interpreter: Add a get_gir() method for BuildTarget

Which allows us to handle Vala targets that generate gir as a side
effect in a more natural way, instead of generating strings using
`meson.source_dir()`

Author: dcbaker

docs/markdown/Vala.md

@@ -342,7 +342,7 @@ To then generate a typelib file use a custom target with the
 ```meson
 g_ir_compiler = find_program('g-ir-compiler')
 custom_target('foo typelib', command: [g_ir_compiler, '--output', '@OUTPUT@', '@INPUT@'],
-              input: meson.current_build_dir() / 'Foo-1.0.gir',
+              input: foo_lib.get_gir(),  # For Meson < 1.4 support: meson.current_build_dir() / 'Foo-1.0.gir',
               output: 'Foo-1.0.typelib',
               depends: foo_lib,
               install: true,

docs/markdown/snippets/vala-get-gir.md

@@ -0,0 +1,5 @@
+## Add way to get Vala GIR file
+
+Our current recomendation is to create a string via `meson.current_build_dir() /
+Foo-1.0.gir`, which is not the Meson way. A new `BuildTarget.get_gir()` method
+has been added which returns a `File` object pointing to the GIR file.

mesonbuild/interpreter/interpreterobjects.py

@@ -863,6 +863,7 @@ def __init__(self, target: _BuildTarget, interp: 'Interpreter'):
                              'path': self.path_method,
                              'found': self.found_method,
                              'private_dir_include': self.private_dir_include_method,
+                             'get_gir': self.get_gir_method,
                              })
 
     def __repr__(self) -> str:
@@ -944,6 +945,16 @@ def get_id_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> str:
     def name_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> str:
         return self._target_object.name
 
+    @FeatureNew('get_gir', '1.4.0')
+    @noPosargs
+    @noKwargs
+    def get_gir_method(self, args: T.List[TYPE_var], kwargs: TYPE_kwargs) -> mesonlib.File:
+        obj = self._target_object
+        if not hasattr(obj, 'vala_gir') or obj.vala_gir is None:
+            raise InterpreterException(f'target {obj.name} does not generate GIR')
+        return mesonlib.File.from_built_file(obj.subdir, obj.vala_gir)
+
+
 class ExecutableHolder(BuildTargetHolder[build.Executable]):
     pass
 

test cases/vala/9 gir/meson.build

@@ -12,6 +12,6 @@ foo = shared_library('foo', 'foo.vala',
 
 custom_target('foo-typelib',
               command: [g_ir_compiler, '--output', '@OUTPUT@', '@INPUT@'],
-              input: meson.current_build_dir() + '/Foo-1.0.gir',
+              input: foo.get_gir(),
               output: 'Foo-1.0.typelib',
               depends: foo)