Rollup merge of #115499 - msizanoen1:riscv-fix-transparent-union-abi, r=bjorn3

rustc_target/riscv: 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 https://github.com/rust-lang/rust/issues/115481.

r? `@RalfJung`

compiler/rustc_middle/src/ty/layout.rs

@@ -1118,6 +1118,10 @@ where
     fn is_unit(this: TyAndLayout<'tcx>) -> bool {
         matches!(this.ty.kind(), ty::Tuple(list) if list.len() == 0)
     }
+
+    fn is_transparent(this: TyAndLayout<'tcx>) -> bool {
+        matches!(this.ty.kind(), ty::Adt(def, _) if def.repr().transparent())
+    }
 }
 
 /// Calculates whether a function's ABI can unwind or not.

compiler/rustc_target/src/abi/call/riscv.rs

@@ -89,6 +89,17 @@ 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);
                 }
             }

compiler/rustc_target/src/abi/mod.rs

@@ -66,6 +66,7 @@ pub trait TyAbiInterface<'a, C>: Sized + std::fmt::Debug {
     fn is_never(this: TyAndLayout<'a, Self>) -> bool;
     fn is_tuple(this: TyAndLayout<'a, Self>) -> bool;
     fn is_unit(this: TyAndLayout<'a, Self>) -> bool;
+    fn is_transparent(this: TyAndLayout<'a, Self>) -> bool;
 }
 
 impl<'a, Ty> TyAndLayout<'a, Ty> {
@@ -136,6 +137,13 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
         Ty::is_unit(self)
     }
 
+    pub fn is_transparent<C>(self) -> bool
+    where
+        Ty: TyAbiInterface<'a, C>,
+    {
+        Ty::is_transparent(self)
+    }
+
     pub fn offset_of_subfield<C>(self, cx: &C, indices: impl Iterator<Item = usize>) -> Size
     where
         Ty: TyAbiInterface<'a, C>,

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
-// riscv64: https://github.com/rust-lang/rust/issues/115481
 // loongarch64: https://github.com/rust-lang/rust/issues/115509
 
 macro_rules! assert_abi_compatible {
@@ -110,7 +109,7 @@ 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(any(target_arch = "riscv64", target_arch = "loongarch64")))]
+            #[cfg(not(target_arch = "loongarch64"))]
             test_abi_compatible!(wrap4, $t, WrapperUnion<$t>);
         }
     };