refactor: Refactor BothLibraries logic

This commit introduces a new type of `HoldableObject`: The
`SecondLevelHolder`. The primary purpose of this class is
to handle cases where two (or more) `HoldableObject`s are
stored at the same time (with one default object). The
best (and currently only) example here is the `BothLibraries`
class.

2 files changed, 6 insertions, 5 deletions

diff --git a/mesonbuild/interpreter/interpreter.py b/mesonbuild/interpreter/interpreter.py
index d9ecd25..e4fb0d4 100644
--- a/mesonbuild/interpreter/interpreter.py
+++ b/mesonbuild/interpreter/interpreter.py
@@ -26,7 +26,7 @@ from ..programs import ExternalProgram, NonExistingExternalProgram
 from ..dependencies import Dependency
 from ..depfile import DepFile
 from ..interpreterbase import ContainerTypeInfo, InterpreterBase, KwargInfo, typed_kwargs, typed_pos_args
-from ..interpreterbase import noPosargs, noKwargs, stringArgs, permittedKwargs, noArgsFlattening, unholder_return
+from ..interpreterbase import noPosargs, noKwargs, stringArgs, permittedKwargs, noArgsFlattening, noSecondLevelHolderResolving, unholder_return
 from ..interpreterbase import InterpreterException, InvalidArguments, InvalidCode, SubdirDoneRequest
 from ..interpreterbase import Disabler, disablerIfNotFound
 from ..interpreterbase import FeatureNew, FeatureDeprecated, FeatureNewKwargs, FeatureDeprecatedKwargs
@@ -2537,8 +2537,6 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
             sources = [sources]
         results: T.List['SourceOutputs'] = []
         for s in sources:
-            if isinstance(s, build.BothLibraries):
-                s = s.get_preferred_library()
             if isinstance(s, str):
                 self.validate_within_subproject(self.subdir, s)
                 results.append(mesonlib.File.from_source_file(self.environment.source_dir, self.subdir, s))
@@ -2713,6 +2711,7 @@ This will become a hard error in the future.''', location=self.current_node)
 
     @noKwargs
     @noArgsFlattening
+    @noSecondLevelHolderResolving
     def func_set_variable(self, node, args, kwargs):
         if len(args) != 2:
             raise InvalidCode('Set_variable takes two arguments.')
diff --git a/mesonbuild/interpreter/interpreterobjects.py b/mesonbuild/interpreter/interpreterobjects.py
index 261d781..03725b9 100644
--- a/mesonbuild/interpreter/interpreterobjects.py
+++ b/mesonbuild/interpreter/interpreterobjects.py
@@ -19,7 +19,7 @@ from ..interpreterbase import (
                                FeatureCheckBase, FeatureNewKwargs, FeatureNew, FeatureDeprecated,
                                typed_pos_args, typed_kwargs, KwargInfo, stringArgs, permittedKwargs,
                                noArgsFlattening, noPosargs, noKwargs, unholder_return, TYPE_var, TYPE_kwargs, TYPE_nvar, TYPE_nkwargs,
-                               flatten, InterpreterException, InvalidArguments, InvalidCode)
+                               flatten, resolve_second_level_holders, InterpreterException, InvalidArguments, InvalidCode)
 from ..dependencies import Dependency, ExternalLibrary, InternalDependency
 from ..programs import ExternalProgram
 from ..mesonlib import HoldableObject, MesonException, OptionKey, listify, Popen_safe
@@ -754,6 +754,8 @@ class ModuleObjectHolder(ObjectHolder[ModuleObject]):
             raise InvalidCode(f'Unknown method {method_name!r} in object.')
         if not getattr(method, 'no-args-flattening', False):
             args = flatten(args)
+        if not getattr(method, 'no-second-level-holder-flattening', False):
+            args, kwargs = resolve_second_level_holders(args, kwargs)
         state = ModuleState(self.interpreter)
         # Many modules do for example self.interpreter.find_program_impl(),
         # so we have to ensure they use the current interpreter and not the one
@@ -798,7 +800,7 @@ class BuildTargetHolder(ObjectHolder[_BuildTarget]):
     @property
     def _target_object(self) -> build.BuildTarget:
         if isinstance(self.held_object, build.BothLibraries):
-            return self.held_object.get_preferred_library()
+            return self.held_object.get_default_object()
         assert isinstance(self.held_object, build.BuildTarget)
         return self.held_object