suggest qualifying bare associated constants

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -227,20 +227,27 @@ fn make_base_error(
                     && let Some(FnCtxt::Assoc(_)) = fn_kind.ctxt()
                     && let Some(items) = self.diagnostic_metadata.current_impl_items
                     && let Some(item) = items.iter().find(|i| {
-                        if let AssocItemKind::Fn(_) = &i.kind && i.ident.name == item_str.name
+                        if let AssocItemKind::Fn(..) | AssocItemKind::Const(..) = &i.kind
+                            && i.ident.name == item_str.name
                         {
                             debug!(?item_str.name);
                             return true
                         }
                         false
                     })
-                    && let AssocItemKind::Fn(fn_) = &item.kind
                 {
-                    debug!(?fn_);
+                    let self_sugg = match &item.kind {
-                    let self_sugg = if fn_.sig.decl.has_self() { "self." } else { "Self::" };
+                        AssocItemKind::Fn(fn_) if fn_.sig.decl.has_self() => "self.",
+                        _ => "Self::",
+                    };
+
                     Some((
                         item_span.shrink_to_lo(),
-                        "consider using the associated function",
+                        match &item.kind {
+                            AssocItemKind::Fn(..) => "consider using the associated function",
+                            AssocItemKind::Const(..) => "consider using the associated constant",
+                            _ => unreachable!("item kind was filtered above"),
+                        },
                         self_sugg.to_string()
                     ))
                 } else {

tests/ui/suggestions/assoc-const-without-self.rs

@@ -0,0 +1,11 @@
+struct Foo;
+
+impl Foo {
+    const A_CONST: usize = 1;
+
+    fn foo() -> usize {
+        A_CONST //~ ERROR cannot find value `A_CONST` in this scope
+    }
+}
+
+fn main() {}

tests/ui/suggestions/assoc-const-without-self.stderr

@@ -0,0 +1,14 @@
+error[E0425]: cannot find value `A_CONST` in this scope
+  --> $DIR/assoc-const-without-self.rs:7:9
+   |
+LL |         A_CONST
+   |         ^^^^^^^ not found in this scope
+   |
+help: consider using the associated constant
+   |
+LL |         Self::A_CONST
+   |         ++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0425`.