Auto merge of rust-lang#125457 - fmease:gacs-diag-infer-plac-missing-…

…ty, r=compiler-errors

Properly deal with missing/placeholder types inside GACs

Fixes rust-lang#124833.

r? oli-obk (rust-lang#123130)

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`.