modules/pkg-config: Add support for Cflags.private

Cflags.private are compiler flags which are retrieved by pkg-config
when static linking is requested.

Currently, not all pkg-config implementations support Cflags.private;
FreeDesktop.Org's pkg-config doesn't support it [1], but pkgconf does.

Fixes #14749

[1] https://gitlab.freedesktop.org/pkg-config/pkg-config/-/issues/38

Author: lb90

docs/markdown/Pkgconfig-module.md

@@ -46,6 +46,9 @@ keyword arguments.
 - `requires` list of strings, pkgconfig-dependencies or libraries that
    `pkgconfig.generate()` was used on to put in the `Requires` field
 - `requires_private` the same as `requires` but for the `Requires.private` field
+- `cflags_private` (*Since 1.9.0*) compiler flags added when linking with a static
+  library. Note: currently, the FreeDesktop.org pkg-config implementation does
+  [not support Cflags.private](https://gitlab.freedesktop.org/pkg-config/pkg-config/-/issues/38)
 - `url` a string with a url for the library
 - `license` (*Since 1.9.0*) a string with a SPDX license to add to the generated file.
 - `variables` a list of strings with custom variables to add to the

mesonbuild/modules/pkgconfig.py

@@ -49,6 +49,7 @@ class GenerateKw(TypedDict):
         install_dir: T.Optional[str]
         d_module_versions: T.List[T.Union[str, int]]
         extra_cflags: T.List[str]
+        cflags_private: T.List[str]
         variables: T.Dict[str, str]
         uninstalled_variables: T.Dict[str, str]
         unescaped_variables: T.Dict[str, str]
@@ -96,6 +97,7 @@ def __init__(self, state: ModuleState, name: str, metadata: T.Dict[str, MetaData
         self.priv_libs: T.List[LIBS] = []
         self.priv_reqs: T.List[str] = []
         self.cflags: T.List[str] = []
+        self.cflags_private: T.List[str] = []
         self.version_reqs: T.DefaultDict[str, T.Set[str]] = defaultdict(set)
         self.link_whole_targets: T.List[T.Union[build.CustomTarget, build.CustomTargetIndex, build.StaticLibrary]] = []
         self.uninstalled_incdirs: mesonlib.OrderedSet[str] = mesonlib.OrderedSet()
@@ -165,6 +167,9 @@ def _process_reqs(self, reqs: T.Sequence[T.Union[str, build.StaticLibrary, build
     def add_cflags(self, cflags: T.List[str]) -> None:
         self.cflags += mesonlib.stringlistify(cflags)
 
+    def add_cflags_private(self, cflags_private: T.List[str]) -> None:
+        self.cflags_private += mesonlib.stringlistify(cflags_private)
+
     def _add_uninstalled_incdirs(self, incdirs: T.List[build.IncludeDirs], subdir: T.Optional[str] = None) -> None:
         for i in incdirs:
             curdir = i.get_curdir()
@@ -372,6 +377,7 @@ def _fn(xs: T.Union[T.List[str], T.List[LIBS]], libs: bool = False) -> T.Union[T
         # Reset exclude list just in case some values can be both cflags and libs.
         exclude = set()
         self.cflags = _fn(self.cflags)
+        self.cflags_private = _fn(self.cflags_private)
 
 class PkgConfigModule(NewExtensionModule):
 
@@ -596,11 +602,16 @@ def generate_libs_flags(libs: T.List[LIBS]) -> T.Iterable[str]:
             if cflags and not dataonly:
                 ofile.write('Cflags: {}\n'.format(' '.join(cflags)))
 
+            cflags_private: T.List[str] = [self._escape(f) for f in deps.cflags_private]
+            if cflags_private and not dataonly:
+                ofile.write('Cflags.private: {}\n'.format(' '.join(cflags_private)))
+
     @typed_pos_args('pkgconfig.generate', optargs=[(build.SharedLibrary, build.StaticLibrary)])
     @typed_kwargs(
         'pkgconfig.generate',
         D_MODULE_VERSIONS_KW.evolve(since='0.43.0'),
         INSTALL_DIR_KW,
+        KwargInfo('cflags_private', ContainerTypeInfo(list, str), default=[], listify=True, since='1.9.0'),
         KwargInfo('conflicts', ContainerTypeInfo(list, str), default=[], listify=True),
         KwargInfo('dataonly', bool, default=False, since='0.54.0'),
         KwargInfo('description', (str, NoneType)),
@@ -651,7 +662,7 @@ def generate(self, state: ModuleState,
         dataonly = kwargs['dataonly']
         if dataonly:
             default_subdirs = []
-            blocked_vars = ['libraries', 'libraries_private', 'requires_private', 'extra_cflags', 'subdirs']
+            blocked_vars = ['libraries', 'libraries_private', 'requires_private', 'extra_cflags', 'cflags_private', 'subdirs']
             # Mypy can't figure out that this TypedDict index is correct, without repeating T.Literal for the entire list
             if any(kwargs[k] for k in blocked_vars):  # type: ignore
                 raise mesonlib.MesonException(f'Cannot combine dataonly with any of {blocked_vars}')
@@ -680,6 +691,7 @@ def generate(self, state: ModuleState,
         deps.add_pub_reqs(kwargs['requires'])
         deps.add_priv_reqs(kwargs['requires_private'])
         deps.add_cflags(kwargs['extra_cflags'])
+        deps.add_cflags_private(kwargs['cflags_private'])
 
         dversions = kwargs['d_module_versions']
         if dversions:

test cases/common/44 pkgconfig-gen/meson.build

@@ -36,6 +36,7 @@ pkgg.generate(
   requires : 'glib-2.0', # Not really, but only here to test that this works.
   requires_private : ['gio-2.0', 'gobject-2.0'],
   libraries_private : [lib, '-lz'],
+  cflags_private: '-DSIMPLE_STATIC',
   license: 'Apache-2.0',
 )