From 257f68777f469bdd26d4763dc0cbd99c99b74e1d Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Wed, 19 Mar 2025 17:53:43 +0000
Subject: [PATCH] Use the new solver in the impossible_predicates

---
 .../rustc_trait_selection/src/traits/mod.rs   | 14 +++----
 tests/ui/traits/vtable/impossible-method.rs   | 38 +++++++++++++++++++
 2 files changed, 43 insertions(+), 9 deletions(-)
 create mode 100644 tests/ui/traits/vtable/impossible-method.rs

diff --git a/compiler/rustc_trait_selection/src/traits/mod.rs b/compiler/rustc_trait_selection/src/traits/mod.rs
index 0987c5b42d881..7a744dfc2ad8d 100644
--- a/compiler/rustc_trait_selection/src/traits/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/mod.rs
@@ -690,8 +690,11 @@ fn replace_param_and_infer_args_with_placeholder<'tcx>(
 /// used during analysis.
 pub fn impossible_predicates<'tcx>(tcx: TyCtxt<'tcx>, predicates: Vec<ty::Clause<'tcx>>) -> bool {
     debug!("impossible_predicates(predicates={:?})", predicates);
-    let (infcx, param_env) =
-        tcx.infer_ctxt().build_with_typing_env(ty::TypingEnv::fully_monomorphized());
+    let (infcx, param_env) = tcx
+        .infer_ctxt()
+        .with_next_trait_solver(true)
+        .build_with_typing_env(ty::TypingEnv::fully_monomorphized());
+
     let ocx = ObligationCtxt::new(&infcx);
     let predicates = ocx.normalize(&ObligationCause::dummy(), param_env, predicates);
     for predicate in predicates {
@@ -704,13 +707,6 @@ pub fn impossible_predicates<'tcx>(tcx: TyCtxt<'tcx>, predicates: Vec<ty::Clause
         return true;
     }
 
-    // Leak check for any higher-ranked trait mismatches.
-    // We only need to do this in the old solver, since the new solver already
-    // leak-checks.
-    if !infcx.next_trait_solver() && infcx.leak_check(ty::UniverseIndex::ROOT, None).is_err() {
-        return true;
-    }
-
     false
 }
 
diff --git a/tests/ui/traits/vtable/impossible-method.rs b/tests/ui/traits/vtable/impossible-method.rs
new file mode 100644
index 0000000000000..ff7910cfac86b
--- /dev/null
+++ b/tests/ui/traits/vtable/impossible-method.rs
@@ -0,0 +1,38 @@
+//@ run-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+
+trait Id {
+    type This<'a>;
+}
+impl<T> Id for T {
+    type This<'a> = T;
+}
+
+trait Trait<T> {}
+impl<T: Id> Trait<for<'a> fn(T::This<'a>)> for T {}
+
+trait Method<T: Id> {
+    fn call_me(&self)
+    where
+        T: Trait<for<'a> fn(T::This<'a>)>;
+}
+
+impl<T, U> Method<U> for T {
+    fn call_me(&self) {
+        println!("method was reachable");
+    }
+}
+
+fn generic<T: Id>(x: &dyn Method<T>) {
+    // Proving `T: Trait<for<'a> fn(T::This<'a>)>` holds.
+    x.call_me();
+}
+
+fn main() {
+    // Proving `u32: Trait<fn(u32)>` fails due to incompleteness.
+    // We don't add the method to the vtable of `dyn Method`, so
+    // calling it causes UB.
+    generic::<u32>(&());
+}