Fix codegen of uninhabited PassMode::Indirect return types.

Add codegen test for uninhabited PassMode::Indirect return types.

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -4,9 +4,7 @@ use rustc_abi::{BackendRepr, ExternAbi, HasDataLayout, Reg, WrappingRange};
 use rustc_ast as ast;
 use rustc_ast::{InlineAsmOptions, InlineAsmTemplatePiece};
 use rustc_hir::lang_items::LangItem;
-use rustc_middle::mir::{
-    self, AssertKind, BasicBlock, InlineAsmMacro, SwitchTargets, UnwindTerminateReason,
-};
+use rustc_middle::mir::{self, AssertKind, InlineAsmMacro, SwitchTargets, UnwindTerminateReason};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, ValidityRequirement};
 use rustc_middle::ty::print::{with_no_trimmed_paths, with_no_visible_paths};
 use rustc_middle::ty::{self, Instance, Ty};
@@ -919,7 +917,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     &fn_abi.ret,
                     &mut llargs,
                     Some(intrinsic),
-                    target,
                 );
                 let dest = match ret_dest {
                     _ if fn_abi.ret.is_indirect() => llargs[0],
@@ -975,19 +972,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         };
 
         let mut llargs = Vec::with_capacity(arg_count);
-        let destination = target.as_ref().map(|&target| {
-            (
-                self.make_return_dest(
-                    bx,
-                    destination,
-                    &fn_abi.ret,
-                    &mut llargs,
-                    None,
-                    Some(target),
-                ),
-                target,
-            )
-        });
+
+        // We still need to call `make_return_dest` even if there's no `target`, since
+        // `fn_abi.ret` could be `PassMode::Indirect`, even if it is uninhabited,
+        // and `make_return_dest` adds the return-place indirect pointer to `llargs`.
+        let return_dest = self.make_return_dest(bx, destination, &fn_abi.ret, &mut llargs, None);
+        let destination = target.map(|target| (return_dest, target));
 
         // Split the rust-call tupled arguments off.
         let (first_args, untuple) = if abi == ExternAbi::RustCall && !args.is_empty() {
@@ -1790,11 +1780,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         fn_ret: &ArgAbi<'tcx, Ty<'tcx>>,
         llargs: &mut Vec<Bx::Value>,
         intrinsic: Option<ty::IntrinsicDef>,
-        target: Option<BasicBlock>,
     ) -> ReturnDest<'tcx, Bx::Value> {
-        if target.is_none() {
-            return ReturnDest::Nothing;
-        }
         // If the return is ignored, we can just return a do-nothing `ReturnDest`.
         if fn_ret.is_ignore() {
             return ReturnDest::Nothing;

tests/codegen/uninhabited-transparent-return-abi.rs

@@ -0,0 +1,42 @@
+// See https://github.com/rust-lang/rust/issues/135802
+
+#![crate_type = "lib"]
+
+enum Void {}
+
+// Should be ABI-compatible with T, but wasn't prior to the PR adding this test.
+#[repr(transparent)]
+struct NoReturn<T>(T, Void);
+
+// Returned by invisible reference (in most ABIs)
+#[allow(dead_code)]
+struct Large(u64, u64, u64);
+
+extern "Rust" {
+    fn opaque() -> NoReturn<Large>;
+    fn opaque_with_arg(rsi: u32) -> NoReturn<Large>;
+}
+
+// CHECK-LABEL: @test_uninhabited_ret_by_ref
+#[no_mangle]
+pub fn test_uninhabited_ret_by_ref() {
+    // CHECK: %_1 = alloca [24 x i8], align {{8|4}}
+    // CHECK-NEXT: call void @llvm.lifetime.start.p0(i64 24, ptr nonnull %_1)
+    // CHECK-NEXT: call void @opaque(ptr noalias nocapture noundef nonnull sret([24 x i8]) align {{8|4}} dereferenceable(24) %_1) #2
+    // CHECK-NEXT: unreachable
+    unsafe {
+        opaque();
+    }
+}
+
+// CHECK-LABEL: @test_uninhabited_ret_by_ref_with_arg
+#[no_mangle]
+pub fn test_uninhabited_ret_by_ref_with_arg(rsi: u32) {
+    // CHECK: %_2 = alloca [24 x i8], align {{8|4}}
+    // CHECK-NEXT: call void @llvm.lifetime.start.p0(i64 24, ptr nonnull %_2)
+    // CHECK-NEXT: call void @opaque_with_arg(ptr noalias nocapture noundef nonnull sret([24 x i8]) align {{8|4}} dereferenceable(24) %_2, i32 noundef %rsi) #2
+    // CHECK-NEXT: unreachable
+    unsafe {
+        opaque_with_arg(rsi);
+    }
+}