From 58d7c2d5a760c1adfa4c3984eeb12787f5ad5b1d Mon Sep 17 00:00:00 2001
From: Scott McMurray <scottmcm@users.noreply.github.com>
Date: Wed, 9 Jul 2025 22:30:15 -0700
Subject: Make UB transmutes really UB in LLVM

Ralf suggested in <https://github.com/rust-lang/rust/pull/143410#discussion_r2184928123> that UB transmutes shouldn't be trapping, which happened for the one path that PR was changing, but there's another path as well, so this PR changes that other path to match.
---
 tests/codegen/intrinsics/transmute.rs | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

(limited to 'tests/codegen')

diff --git a/tests/codegen/intrinsics/transmute.rs b/tests/codegen/intrinsics/transmute.rs
index e375724bc1b..36d6a2f722f 100644
--- a/tests/codegen/intrinsics/transmute.rs
+++ b/tests/codegen/intrinsics/transmute.rs
@@ -58,9 +58,9 @@ pub unsafe fn check_bigger_array(x: [u32; 3]) -> [u32; 7] {
 #[no_mangle]
 #[custom_mir(dialect = "runtime", phase = "optimized")]
 pub unsafe fn check_to_empty_array(x: [u32; 5]) -> [u32; 0] {
-    // CHECK-NOT: trap
-    // CHECK: call void @llvm.trap
-    // CHECK-NOT: trap
+    // CHECK-NOT: call
+    // CHECK: call void @llvm.assume(i1 false)
+    // CHECK-NOT: call
     mir! {
         {
             RET = CastTransmute(x);
@@ -88,9 +88,9 @@ pub unsafe fn check_from_empty_array(x: [u32; 0]) -> [u32; 5] {
 #[no_mangle]
 #[custom_mir(dialect = "runtime", phase = "optimized")]
 pub unsafe fn check_to_uninhabited(x: u16) {
-    // CHECK-NOT: trap
-    // CHECK: call void @llvm.trap
-    // CHECK-NOT: trap
+    // CHECK-NOT: call
+    // CHECK: call void @llvm.assume(i1 false)
+    // CHECK-NOT: call
     mir! {
         let temp: BigNever;
         {
@@ -104,6 +104,9 @@ pub unsafe fn check_to_uninhabited(x: u16) {
 #[no_mangle]
 #[custom_mir(dialect = "runtime", phase = "optimized")]
 pub unsafe fn check_from_uninhabited(x: BigNever) -> u16 {
+    // CHECK-NOT: call
+    // CHECK: call void @llvm.assume(i1 false)
+    // CHECK-NOT: call
     // CHECK: ret i16 poison
     mir! {
         {
@@ -401,9 +404,9 @@ pub unsafe fn check_issue_109992(x: ()) -> [(); 1] {
 pub unsafe fn check_unit_to_never(x: ()) {
     // This uses custom MIR to avoid MIR optimizations having removed ZST ops.
 
-    // CHECK-NOT: trap
-    // CHECK: call void @llvm.trap
-    // CHECK-NOT: trap
+    // CHECK-NOT: call
+    // CHECK: call void @llvm.assume(i1 false)
+    // CHECK-NOT: call
     mir! {
         let temp: ZstNever;
         {
@@ -420,6 +423,7 @@ pub unsafe fn check_unit_from_never(x: ZstNever) -> () {
     // This uses custom MIR to avoid MIR optimizations having removed ZST ops.
 
     // CHECK: start
+    // CHECK-NEXT: call void @llvm.assume(i1 false)
     // CHECK-NEXT: ret void
     mir! {
         {
-- 
cgit 1.4.1-3-g733a5