rust: build: restrict --cfg generation to only required symbols

Parse the Cargo.toml file, looking for the unexpected_cfgs
configuration.  When generating --cfg options from the
config-host.h file, only use those that are included in the
configuration.

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

rust/qemu-api/meson.build

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

scripts/rust/rustc_args.py

@@ -26,30 +26,51 @@
 
 import argparse
 import logging
+from pathlib import Path
+from typing import Any, Iterable, Mapping, Optional, Set
 
-from typing import List
+try:
+    import tomllib
+except ImportError:
+    import tomli as tomllib
 
 
-def generate_cfg_flags(header: str) -> List[str]:
-    """Converts defines from config[..].h headers to rustc --cfg flags."""
+class CargoTOML:
+    tomldata: Mapping[Any, Any]
+    check_cfg: Set[str]
+
+    def __init__(self, path: str):
+        with open(path, 'rb') as f:
+            self.tomldata = tomllib.load(f)
+
+        self.check_cfg = set(self.find_check_cfg())
+
+    def find_check_cfg(self) -> Iterable[str]:
+        toml_lints = self.lints
+        rust_lints = toml_lints.get("rust", {})
+        cfg_lint = rust_lints.get("unexpected_cfgs", {})
+        return cfg_lint.get("check-cfg", [])
+
+    @property
+    def lints(self) -> Mapping[Any, Any]:
+        return self.get_table("lints")
+
+    def get_table(self, key: str) -> Mapping[Any, Any]:
+        table = self.tomldata.get(key, {})
+
+        return table
 
-    def cfg_name(name: str) -> str:
-        """Filter function for C #defines"""
-        if (
-            name.startswith("CONFIG_")
-            or name.startswith("TARGET_")
-            or name.startswith("HAVE_")
-        ):
-            return name
-        return ""
+
+def generate_cfg_flags(header: str, cargo_toml: CargoTOML) -> Iterable[str]:
+    """Converts defines from config[..].h headers to rustc --cfg flags."""
 
     with open(header, encoding="utf-8") as cfg:
         config = [l.split()[1:] for l in cfg if l.startswith("#define")]
 
     cfg_list = []
     for cfg in config:
-        name = cfg_name(cfg[0])
-        if not name:
+        name = cfg[0]
+        if f'cfg({name})' not in cargo_toml.check_cfg:
             continue
         if len(cfg) >= 2 and cfg[1] != "1":
             continue
@@ -59,7 +80,6 @@ def cfg_name(name: str) -> str:
 
 
 def main() -> None:
-    # pylint: disable=missing-function-docstring
     parser = argparse.ArgumentParser()
     parser.add_argument("-v", "--verbose", action="store_true")
     parser.add_argument(
@@ -71,12 +91,21 @@ def main() -> None:
         required=False,
         default=[],
     )
+    parser.add_argument(
+        metavar="TOML_FILE",
+        action="store",
+        dest="cargo_toml",
+        help="path to Cargo.toml file",
+    )
     args = parser.parse_args()
     if args.verbose:
         logging.basicConfig(level=logging.DEBUG)
     logging.debug("args: %s", args)
+
+    cargo_toml = CargoTOML(args.cargo_toml)
+
     for header in args.config_headers:
-        for tok in generate_cfg_flags(header):
+        for tok in generate_cfg_flags(header, cargo_toml):
             print(tok)