make -Zwasm-c-abi=legacy suppress the lint

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

@@ -332,7 +332,7 @@ fn wasm_functype<'tcx>(tcx: TyCtxt<'tcx>, fn_abi: &FnAbi<'tcx, Ty<'tcx>>, def_id
     // please also add `wasm32-unknown-unknown` back in `tests/assembly/wasm32-naked-fn.rs`
     // basically the commit introducing this comment should be reverted
     if let PassMode::Pair { .. } = fn_abi.ret.mode {
-        let _ = WasmCAbi::Legacy;
+        let _ = WasmCAbi::Legacy { with_lint: true };
         span_bug!(
             tcx.def_span(def_id),
             "cannot return a pair (the wasm32-unknown-unknown ABI is broken, see https://github.com/rust-lang/rust/issues/115666"
@@ -384,7 +384,7 @@ fn wasm_type<'tcx>(
                 BackendRepr::SimdVector { .. } => "v128",
                 BackendRepr::Memory { .. } => {
                     // FIXME: remove this branch once the wasm32-unknown-unknown ABI is fixed
-                    let _ = WasmCAbi::Legacy;
+                    let _ = WasmCAbi::Legacy { with_lint: true };
                     span_bug!(
                         tcx.def_span(def_id),
                         "cannot use memory args (the wasm32-unknown-unknown ABI is broken, see https://github.com/rust-lang/rust/issues/115666"

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

@@ -92,6 +92,7 @@ fn do_check_simd_vector_abi<'tcx>(
     }
 }
 
+/// Determines whether the given argument is passed the same way on the old and new wasm ABIs.
 fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool {
     if matches!(arg.layout.backend_repr, BackendRepr::Scalar(_)) {
         return true;
@@ -120,16 +121,16 @@ fn do_check_wasm_abi<'tcx>(
     is_call: bool,
     span: impl Fn() -> Span,
 ) {
-    // Only proceed for `extern "C" fn` on wasm32-unknown-unknown (same check as what `adjust_for_foreign_abi` uses to call `compute_wasm_abi_info`).
+    // Only proceed for `extern "C" fn` on wasm32-unknown-unknown (same check as what `adjust_for_foreign_abi` uses to call `compute_wasm_abi_info`),
+    // and only proceed if `wasm_c_abi_opt` indicates we should emit the lint.
     if !(tcx.sess.target.arch == "wasm32"
         && tcx.sess.target.os == "unknown"
-        && tcx.wasm_c_abi_opt() == WasmCAbi::Legacy
+        && tcx.wasm_c_abi_opt() == WasmCAbi::Legacy { with_lint: true }
         && abi.conv == Conv::C)
     {
         return;
     }
-    // Warn against all types whose ABI will change. That's all arguments except for things passed as scalars.
+    // Warn against all types whose ABI will change. Return values are not affected by this change.
-    // Return values are not affected by this change.
     for arg_abi in abi.args.iter() {
         if wasm_abi_safe(tcx, arg_abi) {
             continue;

compiler/rustc_session/src/options.rs

@@ -1886,7 +1886,8 @@ pub mod parse {
     pub(crate) fn parse_wasm_c_abi(slot: &mut WasmCAbi, v: Option<&str>) -> bool {
         match v {
             Some("spec") => *slot = WasmCAbi::Spec,
-            Some("legacy") => *slot = WasmCAbi::Legacy,
+            // Explicitly setting the `-Z` flag suppresses the lint.
+            Some("legacy") => *slot = WasmCAbi::Legacy { with_lint: false },
             _ => return false,
         }
         true
@@ -2599,7 +2600,7 @@ written to standard error output)"),
         Requires `-Clto[=[fat,yes]]`"),
     wasi_exec_model: Option<WasiExecModel> = (None, parse_wasi_exec_model, [TRACKED],
         "whether to build a wasi command or reactor"),
-    wasm_c_abi: WasmCAbi = (WasmCAbi::Legacy, parse_wasm_c_abi, [TRACKED],
+    wasm_c_abi: WasmCAbi = (WasmCAbi::Legacy { with_lint: true }, parse_wasm_c_abi, [TRACKED],
         "use spec-compliant C ABI for `wasm32-unknown-unknown` (default: legacy)"),
     write_long_types_to_disk: bool = (true, parse_bool, [UNTRACKED],
         "whether long type names should be written to files instead of being printed in errors"),

compiler/rustc_target/src/callconv/mod.rs

@@ -705,7 +705,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "xtensa" => xtensa::compute_abi_info(cx, self),
             "riscv32" | "riscv64" => riscv::compute_abi_info(cx, self),
             "wasm32" => {
-                if spec.os == "unknown" && cx.wasm_c_abi_opt() == WasmCAbi::Legacy {
+                if spec.os == "unknown" && matches!(cx.wasm_c_abi_opt(), WasmCAbi::Legacy { .. }) {
                     wasm::compute_wasm_abi_info(self)
                 } else {
                     wasm::compute_c_abi_info(cx, self)

compiler/rustc_target/src/spec/mod.rs

@@ -2234,7 +2234,10 @@ pub enum WasmCAbi {
     /// Spec-compliant C ABI.
     Spec,
     /// Legacy ABI. Which is non-spec-compliant.
-    Legacy,
+    Legacy {
+        /// Indicates whether the `wasm_c_abi` lint should be emitted.
+        with_lint: bool,
+    },
 }
 
 pub trait HasWasmCAbiOpt {