From 8e7be34ed7fb755a74ecf3cccdbf8411a5005f65 Mon Sep 17 00:00:00 2001
From: johnproblems <ian@busy.email>
Date: Wed, 3 Jun 2026 03:50:38 -0400
Subject: [PATCH] Add a regression test for an unconstrained TransmuteFrom ICE

An unconstrained `TransmuteFrom` obligation used to ICE with `layout_of:
unexpected type` under the next-gen trait solver; it now reports `type
annotations needed` (E0283). Add a regression test for that case, which
the fix's own test suite did not cover.
---
 .../transmute-from-unconstrained.rs           | 20 ++++++++++++++++
 .../transmute-from-unconstrained.stderr       | 23 +++++++++++++++++++
 2 files changed, 43 insertions(+)
 create mode 100644 tests/ui/traits/next-solver/transmute-from-unconstrained.rs
 create mode 100644 tests/ui/traits/next-solver/transmute-from-unconstrained.stderr

diff --git a/tests/ui/traits/next-solver/transmute-from-unconstrained.rs b/tests/ui/traits/next-solver/transmute-from-unconstrained.rs
new file mode 100644
index 0000000000000..892b23d81d45b
--- /dev/null
+++ b/tests/ui/traits/next-solver/transmute-from-unconstrained.rs
@@ -0,0 +1,20 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/151314>.
+//!
+//! Calling a function with an unconstrained `TransmuteFrom` obligation used to
+//! trigger a `layout_of: unexpected type` ICE under the next-gen trait solver
+//! instead of reporting that type annotations are needed.
+
+//@ compile-flags: -Znext-solver=globally
+
+#![feature(transmutability)]
+
+fn assert_transmutable<T>()
+where
+    (): std::mem::TransmuteFrom<T>,
+{
+}
+
+fn main() {
+    assert_transmutable()
+    //~^ ERROR type annotations needed
+}
diff --git a/tests/ui/traits/next-solver/transmute-from-unconstrained.stderr b/tests/ui/traits/next-solver/transmute-from-unconstrained.stderr
new file mode 100644
index 0000000000000..a3a1cee105d59
--- /dev/null
+++ b/tests/ui/traits/next-solver/transmute-from-unconstrained.stderr
@@ -0,0 +1,23 @@
+error[E0283]: type annotations needed
+  --> $DIR/transmute-from-unconstrained.rs:18:5
+   |
+LL |     assert_transmutable()
+   |     ^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `T` declared on the function `assert_transmutable`
+   |
+   = note: cannot satisfy `(): TransmuteFrom<_, Assume { alignment: false, lifetimes: false, safety: false, validity: false }>`
+note: required by a bound in `assert_transmutable`
+  --> $DIR/transmute-from-unconstrained.rs:13:9
+   |
+LL | fn assert_transmutable<T>()
+   |    ------------------- required by a bound in this function
+LL | where
+LL |     (): std::mem::TransmuteFrom<T>,
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `assert_transmutable`
+help: consider specifying a concrete type for the type parameter `T`
+   |
+LL |     assert_transmutable::</* Type */>()
+   |                        ++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.