Rollup merge of #156417 - jakubadamw:issue-137190-reapplied, r=lcnr

Fix an ICE in the vtable iteration for a trait reference in const eval when a supertrait is not implemented

This is a second incarnation of #152287, which was reverted in #152738 as it had exposed another underlying unsoundness (#153596, exhibited indirectly in #152735), which was recently fixed in #155749.

It’s the same fix and the same set of tests. Regression tests for the unsoundness itself were already added in #155749.

Closes #137190.
Closes #135470.

Author: JonathanBrouwer

compiler/rustc_trait_selection/src/traits/vtable.rs

@@ -13,7 +13,7 @@ use rustc_span::DUMMY_SP;
 use smallvec::{SmallVec, smallvec};
 use tracing::debug;
 
-use crate::traits::{impossible_predicates, is_vtable_safe_method};
+use crate::traits::is_vtable_safe_method;
 
 #[derive(Clone, Debug)]
 pub enum VtblSegment<'tcx> {
@@ -276,11 +276,7 @@ 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).
-                    let predicates = tcx.predicates_of(def_id).instantiate_own(tcx, args);
-                    if impossible_predicates(
-                        tcx,
-                        predicates.map(|(predicate, _)| predicate.skip_norm_wip()).collect(),
-                    ) {
+                    if tcx.instantiate_and_check_impossible_predicates((def_id, args)) {
                         debug!("vtable_entries: predicates do not hold");
                         return VtblEntry::Vacant;
                     }

tests/crashes/137190-2.rs

@@ -1,4 +1,8 @@
-//@ known-bug: #135470
+// 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
 //@ compile-flags: -Copt-level=0
 //@ edition: 2021
 

tests/crashes/137190-3.rs

@@ -0,0 +1,25 @@
+// 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/crashes/135470.rs …on/vtable-impossible-predicates-async.rstests/crashes/135470.rs renamed to tests/ui/coercion/vtable-impossible-predicates-async.rs tests/crashes/135470.rs renamed to tests/ui/coercion/vtable-impossible-predicates-async.rs

@@ -0,0 +1,20 @@
+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-generics.rs

@@ -0,0 +1,16 @@
+// 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-generics.stderr

@@ -0,0 +1,20 @@
+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`.