Don't ICE on associated type projection without feature gate

compiler/rustc_trait_selection/src/solve/project_goals.rs

@@ -244,7 +244,21 @@ impl<'tcx> assembly::GoalKind<'tcx> for ProjectionPredicate<'tcx> {
             // Finally we construct the actual value of the associated type.
             let term = match assoc_def.item.kind {
                 ty::AssocKind::Type => tcx.type_of(assoc_def.item.def_id).map_bound(|ty| ty.into()),
-                ty::AssocKind::Const => bug!("associated const projection is not supported yet"),
+                ty::AssocKind::Const => {
+                    if tcx.features().associated_const_equality {
+                        bug!("associated const projection is not supported yet")
+                    } else {
+                        ty::EarlyBinder::bind(
+                            ty::Const::new_error_with_message(
+                                tcx,
+                                tcx.type_of(assoc_def.item.def_id).instantiate_identity(),
+                                DUMMY_SP,
+                                "associated const projection is not supported yet",
+                            )
+                            .into(),
+                        )
+                    }
+                }
                 ty::AssocKind::Fn => unreachable!("we should never project to a fn"),
             };
 

tests/ui/traits/new-solver/dont-ice-on-assoc-projection.rs

@@ -0,0 +1,19 @@
+// compile-flags: -Ztrait-solver=next-coherence
+
+// Makes sure we don't ICE on associated const projection when the feature gate
+// is not enabled, since we should avoid encountering ICEs on stable if possible.
+
+trait Bar {
+    const ASSOC: usize;
+}
+impl Bar for () {
+    const ASSOC: usize = 1;
+}
+
+trait Foo {}
+impl Foo for () {}
+impl<T> Foo for T where T: Bar<ASSOC = 0> {}
+//~^ ERROR associated const equality is incomplete
+//~| ERROR conflicting implementations of trait `Foo` for type `()`
+
+fn main() {}

tests/ui/traits/new-solver/dont-ice-on-assoc-projection.stderr

@@ -0,0 +1,21 @@
+error[E0658]: associated const equality is incomplete
+  --> $DIR/dont-ice-on-assoc-projection.rs:15:32
+   |
+LL | impl<T> Foo for T where T: Bar<ASSOC = 0> {}
+   |                                ^^^^^^^^^
+   |
+   = note: see issue #92827 <https://github.com/rust-lang/rust/issues/92827> for more information
+   = help: add `#![feature(associated_const_equality)]` to the crate attributes to enable
+
+error[E0119]: conflicting implementations of trait `Foo` for type `()`
+  --> $DIR/dont-ice-on-assoc-projection.rs:15:1
+   |
+LL | impl Foo for () {}
+   | --------------- first implementation here
+LL | impl<T> Foo for T where T: Bar<ASSOC = 0> {}
+   | ^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0119, E0658.
+For more information about an error, try `rustc --explain E0119`.