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 rust-lang/rust#152735 but it may still merit team discussion even if this PR is merged.

This reverts commit 8afd63610b, reversing changes made to 19122c03c7.

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;
                     }

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;

tests/ui/coercion/vtable-unsatisfied-supertrait-generics.rs

@@ -1,25 +0,0 @@
-// Regression test for #137190.
-// Variant of vtable-unsatisfied-supertrait.rs with generic traits.
-// Verify that we don't ICE when building vtable entries
-// for a generic trait whose supertrait is not implemented.
-
-//@ compile-flags: --crate-type lib
-
-trait Supertrait<T> {
-    fn method(&self) {}
-}
-
-trait Trait<P>: Supertrait<()> {}
-
-impl<P> Trait<P> for () {}
-//~^ ERROR the trait bound `(): Supertrait<()>` is not satisfied
-
-const fn upcast<P>(x: &dyn Trait<P>) -> &dyn Supertrait<()> {
-    x
-}
-
-const fn foo() -> &'static dyn Supertrait<()> {
-    upcast::<()>(&())
-}
-
-const _: &'static dyn Supertrait<()> = foo();

tests/ui/coercion/vtable-unsatisfied-supertrait-generics.stderr

@@ -1,20 +0,0 @@
-error[E0277]: the trait bound `(): Supertrait<()>` is not satisfied
-  --> $DIR/vtable-unsatisfied-supertrait-generics.rs:14:22
-   |
-LL | impl<P> Trait<P> for () {}
-   |                      ^^ the trait `Supertrait<()>` is not implemented for `()`
-   |
-help: this trait has no implementations, consider adding one
-  --> $DIR/vtable-unsatisfied-supertrait-generics.rs:8:1
-   |
-LL | trait Supertrait<T> {
-   | ^^^^^^^^^^^^^^^^^^^
-note: required by a bound in `Trait`
-  --> $DIR/vtable-unsatisfied-supertrait-generics.rs:12:17
-   |
-LL | trait Trait<P>: Supertrait<()> {}
-   |                 ^^^^^^^^^^^^^^ required by this bound in `Trait`
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0277`.

tests/ui/coercion/vtable-unsatisfied-supertrait.rs

@@ -1,16 +0,0 @@
-// Regression test for #137190.
-// Verify that we don't ICE when building vtable entries
-// for a trait whose supertrait is not implemented.
-
-//@ compile-flags: --crate-type lib
-
-trait Supertrait {
-    fn method(&self) {}
-}
-
-trait Trait: Supertrait {}
-
-impl Trait for () {}
-//~^ ERROR the trait bound `(): Supertrait` is not satisfied
-
-const _: &dyn Supertrait = &() as &dyn Trait as &dyn Supertrait;

tests/ui/coercion/vtable-unsatisfied-supertrait.stderr

@@ -1,20 +0,0 @@
-error[E0277]: the trait bound `(): Supertrait` is not satisfied
-  --> $DIR/vtable-unsatisfied-supertrait.rs:13:16
-   |
-LL | impl Trait for () {}
-   |                ^^ the trait `Supertrait` is not implemented for `()`
-   |
-help: this trait has no implementations, consider adding one
-  --> $DIR/vtable-unsatisfied-supertrait.rs:7:1
-   |
-LL | trait Supertrait {
-   | ^^^^^^^^^^^^^^^^
-note: required by a bound in `Trait`
-  --> $DIR/vtable-unsatisfied-supertrait.rs:11:14
-   |
-LL | trait Trait: Supertrait {}
-   |              ^^^^^^^^^^ required by this bound in `Trait`
-
-error: aborting due to 1 previous error
-
-For more information about this error, try `rustc --explain E0277`.