From 751ecde0640f1d079515964e72e2646e82cb25f4 Mon Sep 17 00:00:00 2001
From: msizanoen <msizanoen@qtmlabs.xyz>
Date: Sun, 3 Sep 2023 15:58:21 +0700
Subject: [PATCH] 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.
---
 compiler/rustc_middle/src/ty/layout.rs      |  4 ++++
 compiler/rustc_target/src/abi/call/riscv.rs | 11 +++++++++++
 compiler/rustc_target/src/abi/mod.rs        |  8 ++++++++
 3 files changed, 23 insertions(+)

diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index 9a0e72d7b640..8b425ce0267d 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/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.
diff --git a/compiler/rustc_target/src/abi/call/riscv.rs b/compiler/rustc_target/src/abi/call/riscv.rs
index d90dce2a0878..93a2045632a8 100644
--- a/compiler/rustc_target/src/abi/call/riscv.rs
+++ b/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);
                 }
             }
diff --git a/compiler/rustc_target/src/abi/mod.rs b/compiler/rustc_target/src/abi/mod.rs
index 636adcf6b178..74fe98920c45 100644
--- a/compiler/rustc_target/src/abi/mod.rs
+++ b/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>,