Auto merge of rust-lang#136988 - compiler-errors:impossible_predicates, r=lcnr

Use the new solver in the `impossible_predicates`

The old solver is unsound for many reasons. One of which was weaponized by `@lcnr` in rust-lang#140212, where the old solver was incompletely considering a dyn vtable method to be impossible and replacing its vtable entry with a null value. This null function could be called post-mono.

The new solver is expected to be less incomplete due to its correct handling of higher-ranked aliases in relate. This PR switches the `impossible_predicates` query to use the new solver, which patches this UB.

r? lcnr

Author: bors

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
 }
 

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>(&());
+}