fix supposedly unreachable bug! being reachable

compiler/rustc_trait_selection/src/traits/wf.rs

@@ -504,7 +504,16 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         // (*) The predicates of an inherent associated type include the
         //     predicates of the impl that it's contained in.
 
-        if !data.self_ty().has_escaping_bound_vars() {
+        // In an ideal world, there are no escaping bound vars here. However, WF is jank, and
+        // sometimes there are. We can only `compute_inherent_assoc_term_args` if the Self ty in the
+        // args has no escaping bound vars. If we already have impl format args, though,
+        // `compute_inherent_assoc_term_args` is a no-op (and we have no Self type), so no need to
+        // check for escaping bound vars.
+        let can_compute_impl_args =
+            matches!(data.kind, ty::AliasTermKind::InherentConstImpl { .. })
+                || !data.self_ty().has_escaping_bound_vars();
+
+        if can_compute_impl_args {
             // FIXME(inherent_associated_types): Should this happen inside of a snapshot?
             // FIXME(inherent_associated_types): This is incompatible with the new solver and lazy norm!
             let args = traits::project::compute_inherent_assoc_term_args(
@@ -1099,10 +1108,12 @@ impl<'a, 'tcx> TypeVisitor<TyCtxt<'tcx>> for WfPredicates<'a, 'tcx> {
                             self.add_wf_preds_for_inherent_projection(alias_const.into());
                             return; // Subtree is handled by above function
                         }
-                        // please ping khyperia and/or BoxyUwU if this `bug!` fires
-                        ty::AliasConstKind::InherentImpl { .. } => bug!(
-                            "This ought to be unreachable, the entrypoints of WF should still have InherentSelf-form alias consts."
-                        ),
+                        // FIXME: This should be unreachable but isn't because we normalize in item
+                        // wfck before computing wf requirements
+                        ty::AliasConstKind::InherentImpl { .. } => {
+                            self.add_wf_preds_for_inherent_projection(alias_const.into());
+                            return;
+                        }
                         ty::AliasConstKind::Projection { def_id }
                         | ty::AliasConstKind::Free { def_id }
                         | ty::AliasConstKind::Anon { def_id } => {

compiler/rustc_type_ir/src/term_kind.rs

@@ -1,3 +1,5 @@
+use std::debug_assert_matches;
+
 use derive_where::derive_where;
 #[cfg(feature = "nightly")]
 use rustc_macros::{Decodable_NoContext, Encodable_NoContext, StableHash_NoContext};
@@ -265,11 +267,25 @@ impl<I: Interner> AliasTerm<I> {
 /// The following methods work only with (trait) associated term projections.
 // FIXME: Replace by an impl on Alias<ProjectionAliasTermKind>
 impl<I: Interner> AliasTerm<I> {
+    fn debug_assert_has_self(self) {
+        // InherentConstImpl is deliberately omitted here, it is not self-format args
+        debug_assert_matches!(
+            self.kind,
+            AliasTermKind::ProjectionTy { .. }
+                | AliasTermKind::ProjectionConst { .. }
+                | AliasTermKind::InherentTy { .. }
+                | AliasTermKind::InherentConstSelf { .. },
+            "AliasTerm::self_ty is only valid on projection and inherent aliases"
+        );
+    }
+
     pub fn self_ty(self) -> I::Ty {
+        self.debug_assert_has_self();
         self.args.type_at(0)
     }
 
     pub fn with_replaced_self_ty(self, interner: I, self_ty: I::Ty) -> Self {
+        self.debug_assert_has_self();
         AliasTerm::new(
             interner,
             self.kind,

tests/ui/const-generics/gca/wf-inherentimpl.old.stderr

@@ -0,0 +1,10 @@
+error: `generic_const_args` requires -Znext-solver=globally to be enabled
+  --> $DIR/wf-inherentimpl.rs:7:12
+   |
+LL | #![feature(generic_const_args, min_generic_const_args)]
+   |            ^^^^^^^^^^^^^^^^^^
+   |
+   = help: enable all of these features
+
+error: aborting due to 1 previous error
+

tests/ui/const-generics/gca/wf-inherentimpl.rs

@@ -0,0 +1,16 @@
+//@[next] check-pass
+//@ revisions: next old
+//@[next] compile-flags: -Znext-solver
+//@ ignore-compare-mode-next-solver (explicit revisions)
+#![feature(inherent_associated_types)]
+#![feature(macroless_generic_const_args)]
+#![feature(generic_const_args, min_generic_const_args)]
+//[old]~^ ERROR `generic_const_args` requires -Znext-solver=globally to be enabled
+struct Foo<const A: usize>;
+impl<const A: usize> Foo<A> {
+    const SIZE: usize = { todo!() };
+    fn to_bytes() -> [u8; Self::SIZE] {
+        todo!()
+    }
+}
+fn main() {}