Add a helper to simplify the usage of PerMachineDefaultable

3 files changed, 29 insertions, 24 deletions

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index a60b2b7..13eb0c1 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -206,21 +206,14 @@ class Build:
         self.projects = {}
         self.targets: T.MutableMapping[str, 'Target'] = OrderedDict()
         self.run_target_names: T.Set[T.Tuple[str, str]] = set()
-
-        global_args: PerMachineDefaultable[T.Dict[str, T.List[str]]] = PerMachineDefaultable({})
-        global_link_args: PerMachineDefaultable[T.Dict[str, T.List[str]]] = PerMachineDefaultable({})
-        project_args: PerMachineDefaultable[T.Dict[str, T.List[str]]] = PerMachineDefaultable({})
-        project_link_args: PerMachineDefaultable[T.Dict[str, T.List[str]]] = PerMachineDefaultable({})
-        if environment.is_cross_build():
-            global_args.host = {}
-            global_link_args.host = {}
-            project_args.host = {}
-            project_link_args.host = {}
-        self.global_args = global_args.default_missing()
-        self.projects_args = project_args.default_missing()
-        self.global_link_args = global_link_args.default_missing()
-        self.projects_link_args = project_link_args.default_missing()
-
+        self.global_args: PerMachine[T.Dict[str, T.List[str]]] = PerMachineDefaultable.default(
+            environment.is_cross_build(), {}, {})
+        self.global_link_args: PerMachine[T.Dict[str, T.List[str]]] = PerMachineDefaultable.default(
+            environment.is_cross_build(), {}, {})
+        self.projects_args: PerMachine[T.Dict[str, T.List[str]]] = PerMachineDefaultable.default(
+            environment.is_cross_build(), {}, {})
+        self.projects_link_args: PerMachine[T.Dict[str, T.List[str]]] = PerMachineDefaultable.default(
+            environment.is_cross_build(), {}, {})
         self.tests: T.List['Test'] = []
         self.benchmarks: T.List['Test'] = []
         self.headers: T.List[Headers] = []
@@ -243,10 +236,8 @@ class Build:
 
         # If we are doing a cross build we need two caches, if we're doing a
         # build == host compilation the both caches should point to the same place.
-        dependency_overrides: PerMachineDefaultable[T.Dict[T.Tuple, DependencyOverride]] = PerMachineDefaultable({})
-        if environment.is_cross_build():
-            dependency_overrides.host = {}
-        self.dependency_overrides = dependency_overrides.default_missing()
+        self.dependency_overrides: PerMachine[T.Dict[T.Tuple, DependencyOverride]] = PerMachineDefaultable.default(
+            environment.is_cross_build(), {}, {})
         self.devenv: T.List[EnvironmentVariables] = []
 
     def get_build_targets(self):
diff --git a/mesonbuild/coredata.py b/mesonbuild/coredata.py
index 7f94d18..5e726a4 100644
--- a/mesonbuild/coredata.py
+++ b/mesonbuild/coredata.py
@@ -407,11 +407,10 @@ class CoreData:
         self.initialized_subprojects: T.Set[str] = set()
 
         # For host == build configuraitons these caches should be the same.
-        deps: PerMachineDefaultable[DependencyCache] = PerMachineDefaultable(
-            DependencyCache(self.options, MachineChoice.BUILD))
-        if self.is_cross_build():
-            deps.host = DependencyCache(self.options, MachineChoice.HOST)
-        self.deps = deps.default_missing()
+        self.deps: PerMachine[DependencyCache] = PerMachineDefaultable.default(
+            self.is_cross_build(),
+            DependencyCache(self.options, MachineChoice.BUILD),
+            DependencyCache(self.options, MachineChoice.HOST))
 
         self.compiler_check_cache = OrderedDict()  # type: T.Dict[CompilerCheckCacheKey, compiler.CompileResult]
 
diff --git a/mesonbuild/mesonlib/universal.py b/mesonbuild/mesonlib/universal.py
index 684d223..66149f9 100644
--- a/mesonbuild/mesonlib/universal.py
+++ b/mesonbuild/mesonlib/universal.py
@@ -547,6 +547,21 @@ class PerMachineDefaultable(PerMachine[T.Optional[_T]]):
     def __repr__(self) -> str:
         return f'PerMachineDefaultable({self.build!r}, {self.host!r})'
 
+    @classmethod
+    def default(cls, is_cross: bool, build: _T, host: _T) -> PerMachine[_T]:
+        """Easy way to get a defaulted value
+
+        This allows simplifying the case where you can control whether host and
+        build are separate or not with a boolean. If the is_cross value is set
+        to true then the optional host value will be used, otherwise the host
+        will be set to the build value.
+        """
+        m = cls(build)
+        if is_cross:
+            m.host = host
+        return m.default_missing()
+
+
 
 class PerThreeMachineDefaultable(PerMachineDefaultable, PerThreeMachine[T.Optional[_T]]):
     """Extends `PerThreeMachine` with the ability to default from `None`s.