Don't warn about v128 in wasm ABI transition

This has other warnings if necessary and doesn't need extra warnings
from this FCW.

cc #138762

Author: alexcrichton

compiler/rustc_monomorphize/src/mono_checks/abi_check.rs

@@ -99,6 +99,12 @@ fn wasm_abi_safe<'tcx>(tcx: TyCtxt<'tcx>, arg: &ArgAbi<'tcx, Ty<'tcx>>) -> bool
         return true;
     }
 
+    // If this is a simd argument let other rustc warnings kick in and don't
+    // provide extra warnings here
+    if uses_vector_registers(&arg.mode, &arg.layout.backend_repr) {
+        return true;
+    }
+
     // This matches `unwrap_trivial_aggregate` in the wasm ABI logic.
     if arg.layout.is_aggregate() {
         let cx = LayoutCx::new(tcx, TypingEnv::fully_monomorphized());

tests/ui/lint/wasm_c_abi_transition.rs

@@ -3,7 +3,7 @@
 //@ add-core-stubs
 //@ build-fail
 
-#![feature(no_core)]
+#![feature(no_core, repr_simd)]
 #![no_core]
 #![crate_type = "lib"]
 #![deny(wasm_c_abi)]
@@ -39,3 +39,11 @@ pub fn call_other_fun(x: MyType) {
     unsafe { other_fun(x) } //~ERROR: wasm ABI transition
     //~^WARN: previously accepted
 }
+
+// Don't warn about simd types
+#[repr(simd)]
+#[allow(non_camel_case_types)]
+pub struct v128([i32; 4]);
+#[target_feature(enable = "simd128")]
+#[allow(improper_ctypes_definitions)]
+pub extern "C" fn my_safe_simd(x: v128) -> v128 { x }