rust: cargo: store desired warning levels in workspace Cargo.toml

An extra benefit of workspaces is that they allow to place lint level
settings in a single Cargo.toml; the settings are then inherited by
packages in the workspace.

Correspondingly, teach rustc_args.py to get the unexpected_cfgs
configuration from the workspace Cargo.toml.

Note that it is still possible to allow or deny warnings per crate or
module, via the #![] attribute syntax.  The rust/qemu-api/src/bindings.rs
file is an example.

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

meson.build

@@ -121,11 +121,12 @@ endif
 
 if have_rust
   rustc_args = [find_program('scripts/rust/rustc_args.py'),
-    '--rustc-version', rustc.version()]
+    '--rustc-version', rustc.version(),
+    '--workspace', meson.project_source_root() / 'rust']
   rustfmt = find_program('rustfmt', required: false)
 
-  # Prohibit code that is forbidden in Rust 2024
-  rustc_lint_args = ['-D', 'unsafe_op_in_unsafe_fn']
+  rustc_lint_args = run_command(rustc_args, '--lints',
+     capture: true, check: true).stdout().strip().splitlines()
 
   # Occasionally, we may need to silence warnings and clippy lints that
   # were only introduced in newer Rust compiler versions.  Do not croak

rust/Cargo.toml

@@ -5,3 +5,11 @@ members = [
     "qemu-api",
     "hw/char/pl011",
 ]
+
+[workspace.lints.rust]
+unexpected_cfgs = { level = "deny", check-cfg = [
+    'cfg(MESON)', 'cfg(HAVE_GLIB_WITH_ALIGNED_ALLOC)',
+    'cfg(has_offset_of)'] }
+
+# Prohibit code that is forbidden in Rust 2024
+unsafe_op_in_unsafe_fn = "deny"

rust/hw/char/pl011/Cargo.toml

@@ -21,3 +21,6 @@ bilge = { version = "0.2.0" }
 bilge-impl = { version = "0.2.0" }
 qemu_api = { path = "../../../qemu-api" }
 qemu_api_macros = { path = "../../../qemu-api-macros" }
+
+[lints]
+workspace = true

rust/qemu-api-macros/Cargo.toml

@@ -20,3 +20,6 @@ proc-macro = true
 proc-macro2 = "1"
 quote = "1"
 syn = { version = "2", features = ["extra-traits"] }
+
+[lints]
+workspace = true

rust/qemu-api/Cargo.toml

@@ -23,6 +23,5 @@ version_check = "~0.9"
 default = []
 allocator = []
 
-[lints.rust]
-unexpected_cfgs = { level = "warn", check-cfg = ['cfg(MESON)', 'cfg(HAVE_GLIB_WITH_ALIGNED_ALLOC)',
-                                                 'cfg(has_offset_of)'] }
+[lints]
+workspace = true

rust/qemu-api/meson.build

@@ -1,5 +1,5 @@
 _qemu_api_cfg = run_command(rustc_args,
-  '--config-headers', config_host_h, '--features', '--lints', files('Cargo.toml'),
+  '--config-headers', config_host_h, '--features', files('Cargo.toml'),
   capture: true, check: true).stdout().strip().splitlines()
 
 # _qemu_api_cfg += ['--cfg', 'feature="allocator"']

scripts/rust/rustc_args.py

@@ -38,11 +38,21 @@ except ImportError:
 
 class CargoTOML:
     tomldata: Mapping[Any, Any]
+    workspace_data: Mapping[Any, Any]
     check_cfg: Set[str]
 
-    def __init__(self, path: str):
-        with open(path, 'rb') as f:
-            self.tomldata = tomllib.load(f)
+    def __init__(self, path: Optional[str], workspace: Optional[str]):
+        if path is not None:
+            with open(path, 'rb') as f:
+                self.tomldata = tomllib.load(f)
+        else:
+            self.tomldata = {"lints": {"workspace": True}}
+
+        if workspace is not None:
+            with open(workspace, 'rb') as f:
+                self.workspace_data = tomllib.load(f)
+            if "workspace" not in self.workspace_data:
+                self.workspace_data["workspace"] = {}
 
         self.check_cfg = set(self.find_check_cfg())
 
@@ -54,10 +64,12 @@ class CargoTOML:
 
     @property
     def lints(self) -> Mapping[Any, Any]:
-        return self.get_table("lints")
+        return self.get_table("lints", True)
 
-    def get_table(self, key: str) -> Mapping[Any, Any]:
+    def get_table(self, key: str, can_be_workspace: bool = False) -> Mapping[Any, Any]:
         table = self.tomldata.get(key, {})
+        if can_be_workspace and table.get("workspace", False) is True:
+            table = self.workspace_data["workspace"].get(key, {})
 
         return table
 
@@ -136,6 +148,16 @@ def main() -> None:
         action="store",
         dest="cargo_toml",
         help="path to Cargo.toml file",
+        nargs='?',
+    )
+    parser.add_argument(
+        "--workspace",
+        metavar="DIR",
+        action="store",
+        dest="workspace",
+        help="path to root of the workspace",
+        required=False,
+        default=None,
     )
     parser.add_argument(
         "--features",
@@ -168,7 +190,11 @@ def main() -> None:
     logging.debug("args: %s", args)
 
     rustc_version = tuple((int(x) for x in args.rustc_version.split('.')[0:2]))
-    cargo_toml = CargoTOML(args.cargo_toml)
+    if args.workspace:
+        workspace_cargo_toml = Path(args.workspace, "Cargo.toml").resolve()
+        cargo_toml = CargoTOML(args.cargo_toml, str(workspace_cargo_toml))
+    else:
+        cargo_toml = CargoTOML(args.cargo_toml, None)
 
     if args.lints:
         for tok in generate_lint_flags(cargo_toml):