Make Cargo use unknown_lints = "allow" as well.  This is more future
proof as we might add new lints to rust/Cargo.toml that are not supported
by older versions of rustc or clippy.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 meson.build                | 12 ++++--------
 rust/Cargo.toml            |  6 ++++++
 scripts/rust/rustc_args.py | 19 ++++++++++++++++---
 3 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/meson.build b/meson.build
index 5726135b324..1239f5c48c1 100644
--- a/meson.build
+++ b/meson.build
@@ -122,20 +122,16 @@ endif
 if have_rust
   rustc_args = [find_program('scripts/rust/rustc_args.py'),
     '--workspace', meson.project_source_root() / 'rust']
+  if get_option('strict_rust_lints')
+    rustc_args += ['--strict-lints']
+  endif
+
   rustfmt = find_program('rustfmt', required: false)
 
   rustc_lint_args = run_command(rustc_args,
      '--lints', files('rust/Cargo.toml'),
      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
-  # in that case; a CI job with rust_strict_lints == true ensures that
-  # we do not have misspelled allow() attributes.
-  if not get_option('strict_rust_lints')
-    rustc_lint_args += ['-A', 'unknown_lints']
-  endif
-
   # Apart from procedural macros, our Rust executables will often link
   # with C code, so include all the libraries that C code needs.  This
   # is safe; https://github.com/rust-lang/rust/pull/54675 says that
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index 0230b92a9fa..1ff8f5c2781 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -11,5 +11,11 @@ unexpected_cfgs = { level = "warn", check-cfg = [
     'cfg(MESON)', 'cfg(HAVE_GLIB_WITH_ALIGNED_ALLOC)',
     'cfg(has_offset_of)'] }
 
+# Occasionally, we may need to silence warnings and clippy lints that
+# were only introduced in newer Rust compiler versions.  Do not croak
+# in that case; a CI job with rust_strict_lints == true disables this
+# and ensures that we do not have misspelled allow() attributes.
+unknown_lints = "allow"
+
 # Prohibit code that is forbidden in Rust 2024
 unsafe_op_in_unsafe_fn = "deny"
diff --git a/scripts/rust/rustc_args.py b/scripts/rust/rustc_args.py
index 26733439ed4..1f8b05b8001 100644
--- a/scripts/rust/rustc_args.py
+++ b/scripts/rust/rustc_args.py
@@ -35,6 +35,8 @@
 except ImportError:
     import tomli as tomllib
 
+STRICT_LINTS = {"unknown_lints", "warnings"}
+
 
 class CargoTOML:
     tomldata: Mapping[Any, Any]
@@ -77,7 +79,7 @@ class LintFlag:
     priority: int
 
 
-def generate_lint_flags(cargo_toml: CargoTOML) -> Iterable[str]:
+def generate_lint_flags(cargo_toml: CargoTOML, strict_lints: bool) -> Iterable[str]:
     """Converts Cargo.toml lints to rustc -A/-D/-F/-W flags."""
 
     toml_lints = cargo_toml.lints
@@ -102,9 +104,13 @@ def generate_lint_flags(cargo_toml: CargoTOML) -> Iterable[str]:
             # This may change if QEMU ever invokes clippy-driver or rustdoc by
             # hand.  For now, check the syntax but do not add non-rustc lints to
             # the command line.
-            if k == "rust":
+            if k == "rust" and not (strict_lints and lint in STRICT_LINTS):
                 lint_list.append(LintFlag(flags=[flag, prefix + lint], priority=priority))
 
+    if strict_lints:
+        for lint in STRICT_LINTS:
+            lint_list.append(LintFlag(flags=["-D", lint], priority=1000000))
+
     lint_list.sort(key=lambda x: x.priority)
     for lint in lint_list:
         yield from lint.flags
@@ -181,6 +187,13 @@ def main() -> None:
         required=False,
         default="1.0.0",
     )
+    parser.add_argument(
+        "--strict-lints",
+        action="store_true",
+        dest="strict_lints",
+        help="apply stricter checks (for nightly Rust)",
+        default=False,
+    )
     args = parser.parse_args()
     if args.verbose:
         logging.basicConfig(level=logging.DEBUG)
@@ -194,7 +207,7 @@ def main() -> None:
         cargo_toml = CargoTOML(args.cargo_toml, None)
 
     if args.lints:
-        for tok in generate_lint_flags(cargo_toml):
+        for tok in generate_lint_flags(cargo_toml, args.strict_lints):
             print(tok)
 
     if rustc_version >= (1, 80):
-- 
2.47.0