From ca30c6715d65042990a97d9c8f89f96ae9dced9c Mon Sep 17 00:00:00 2001
From: lapla <me@lapla.dev>
Date: Sun, 23 Nov 2025 07:50:58 +0900
Subject: [PATCH] transmutability: only short-circuit `is_transmutable` for
 non-region infers

---
 compiler/rustc_transmute/src/lib.rs           |  6 +++++-
 .../ice-141400-transmutability-coherence.rs   | 10 +++++++++
 ...ce-141400-transmutability-coherence.stderr | 21 +++++++++++++++++++
 3 files changed, 36 insertions(+), 1 deletion(-)
 create mode 100644 tests/ui/transmutability/ice-141400-transmutability-coherence.rs
 create mode 100644 tests/ui/transmutability/ice-141400-transmutability-coherence.stderr

diff --git a/compiler/rustc_transmute/src/lib.rs b/compiler/rustc_transmute/src/lib.rs
index 36281ff16bceb..5c7a4e1e33105 100644
--- a/compiler/rustc_transmute/src/lib.rs
+++ b/compiler/rustc_transmute/src/lib.rs
@@ -89,7 +89,7 @@ pub enum Reason<T> {
 #[cfg(feature = "rustc")]
 mod rustc {
     use rustc_hir::lang_items::LangItem;
-    use rustc_middle::ty::{Const, Region, Ty, TyCtxt};
+    use rustc_middle::ty::{Const, Region, Ty, TyCtxt, TypeVisitableExt};
 
     use super::*;
 
@@ -116,6 +116,10 @@ mod rustc {
             types: Types<'tcx>,
             assume: crate::Assume,
         ) -> crate::Answer<Region<'tcx>, Ty<'tcx>> {
+            if types.src.has_non_region_infer() || types.dst.has_non_region_infer() {
+                return Answer::No(Reason::TypeError);
+            }
+
             crate::maybe_transmutable::MaybeTransmutableQuery::new(
                 types.src, types.dst, assume, self.tcx,
             )
diff --git a/tests/ui/transmutability/ice-141400-transmutability-coherence.rs b/tests/ui/transmutability/ice-141400-transmutability-coherence.rs
new file mode 100644
index 0000000000000..be3a9c0cb64c8
--- /dev/null
+++ b/tests/ui/transmutability/ice-141400-transmutability-coherence.rs
@@ -0,0 +1,10 @@
+#![feature(transmutability)]
+trait OpaqueTrait {}
+
+impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+//~^ ERROR: type provided when a constant was expected
+
+impl<T> OpaqueTrait for &T where T: OpaqueTrait {}
+//~^ ERROR: conflicting implementations of trait `OpaqueTrait` for type `&_`
+
+fn main() {}
diff --git a/tests/ui/transmutability/ice-141400-transmutability-coherence.stderr b/tests/ui/transmutability/ice-141400-transmutability-coherence.stderr
new file mode 100644
index 0000000000000..0c65957283950
--- /dev/null
+++ b/tests/ui/transmutability/ice-141400-transmutability-coherence.stderr
@@ -0,0 +1,21 @@
+error[E0747]: type provided when a constant was expected
+  --> $DIR/ice-141400-transmutability-coherence.rs:4:37
+   |
+LL | impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+   |                                     ^^
+
+error[E0119]: conflicting implementations of trait `OpaqueTrait` for type `&_`
+  --> $DIR/ice-141400-transmutability-coherence.rs:7:1
+   |
+LL | impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+   | ---------------------------------------------------------- first implementation here
+...
+LL | impl<T> OpaqueTrait for &T where T: OpaqueTrait {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `&_`
+   |
+   = note: downstream crates may implement trait `std::mem::TransmuteFrom<(), std::mem::Assume { alignment: false, lifetimes: false, safety: false, validity: false }>` for type `&_`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0747.
+For more information about an error, try `rustc --explain E0119`.