Revert "Fix an ICE in the vtable iteration for a trait reference"

The ICE fix appears to be unsound, causing a miscompilation involving
`dyn Trait` and `async {}` which induces segfaults in safe Rust code.
As the patch only hid an ICE, it does not seem worth the risk.

This reverts commit 8afd636, reversing
changes made to 19122c0.

Author: workingjubilee

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -12,7 +12,7 @@ use rustc_span::DUMMY_SP;
 use smallvec::{SmallVec, smallvec};
 use tracing::debug;
 
-use crate::traits::is_vtable_safe_method;
+use crate::traits::{impossible_predicates, is_vtable_safe_method};
 
 #[derive(Clone, Debug)]
 pub enum VtblSegment<'tcx> {
@@ -271,7 +271,11 @@ fn vtable_entries<'tcx>(
                     // do not hold for this particular set of type parameters.
                     // Note that this method could then never be called, so we
                     // do not want to try and codegen it, in that case (see #23435).
-                    if tcx.instantiate_and_check_impossible_predicates((def_id, args)) {
+                    let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, args);
+                    if impossible_predicates(
+                        tcx,
+                        predicates.map(|(predicate, _)| predicate).collect(),
+                    ) {
                         debug!("vtable_entries: predicates do not hold");
                         return VtblEntry::Vacant;
                     }

…on/vtable-impossible-predicates-async.rs tests/crashes/135470.rstests/ui/coercion/vtable-impossible-predicates-async.rs renamed to tests/crashes/135470.rs tests/ui/coercion/vtable-impossible-predicates-async.rs renamed to tests/crashes/135470.rs

@@ -1,8 +1,4 @@
-// Regression test for #135470.
-// Verify that we don't ICE when building vtable entries
-// for a blanket impl involving async and impossible predicates.
-
-//@ check-pass
+//@ known-bug: #135470
 //@ compile-flags: -Copt-level=0
 //@ edition: 2021
 

tests/crashes/137190-2.rs

@@ -0,0 +1,18 @@
+//@ known-bug: #137190
+trait Supertrait<T> {
+    fn method(&self) {}
+}
+
+trait Trait<P>: Supertrait<()> {}
+
+impl<P> Trait<P> for () {}
+
+const fn upcast<P>(x: &dyn Trait<P>) -> &dyn Supertrait<()> {
+    x
+}
+
+const fn foo() -> &'static dyn Supertrait<()> {
+    upcast::<()>(&())
+}
+
+const _: &'static dyn Supertrait<()> = foo();

tests/crashes/137190-3.rs

@@ -0,0 +1,10 @@
+//@ known-bug: #137190
+trait Supertrait {
+    fn method(&self) {}
+}
+
+trait Trait: Supertrait {}
+
+impl Trait for () {}
+
+const _: &dyn Supertrait = &() as &dyn Trait as &dyn Supertrait;