rust: Store absolute paths in rust-project.json

As meson requires source_dir!=build_dir and stores the rust-project.json
inside the build directory, while software like rust-analyzer expects it
at the root of the source directory, manual steps are needed for making
them work together.

One option, as described in the documentation, is per project
configuration. Another option, that works correctly with
compile-commands.json and clangd, is to store a symlink to the file in
the build directory at the root of the source directory.

As currently rust-project.json stores paths relative to the location of
the file itself and rust-analyzer does not resolve symlinks, this does
not work.

To solve this, store absolute paths in rust-project.json as is already
done in compile_commands.json for the directory.

mesonbuild/backend/ninjabackend.py

@@ -2021,10 +2021,14 @@ class NinjaBackend(backends.Backend):
             for rpath_arg in rpath_args:
                 args += ['-C', 'link-arg=' + rpath_arg + ':' + os.path.join(rustc.get_sysroot(), 'lib')]
 
-        self._add_rust_project_entry(target.name, main_rust_file, args, bool(target.subproject),
+        self._add_rust_project_entry(target.name,
+                                     os.path.abspath(os.path.join(self.environment.build_dir, main_rust_file)),
+                                     args,
+                                     bool(target.subproject),
                                      #XXX: There is a fix for this pending
                                      getattr(target, 'rust_crate_type', '') == 'procmacro',
-                                     output, project_deps)
+                                     output,
+                                     project_deps)
 
         compiler_name = self.compiler_to_rule_name(rustc)
         element = NinjaBuildElement(self.all_outputs, target_name, compiler_name, main_rust_file)