rust: silence unknown warnings for the sake of old compilers

Occasionally, we may need to silence warnings and clippy lints that
were only introduced in newer Rust compiler versions.  However, this
would fail when compiling with an older rustc:

error: unknown lint: `non_local_definitions`
   --> rust/qemu-api/rust-qemu-api-tests.p/structured/offset_of.rs:79:17

So by default we need to block the unknown_lints warning.  To avoid
misspelled lints or other similar issues, re-enable it in the CI job
that uses nightly rust.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson.build

@@ -3346,6 +3346,14 @@ if have_rust and have_system
   # Prohibit code that is forbidden in Rust 2024
   rustc_args += ['-D', 'unsafe_op_in_unsafe_fn']
 
+  # 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_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