cargo: load a toplevel Cargo.lock

Author: bonzini

docs/markdown/snippets/main-project-cargo-lock.md

@@ -0,0 +1,12 @@
+## Common `Cargo.lock` for all Cargo subprojects
+
+Whenever Meson finds a `Cargo.lock` file in the toplevel directory
+of the project, it will use it to resolve the versions of Cargo
+subprojects in preference to per-subproject `Cargo.lock` files.
+Per-subproject lock files are only used if the invoking project
+did not have a `Cargo.lock` file itself.
+
+If you wish to experiment with Cargo subprojects, it is recommended
+to use `cargo` to set up `Cargo.lock` and `Cargo.toml` files,
+encompassing all Rust targets, in the toplevel source directory.
+Cargo subprojects remain unstable and subject to change.

mesonbuild/interpreter/interpreter.py

@@ -316,6 +316,18 @@ def __init__(
     def __getnewargs_ex__(self) -> T.Tuple[T.Tuple[object], T.Dict[str, object]]:
         raise MesonBugException('This class is unpicklable')
 
+    def load_root_cargo_lock_file(self) -> None:
+        cargo_lock_filename = os.path.join(self.subdir, 'Cargo.lock')
+        cargo_lock = os.path.join(self.source_root, cargo_lock_filename)
+        if not os.path.isfile(cargo_lock):
+            return
+        from .. import cargo
+        try:
+            self.cargo = cargo.Interpreter(self.environment, self.subdir, self.subproject_dir)
+        except cargo.TomlImplementationMissing as e:
+            # error delayed to actual usage of a Cargo subproject
+            mlog.warning(f'cannot load Cargo.lock: {e}', fatal=False)
+
     def _redetect_machines(self) -> None:
         # Re-initialize machine descriptions. We can do a better job now because we
         # have the compilers needed to gain more knowledge, so wipe out old
@@ -1301,6 +1313,9 @@ def func_project(self, node: mparser.FunctionNode, args: T.Tuple[str, T.List[str
             assert self.environment.wrap_resolver is not None, 'for mypy'
             self.environment.wrap_resolver.load_and_merge(subprojects_dir, self.subproject)
 
+        if self.cargo is None:
+            self.load_root_cargo_lock_file()
+
         self.build.projects[self.subproject] = proj_name
         mlog.log('Project name:', mlog.bold(proj_name))
         mlog.log('Project version:', mlog.bold(self.project_version))