Only allow PassMode::Direct for aggregates on wasm when using the C ABI

For the Rust ABI we don't have any ABI compat reasons to allow
PassMode::Direct for aggregates.

compiler/rustc_ty_utils/src/abi.rs

@@ -473,20 +473,30 @@ fn fn_abi_sanity_check<'tcx>(
                         // This really shouldn't happen even for sized aggregates, since
                         // `immediate_llvm_type` will use `layout.fields` to turn this Rust type into an
                         // LLVM type. This means all sorts of Rust type details leak into the ABI.
-                        // However wasm sadly *does* currently use this mode so we have to allow it --
-                        // but we absolutely shouldn't let any more targets do that.
-                        // (Also see <https://github.com/rust-lang/rust/issues/115666>.)
+                        // However wasm sadly *does* currently use this mode for it's "C" ABI so we
+                        // have to allow it -- but we absolutely shouldn't let any more targets do
+                        // that. (Also see <https://github.com/rust-lang/rust/issues/115666>.)
                         //
                         // The unstable abi `PtxKernel` also uses Direct for now.
                         // It needs to switch to something else before stabilization can happen.
                         // (See issue: https://github.com/rust-lang/rust/issues/117271)
-                        assert!(
-                            matches!(&*tcx.sess.target.arch, "wasm32" | "wasm64")
-                                || matches!(spec_abi, ExternAbi::PtxKernel | ExternAbi::Unadjusted),
-                            "`PassMode::Direct` for aggregates only allowed for \"unadjusted\" and \"ptx-kernel\" functions and on wasm\n\
-                          Problematic type: {:#?}",
-                            arg.layout,
-                        );
+                        //
+                        // And finally the unadjusted ABI is ill specified and uses Direct for all
+                        // args, but unfortunately we need it for calling certain LLVM intrinsics.
+
+                        match spec_abi {
+                            ExternAbi::Unadjusted => {}
+                            ExternAbi::PtxKernel => {}
+                            ExternAbi::C { unwind: _ }
+                                if matches!(&*tcx.sess.target.arch, "wasm32" | "wasm64") => {}
+                            _ => {
+                                panic!(
+                                    "`PassMode::Direct` for aggregates only allowed for \"unadjusted\" and \"ptx-kernel\" functions and on wasm\n\
+                                      Problematic type: {:#?}",
+                                    arg.layout,
+                                );
+                            }
+                        }
                     }
                 }
             }