiler
     from ..envconfig import MachineInfo
     from ..environment import Environment
-    from ..dependencies import MissingCompiler
     from ..interpreter.kwargs import Dependency as DependencyKw
 
 
@@ -132,7 +131,7 @@ class _QtBase:
     """Mixin class for shared components between PkgConfig and Qmake."""
 
     link_args: T.List[str]
-    clib_compiler: T.Union['MissingCompiler', 'Compiler']
+    clib_compiler: Compiler
     env: 'Environment'
     libexecdir: T.Optional[str] = None