Rollup merge of #162475 - lsunsi:issue296, r=BoxyUwU

Fix unsoundness bug on next trait solver for dyn const generics placeholder

This seem to fix the related test, which would compile on next trait solver and should not.
It's related to the linked issue where it'd lead to a segmentation fault.

Closes rust-lang/trait-system-refactor-initiative#296

r? @BoxyUwU

Author: JonathanBrouwer

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -1003,7 +1003,10 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                         let kind = obligation.predicate.kind().skip_binder();
                         let keep = match kind {
                             ty::PredicateKind::Clause(ty::ClauseKind::ConstArgHasType(ct, _))
-                                if matches!(ct.kind(), ty::ConstKind::Param(..)) =>
+                                if matches!(
+                                    ct.kind(),
+                                    ty::ConstKind::Param(..) | ty::ConstKind::Placeholder(..)
+                                ) =>
                             {
                                 // ConstArgHasType clauses are not higher kinded. Assert as
                                 // such so we can fix this up if that ever changes.

tests/ui/const-generics/dyn-trait-ill-typed.rs

@@ -0,0 +1,15 @@
+//! Regression test for <https://github.com/rust-lang/trait-system-refactor-initiative/issues/296>.
+//@ compile-flags: -Znext-solver=globally
+//@ check-fail
+
+// CHECK PASS TO SHOW IT PASSES, BUT IT SHOULD NOT
+// THIS CODE SEGFAULTS, WITH REASON
+
+//~v ERROR: the constant `M` is not of type `usize`
+fn foo<const M: u32>() -> Box<dyn Tr<M>> {
+    loop {}
+}
+
+trait Tr<const N: usize> {}
+
+fn main() {}

tests/ui/const-generics/dyn-trait-ill-typed.stderr

@@ -0,0 +1,14 @@
+error: the constant `M` is not of type `usize`
+  --> $DIR/dyn-trait-ill-typed.rs:9:27
+   |
+LL | fn foo<const M: u32>() -> Box<dyn Tr<M>> {
+   |                           ^^^^^^^^^^^^^^ expected `usize`, found `u32`
+   |
+note: required by a const generic parameter in `Tr`
+  --> $DIR/dyn-trait-ill-typed.rs:13:10
+   |
+LL | trait Tr<const N: usize> {}
+   |          ^^^^^^^^^^^^^^ required by this const generic parameter in `Tr`
+
+error: aborting due to 1 previous error
+