[PATCH 12/23] rust: silence unknown warnings for the sake of old compilers

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 meson.build                   | 8 ++++++++
 .gitlab-ci.d/buildtest.yml    | 2 +-
 meson_options.txt             | 2 ++
 scripts/meson-buildoptions.sh | 4 ++++
 4 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/meson.build b/meson.build
index e819148f36c..69b8367eb3c 100644
--- a/meson.build
+++ b/meson.build
@@ -3327,6 +3327,14 @@ 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
diff --git a/.gitlab-ci.d/buildtest.yml b/.gitlab-ci.d/buildtest.yml
index 19ba5b9c818..aba65ff833a 100644
--- a/.gitlab-ci.d/buildtest.yml
+++ b/.gitlab-ci.d/buildtest.yml
@@ -128,7 +128,7 @@ build-system-fedora-rust-nightly:
     job: amd64-fedora-rust-nightly-container
   variables:
     IMAGE: fedora-rust-nightly
-    CONFIGURE_ARGS: --disable-docs --enable-rust
+    CONFIGURE_ARGS: --disable-docs --enable-rust --enable-strict-rust-lints
     TARGETS: aarch64-softmmu
     MAKE_CHECK_ARGS: check-build
   allow_failure: true
diff --git a/meson_options.txt b/meson_options.txt
index 0ee4d7bb86b..e46199a3232 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -376,3 +376,5 @@ option('x86_version', type : 'combo', choices : ['0', '1', '2', '3', '4'], value
 
 option('rust', type: 'feature', value: 'disabled',
        description: 'Rust support')
+option('strict_rust_lints', type: 'boolean', value: false,
+       description: 'Enable stricter set of Rust warnings')
diff --git a/scripts/meson-buildoptions.sh b/scripts/meson-buildoptions.sh
index 6d08605b771..e898b20307d 100644
--- a/scripts/meson-buildoptions.sh
+++ b/scripts/meson-buildoptions.sh
@@ -47,6 +47,8 @@ meson_options_help() {
   printf "%s\n" '                           getrandom()'
   printf "%s\n" '  --enable-safe-stack      SafeStack Stack Smash Protection (requires'
   printf "%s\n" '                           clang/llvm and coroutine backend ucontext)'
+  printf "%s\n" '  --enable-strict-rust-lints'
+  printf "%s\n" '                           Enable stricter set of Rust warnings'
   printf "%s\n" '  --enable-strip           Strip targets on install'
   printf "%s\n" '  --enable-tcg-interpreter TCG with bytecode interpreter (slow)'
   printf "%s\n" '  --enable-trace-backends=CHOICES'
@@ -490,6 +492,8 @@ _meson_option_parse() {
     --disable-spice-protocol) printf "%s" -Dspice_protocol=disabled ;;
     --enable-stack-protector) printf "%s" -Dstack_protector=enabled ;;
     --disable-stack-protector) printf "%s" -Dstack_protector=disabled ;;
+    --enable-strict-rust-lints) printf "%s" -Dstrict_rust_lints=true ;;
+    --disable-strict-rust-lints) printf "%s" -Dstrict_rust_lints=false ;;
     --enable-strip) printf "%s" -Dstrip=true ;;
     --disable-strip) printf "%s" -Dstrip=false ;;
     --sysconfdir=*) quote_sh "-Dsysconfdir=$2" ;;
-- 
2.47.0