Properly deal with missing/placeholder types inside GACs

compiler/rustc_hir_typeck/src/lib.rs

@@ -235,11 +235,17 @@ fn infer_type_if_missing<'tcx>(fcx: &FnCtxt<'_, 'tcx>, node: Node<'tcx>) -> Opti
  if let Some(item) = tcx.opt_associated_item(def_id.into())
  && let ty::AssocKind::Const = item.kind
  && let ty::ImplContainer = item.container
- && let Some(trait_item) = item.trait_item_def_id
+ && let Some(trait_item_def_id) = item.trait_item_def_id
  {
- let args =
- tcx.impl_trait_ref(item.container_id(tcx)).unwrap().instantiate_identity().args;
- Some(tcx.type_of(trait_item).instantiate(tcx, args))
+ let impl_def_id = item.container_id(tcx);
+ let impl_trait_ref = tcx.impl_trait_ref(impl_def_id).unwrap().instantiate_identity();
+ let args = ty::GenericArgs::identity_for_item(tcx, def_id).rebase_onto(
+ tcx,
+ impl_def_id,
+ impl_trait_ref.args,
+ );
+ tcx.check_args_compatible(trait_item_def_id, args)
+ .then(|| tcx.type_of(trait_item_def_id).instantiate(tcx, args))
  } else {
  Some(fcx.next_ty_var(span))
  }

tests/ui/generic-const-items/assoc-const-missing-type.rs

@@ -0,0 +1,21 @@
+// Ensure that we properly deal with missing/placeholder types inside GACs.
+// issue: rust-lang/rust#124833
+#![feature(generic_const_items)]
+#![allow(incomplete_features)]
+
+trait Trait {
+ const K<T>: T;
+ const Q<'a>: &'a str;
+}
+
+impl Trait for () {
+ const K<T> = ();
+ //~^ ERROR missing type for `const` item
+ //~| ERROR mismatched types
+ //~| ERROR mismatched types
+ const Q = "";
+ //~^ ERROR missing type for `const` item
+ //~| ERROR lifetime parameters or bounds on const `Q` do not match the trait declaration
+}
+
+fn main() {}

tests/ui/generic-const-items/assoc-const-missing-type.stderr

@@ -0,0 +1,48 @@
+error[E0308]: mismatched types
+ --> $DIR/assoc-const-missing-type.rs:12:18
+ |
+LL | const K<T> = ();
+ | - ^^ expected type parameter `T`, found `()`
+ | |
+ | expected this type parameter
+ |
+ = note: expected type parameter `T`
+ found unit type `()`
+
+error: missing type for `const` item
+ --> $DIR/assoc-const-missing-type.rs:12:15
+ |
+LL | const K<T> = ();
+ | ^ help: provide a type for the associated constant: `()`
+
+error[E0195]: lifetime parameters or bounds on const `Q` do not match the trait declaration
+ --> $DIR/assoc-const-missing-type.rs:16:12
+ |
+LL | const Q<'a>: &'a str;
+ | ---- lifetimes in impl do not match this const in trait
+...
+LL | const Q = "";
+ | ^ lifetimes do not match const in trait
+
+error: missing type for `const` item
+ --> $DIR/assoc-const-missing-type.rs:16:12
+ |
+LL | const Q = "";
+ | ^ help: provide a type for the associated constant: `: &str`
+
+error[E0308]: mismatched types
+ --> $DIR/assoc-const-missing-type.rs:12:18
+ |
+LL | const K<T> = ();
+ | - ^^ expected type parameter `T`, found `()`
+ | |
+ | expected this type parameter
+ |
+ = note: expected type parameter `T`
+ found unit type `()`
+ = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 5 previous errors
+
+Some errors have detailed explanations: E0195, E0308.
+For more information about an error, try `rustc --explain E0195`.