From ef367ed3259948e3d738c7205a8dbe09a150fef6 Mon Sep 17 00:00:00 2001
From: lapla <me@lapla.dev>
Date: Mon, 6 Apr 2026 23:00:20 +0900
Subject: [PATCH] transmutability: Stall new solver when inference variables
 remain

---
 .../src/solve/eval_ctxt/mod.rs                |  3 +++
 .../transmutability-coherence.rs              | 10 ++++++++++
 .../transmutability-coherence.stderr          | 19 +++++++++++++++++++
 3 files changed, 32 insertions(+)
 create mode 100644 tests/ui/transmutability/transmutability-coherence.rs
 create mode 100644 tests/ui/transmutability/transmutability-coherence.stderr

diff --git a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
index 6841fe1c5124e..36cee1ac01b6a 100644
--- a/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
+++ b/compiler/rustc_next_trait_solver/src/solve/eval_ctxt/mod.rs
@@ -1174,6 +1174,9 @@ where
         dst: I::Ty,
         assume: I::Const,
     ) -> Result<Certainty, NoSolution> {
+        if src.has_non_region_infer() || dst.has_non_region_infer() {
+            return Ok(Certainty::AMBIGUOUS);
+        }
         self.delegate.is_transmutable(dst, src, assume)
     }
 
diff --git a/tests/ui/transmutability/transmutability-coherence.rs b/tests/ui/transmutability/transmutability-coherence.rs
new file mode 100644
index 0000000000000..be3a9c0cb64c8
--- /dev/null
+++ b/tests/ui/transmutability/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/transmutability-coherence.stderr b/tests/ui/transmutability/transmutability-coherence.stderr
new file mode 100644
index 0000000000000..3d8af36432608
--- /dev/null
+++ b/tests/ui/transmutability/transmutability-coherence.stderr
@@ -0,0 +1,19 @@
+error[E0747]: type provided when a constant was expected
+  --> $DIR/transmutability-coherence.rs:4:37
+   |
+LL | impl<T: std::mem::TransmuteFrom<(), ()>> OpaqueTrait for T {}
+   |                                     ^^
+
+error[E0119]: conflicting implementations of trait `OpaqueTrait` for type `&_`
+  --> $DIR/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 `&_`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0747.
+For more information about an error, try `rustc --explain E0119`.