utils: fix annotation of pickle_load

It's actually Generic, and we should use Generic annotations to get the
correct result. This means that we don't have to assert or cast the
return type, because mypy just knowns

mesonbuild/coredata.py

@@ -1056,9 +1056,7 @@ def major_versions_differ(v1: str, v2: str) -> bool:
 
 def load(build_dir: str) -> CoreData:
     filename = os.path.join(build_dir, 'meson-private', 'coredata.dat')
-    obj = pickle_load(filename, 'Coredata', CoreData)
-    assert isinstance(obj, CoreData), 'for mypy'
-    return obj
+    return pickle_load(filename, 'Coredata', CoreData)
 
 
 def save(obj: CoreData, build_dir: str) -> str:

mesonbuild/minstall.py

@@ -131,9 +131,7 @@ class DirMaker:
 
 
 def load_install_data(fname: str) -> InstallData:
-    obj = pickle_load(fname, 'InstallData', InstallData)
-    assert isinstance(obj, InstallData), 'fo mypy'
-    return obj
+    return pickle_load(fname, 'InstallData', InstallData)
 
 def is_executable(path: str, follow_symlinks: bool = False) -> bool:
     '''Checks whether any of the "x" bits are set in the source file mode.'''

mesonbuild/utils/universal.py

@@ -37,13 +37,26 @@ from mesonbuild import mlog
 from .core import MesonException, HoldableObject
 
 if T.TYPE_CHECKING:
-    from typing_extensions import Literal
+    from typing_extensions import Literal, Protocol
 
     from .._typing import ImmutableListProtocol
     from ..build import ConfigurationData
     from ..coredata import KeyedOptionDictType, UserOption
+    from ..environment import Environment
     from ..compilers.compilers import Compiler
 
+    class _EnvPickleLoadable(Protocol):
+
+        environment: Environment
+
+    class _VerPickleLoadable(Protocol):
+
+        version: str
+
+    # A generic type for pickle_load. This allows any type that has either a
+    # .version or a .environment to be passed.
+    _PL = T.TypeVar('_PL', bound=T.Union[_EnvPickleLoadable, _VerPickleLoadable])
+
 FileOrString = T.Union['File', str]
 
 _T = T.TypeVar('_T')
@@ -2326,7 +2339,8 @@ class OptionKey:
         """Convenience method to check if this is a base option."""
         return self.type is OptionType.BASE
 
-def pickle_load(filename: str, object_name: str, object_type: T.Type) -> T.Any:
+
+def pickle_load(filename: str, object_name: str, object_type: T.Type[_PL]) -> _PL:
     load_fail_msg = f'{object_name} file {filename!r} is corrupted. Try with a fresh build tree.'
     try:
         with open(filename, 'rb') as f:
@@ -2342,11 +2356,18 @@ def pickle_load(filename: str, object_name: str, object_type: T.Type) -> T.Any:
             f'meson setup {build_dir} --wipe')
     if not isinstance(obj, object_type):
         raise MesonException(load_fail_msg)
+
+    # Because these Protocols are not available at runtime (and cannot be made
+    # available at runtime until we drop support for Python < 3.8), we have to
+    # do a bit of hackery so that mypy understands what's going on here
+    version: str
+    if hasattr(obj, 'version'):
+        version = T.cast('_VerPickleLoadable', obj).version
+    else:
+        version = T.cast('_EnvPickleLoadable', obj).environment.coredata.version
+
     from ..coredata import version as coredata_version
     from ..coredata import major_versions_differ, MesonVersionMismatchException
-    version = getattr(obj, 'version', None)
-    if version is None:
-        version = obj.environment.coredata.version
     if major_versions_differ(version, coredata_version):
         raise MesonVersionMismatchException(version, coredata_version)
     return obj