Fix ICE in next-solver TransmuteFrom candidate

Treat TransmuteFrom goals with non-region inference variables as ambiguous before running transmutability, instead of computing layouts for unresolved types. The old solver already treated these goals as ambiguous, so only the next solver could hit this ICE.

Author: cijiugechu

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -665,6 +665,12 @@ where
             return Err(NoSolution);
         }
 
+        // Match the old solver by treating unresolved inference variables as
+        // ambiguous until `rustc_transmute` can compute their layout.
+        if goal.has_non_region_infer() {
+            return ecx.forced_ambiguity(MaybeCause::Ambiguity);
+        }
+
         ecx.probe_builtin_trait_candidate(BuiltinImplSource::Misc).enter(|ecx| {
             let assume = ecx.structurally_normalize_const(
                 goal.param_env,

tests/ui/traits/next-solver/transmute-from-async-closure.rs

@@ -0,0 +1,22 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/153370>.
+
+//@ edition:2024
+//@ compile-flags: -Znext-solver=globally
+
+#![feature(transmutability)]
+
+trait NodeImpl {}
+struct Wrap<F, P>(F, P);
+impl<F, P> Wrap<F, P> {
+    fn new(_: F) -> Self {
+        loop {}
+    }
+}
+
+impl<F, A> NodeImpl for Wrap<F, (A,)> where F: std::mem::TransmuteFrom<()> {}
+fn trigger_ice() {
+    let _: &dyn NodeImpl = &Wrap::<_, (i128,)>::new(async |_: &(), i128| 0);
+    //~^ ERROR type annotations needed
+}
+
+fn main() {}

tests/ui/traits/next-solver/transmute-from-async-closure.stderr

@@ -0,0 +1,23 @@
+error[E0283]: type annotations needed
+  --> $DIR/transmute-from-async-closure.rs:18:29
+   |
+LL |     let _: &dyn NodeImpl = &Wrap::<_, (i128,)>::new(async |_: &(), i128| 0);
+   |                             ^^^^^^^^^^^^^^^^^^^^^^^ cannot infer type of the type parameter `F` declared on the struct `Wrap`
+   |
+   = note: cannot satisfy `{async closure@$DIR/transmute-from-async-closure.rs:18:53: 18:73}: TransmuteFrom<(), Assume { alignment: false, lifetimes: false, safety: false, validity: false }>`
+note: required for `Wrap<{async closure@$DIR/transmute-from-async-closure.rs:18:53: 18:73}, (i128,)>` to implement `NodeImpl`
+  --> $DIR/transmute-from-async-closure.rs:16:12
+   |
+LL | impl<F, A> NodeImpl for Wrap<F, (A,)> where F: std::mem::TransmuteFrom<()> {}
+   |            ^^^^^^^^     ^^^^^^^^^^^^^          --------------------------- unsatisfied trait bound introduced here
+note: required because it appears within the type `Wrap<{async closure@$DIR/transmute-from-async-closure.rs:18:53: 18:73}, (i128,)>`
+  --> $DIR/transmute-from-async-closure.rs:9:8
+   |
+LL | struct Wrap<F, P>(F, P);
+   |        ^^^^
+   = note: required for `&Wrap<{async closure@$DIR/transmute-from-async-closure.rs:18:53: 18:73}, (i128,)>` to implement `CoerceUnsized<&dyn NodeImpl>`
+   = note: required for the cast from `&Wrap<{async closure@$DIR/transmute-from-async-closure.rs:18:53: 18:73}, (i128,)>` to `&dyn NodeImpl`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0283`.