Use full relative paths to generate object names.

This prevents files with the same name from different directories from
producing object files with the same name and breaking ninja.

Fixes #1494.

mesonbuild/backend/backends.py

@@ -309,13 +309,35 @@ class Backend:
         return result
 
     def object_filename_from_source(self, target, source, is_unity):
-        if isinstance(source, mesonlib.File):
-            source = source.fname
+        assert isinstance(source, mesonlib.File)
+        build_dir = self.environment.get_build_dir()
+        rel_src = source.rel_to_builddir(self.build_to_src)
         # foo.vala files compile down to foo.c and then foo.c.o, not foo.vala.o
-        if source.endswith(('.vala', '.gs')):
+        if rel_src.endswith(('.vala', '.gs')):
+            # See description in generate_vala_compile for this logic.
+            if source.is_built:
+                if os.path.isabs(rel_src):
+                    rel_src = rel_src[len(build_dir) + 1:]
+                rel_src = os.path.relpath(rel_src, self.get_target_private_dir(target))
+            else:
+                rel_src = os.path.basename(rel_src)
             if is_unity:
-                return source[:-5] + '.c.' + self.environment.get_object_suffix()
-            source = source[:-5] + '.c'
+                return 'meson-generated_' + rel_src[:-5] + '.c.' + self.environment.get_object_suffix()
+            # A meson- prefixed directory is reserved; hopefully no-one creates a file name with such a weird prefix.
+            source = 'meson-generated_' + rel_src[:-5] + '.c'
+        elif source.is_built:
+            if os.path.isabs(rel_src):
+                rel_src = rel_src[len(build_dir) + 1:]
+            targetdir = self.get_target_private_dir(target)
+            # A meson- prefixed directory is reserved; hopefully no-one creates a file name with such a weird prefix.
+            source = 'meson-generated_' + os.path.relpath(rel_src, targetdir)
+        else:
+            if os.path.isabs(rel_src):
+                # Not from the source directory; hopefully this doesn't conflict with user's source files.
+                source = os.path.basename(rel_src)
+            else:
+                source = os.path.relpath(os.path.join(build_dir, rel_src),
+                                         os.path.join(self.environment.get_source_dir(), target.get_subdir()))
         return source.replace('/', '_').replace('\\', '_') + '.' + self.environment.get_object_suffix()
 
     def determine_ext_objs(self, target, extobj, proj_dir_to_build_root):