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 #115481.

Author: msizanoen1

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

@@ -87,7 +87,31 @@ where
             FieldsShape::Primitive => {
                 unreachable!("aggregates can't have `FieldsShape::Primitive`")
             }
-            FieldsShape::Union(_) => {
+            FieldsShape::Union(count) => {
+                if arg_layout.is_transparent() {
+                    let mut non_1zst_elem = None;
+                    for i in 0..count.get() {
+                        let elem_layout = arg_layout.field(cx, i);
+                        if elem_layout.is_1zst() {
+                            continue;
+                        }
+                        // There can only be one.
+                        if non_1zst_elem.is_some() {
+                            unreachable!("two non-1-ZST fields in repr(transparent) union");
+                        }
+                        non_1zst_elem = Some(elem_layout);
+                    }
+                    if let Some(non_1zst_elem) = non_1zst_elem {
+                        return should_use_fp_conv_helper(
+                            cx,
+                            &non_1zst_elem,
+                            xlen,
+                            flen,
+                            field1_kind,
+                            field2_kind,
+                        );
+                    }
+                }
                 if !arg_layout.is_zst() {
                     return Err(CannotUseFpConv);
                 }

compiler/rustc_target/src/abi/mod.rs

@@ -55,6 +55,7 @@ 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> {
@@ -125,6 +126,13 @@ 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>,