Auto merge of #152738 - workingjubilee:revert-unsound-ice-patch, r=mati865

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 addresses the problem in #152735 but it may still merit team discussion even if this PR is merged.

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

Author: bors

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> {
@@ -276,7 +276,11 @@ 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;