Auto merge of #115987 - loongarch-rs:fix-transparent-union-abi, r=bjorn3

rustc_target/loongarch: Fix passing of transparent unions with only one non-ZST member

This ensures that `MaybeUninit<T>` has the same ABI as `T` when passed through an `extern "C"` function.

Fixes #115509

r? `@bjorn3`

Author: bors

Diff for: compiler/rustc_target/src/abi/call/loongarch.rs

@@ -83,6 +83,17 @@ where
             }
             FieldsShape::Union(_) => {
                 if !arg_layout.is_zst() {
+                    if arg_layout.is_transparent() {
+                        let non_1zst_elem = arg_layout.non_1zst_field(cx).expect("not exactly one non-1-ZST field in non-ZST repr(transparent) union").1;
+                        return should_use_fp_conv_helper(
+                            cx,
+                            &non_1zst_elem,
+                            xlen,
+                            flen,
+                            field1_kind,
+                            field2_kind,
+                        );
+                    }
                     return Err(CannotUseFpConv);
                 }
             }

Diff for: tests/ui/abi/compatibility.rs

@@ -10,7 +10,6 @@ use std::ptr::NonNull;
 // Hence there are `cfg` throughout this test to disable parts of it on those targets.
 // sparc64: https://github.com/rust-lang/rust/issues/115336
 // mips64: https://github.com/rust-lang/rust/issues/115404
-// loongarch64: https://github.com/rust-lang/rust/issues/115509
 
 macro_rules! assert_abi_compatible {
     ($name:ident, $t1:ty, $t2:ty) => {
@@ -109,7 +108,6 @@ macro_rules! test_transparent {
             test_abi_compatible!(wrap1, $t, Wrapper1<$t>);
             test_abi_compatible!(wrap2, $t, Wrapper2<$t>);
             test_abi_compatible!(wrap3, $t, Wrapper3<$t>);
-            #[cfg(not(target_arch = "loongarch64"))]
             test_abi_compatible!(wrap4, $t, WrapperUnion<$t>);
         }
     };