preprocess: Add dependencies kwarg

docs/markdown/snippets/preprocess.md

@@ -0,0 +1,6 @@
+## compiler.preprocess()
+
+Dependencies keyword argument can now be passed to `compiler.preprocess()` to
+add include directories or compiler arguments.
+
+Generated sources such as custom targets are now allowed too.

docs/yaml/objects/compiler.yaml

@@ -617,3 +617,7 @@ methods:
       type: list[str]
       description: |
         Extra flags to pass to the preprocessor
+    dependencies:
+      type: dep | list[dep]
+      description: Additionally dependencies required.
+      since: 1.1.0

mesonbuild/interpreter/compiler.py

@@ -87,6 +87,7 @@ if T.TYPE_CHECKING:
         output: str
         compile_args: T.List[str]
         include_directories: T.List[build.IncludeDirs]
+        dependencies: T.List[dependencies.Dependency]
 
 
 class _TestMode(enum.Enum):
@@ -760,6 +761,7 @@ class CompilerHolder(ObjectHolder['Compiler']):
         KwargInfo('output', str, default='@PLAINNAME@.i'),
         KwargInfo('compile_args', ContainerTypeInfo(list, str), listify=True, default=[]),
         _INCLUDE_DIRS_KW,
+        _DEPENDENCIES_KW.evolve(since='1.1.0'),
     )
     def preprocess_method(self, args: T.Tuple[T.List['mesonlib.FileOrString']], kwargs: 'PreprocessKW') -> T.List[build.CustomTargetIndex]:
         compiler = self.compiler.get_preprocessor()
@@ -771,6 +773,7 @@ class CompilerHolder(ObjectHolder['Compiler']):
             f'{self.compiler.language}_args': kwargs['compile_args'],
             'build_by_default': False,
             'include_directories': kwargs['include_directories'],
+            'dependencies': kwargs['dependencies'],
         }
         tg_name = f'preprocessor_{next(self.preprocess_uid)}'
         tg = build.CompileTarget(

test cases/common/259 preprocess/bar.c

@@ -1,3 +1,3 @@
 int @BAR@(void) {
-    return BAR;
+    return BAR + PLOP;
 }

test cases/common/259 preprocess/meson.build

@@ -13,7 +13,9 @@ bar_c = custom_target(
   capture: true,
 )
 
-pp_files = cc.preprocess('foo.c', bar_c, output: '@PLAINNAME@')
+dep = declare_dependency(compile_args: '-DPLOP=0')
+
+pp_files = cc.preprocess('foo.c', bar_c, output: '@PLAINNAME@', dependencies: dep)
 
 foreach f : pp_files
   message(f.full_path())