Don't allow consts with unconstrained lifetimes in their types

compiler/rustc_hir_analysis/src/impl_wf_check.rs

@@ -94,21 +94,21 @@ fn enforce_impl_params_are_constrained(tcx: TyCtxt<'_>, impl_def_id: LocalDefId)
         &mut input_parameters,
     );
 
-    // Disallow unconstrained lifetimes, but only if they appear in assoc types.
+    // Disallow unconstrained lifetimes, but only if they appear in assoc types or consts.
     let lifetimes_in_associated_types: FxHashSet<_> = tcx
         .associated_item_def_ids(impl_def_id)
         .iter()
         .flat_map(|def_id| {
             let item = tcx.associated_item(def_id);
             match item.kind {
-                ty::AssocKind::Type => {
+                ty::AssocKind::Type | ty::AssocKind::Const => {
                     if item.defaultness(tcx).has_value() {
                         cgp::parameters_for(&tcx.type_of(def_id).instantiate_identity(), true)
                     } else {
                         vec![]
                     }
                 }
-                ty::AssocKind::Fn | ty::AssocKind::Const => vec![],
+                ty::AssocKind::Fn => vec![],
             }
         })
         .collect();

tests/ui/consts/assoc-const-unconstrained-lifetime.rs

@@ -0,0 +1,8 @@
+struct Foo;
+
+impl<'a> Foo {
+    const CONST: &'a str = "";
+    //~^ ERROR the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
+}
+
+fn main() {}

tests/ui/consts/assoc-const-unconstrained-lifetime.stderr

@@ -0,0 +1,9 @@
+error[E0207]: the lifetime parameter `'a` is not constrained by the impl trait, self type, or predicates
+  --> $DIR/assoc-const-unconstrained-lifetime.rs:3:6
+   |
+LL | impl<'a> Foo {
+   |      ^^ unconstrained lifetime parameter
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0207`.

tests/ui/nll/trait-associated-constant.rs

@@ -19,7 +19,7 @@ struct FailStruct { }
 
 impl<'a: 'b, 'b, 'c> Anything<'a, 'b> for FailStruct {
     const AC: Option<&'c str> = None;
-    //~^ ERROR: const not compatible with trait
+    //~^ ERROR: the lifetime parameter `'c` is not constrained by the impl trait, self type, or predicates
 }
 
 struct OKStruct2 { }

tests/ui/nll/trait-associated-constant.stderr

@@ -1,22 +1,9 @@
-error[E0308]: const not compatible with trait
-  --> $DIR/trait-associated-constant.rs:21:5
-   |
-LL |     const AC: Option<&'c str> = None;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ lifetime mismatch
-   |
-   = note: expected enum `Option<&'b str>`
-              found enum `Option<&'c str>`
-note: the lifetime `'c` as defined here...
+error[E0207]: the lifetime parameter `'c` is not constrained by the impl trait, self type, or predicates
   --> $DIR/trait-associated-constant.rs:20:18
    |
 LL | impl<'a: 'b, 'b, 'c> Anything<'a, 'b> for FailStruct {
-   |                  ^^
-note: ...does not necessarily outlive the lifetime `'b` as defined here
-  --> $DIR/trait-associated-constant.rs:20:14
-   |
-LL | impl<'a: 'b, 'b, 'c> Anything<'a, 'b> for FailStruct {
-   |              ^^
+   |                  ^^ unconstrained lifetime parameter
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0308`.
+For more information about this error, try `rustc --explain E0207`.