rust: add warning_level=0 to downloaded Cargo subprojects

This adds --cap-lints allow, matching how Cargo builds them.  In the case of
Cargo, this is only applied to non-path dependencies.

Without this change, clippy will complain about dependencies as well.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/markdown/snippets/cargo_cap_lints.md

@@ -0,0 +1,8 @@
+## `--cap-lints allow` used for Cargo subprojects
+
+Similar to Cargo itself, all Cargo subprojects automatically add the
+`--cap-lints allow` compiler argument, thus hiding any warnings from
+the compiler.
+
+Related to this, `warning_level=0` now translates into `--cap-lints allow`
+for Rust targets instead of `-A warnings`.

mesonbuild/cargo/interpreter.py

@@ -459,8 +459,9 @@ def _extra_deps_varname() -> str:
 
 
 class PackageState:
-    def __init__(self, manifest: Manifest) -> None:
+    def __init__(self, manifest: Manifest, downloaded: bool) -> None:
         self.manifest = manifest
+        self.downloaded = downloaded
         self.features: T.Set[str] = set()
         self.required_deps: T.Set[str] = set()
         self.optional_deps_features: T.Dict[str, T.Set[str]] = collections.defaultdict(set)
@@ -520,7 +521,8 @@ def _fetch_package(self, package_name: str, api: str) -> T.Tuple[PackageState, b
         subprojects_dir = os.path.join(subdir, 'subprojects')
         self.environment.wrap_resolver.load_and_merge(subprojects_dir, T.cast('SubProject', meson_depname))
         manifest = self._load_manifest(subdir)
-        pkg = PackageState(manifest)
+        downloaded = self.environment.wrap_resolver.wraps[meson_depname].type is not None
+        pkg = PackageState(manifest, downloaded)
         self.packages[key] = pkg
         # Fetch required dependencies recursively.
         for depname, dep in manifest.dependencies.items():
@@ -602,6 +604,11 @@ def _create_project(self, pkg: PackageState, build: builder.Builder) -> T.List[m
         :param build: The AST builder
         :return: a list nodes
         """
+        default_options: T.List[mparser.BaseNode] = []
+        default_options.append(build.string(f'rust_std={pkg.manifest.package.edition}'))
+        if pkg.downloaded:
+            default_options.append(build.string('warning_level=0'))
+
         args: T.List[mparser.BaseNode] = []
         args.extend([
             build.string(pkg.manifest.package.name),
@@ -613,7 +620,7 @@ def _create_project(self, pkg: PackageState, build: builder.Builder) -> T.List[m
             # This will warn when when we generate deprecated code, which is helpful
             # for the upkeep of the module
             'meson_version': build.string(f'>= {coredata.stable_version}'),
-            'default_options': build.array([build.string(f'rust_std={pkg.manifest.package.edition}')]),
+            'default_options': build.array(default_options),
         }
         if pkg.manifest.package.license:
             kwargs['license'] = build.string(pkg.manifest.package.license)