Auto merge of #36289 - euclio:self-suggestion, r=jseyfried

resolve: Suggest `use self` when import resolves

Improves errors messages by replacing "Maybe a missing `extern crate`" messages
with "Did you mean `self::...`" when the `self` import would succeed.

Fixes #34191.

Thank you for the help @jseyfried!

Author: bors

src/librustc_resolve/resolve_imports.rs

@@ -588,7 +588,19 @@ impl<'a, 'b:'a> ImportResolver<'a, 'b> {
         let module = match module_result {
             Success(module) => module,
             Indeterminate => return Indeterminate,
-            Failed(err) => return Failed(err),
+            Failed(err) => {
+                let self_module = self.module_map[&self.current_module.normal_ancestor_id.unwrap()];
+
+                let resolve_from_self_result = self.resolve_module_path_from_root(
+                    &self_module, &module_path, 0, Some(span));
+
+                return if let Success(_) = resolve_from_self_result {
+                    let msg = format!("Did you mean `self::{}`?", &names_to_string(module_path));
+                    Failed(Some((span, msg)))
+                } else {
+                    Failed(err)
+                };
+            },
         };
 
         let (name, value_result, type_result) = match directive.subclass {

src/test/compile-fail/unresolved-import.rs

@@ -35,3 +35,23 @@ mod food {
         }
     }
 }
+
+mod m {
+    enum MyEnum {
+        MyVariant
+    }
+
+    use MyEnum::*; //~ ERROR unresolved import `MyEnum::*` [E0432]
+                   //~^ Did you mean `self::MyEnum`?
+}
+
+mod items {
+    enum Enum {
+        Variant
+    }
+
+    use Enum::*; //~ ERROR unresolved import `Enum::*` [E0432]
+                 //~^ Did you mean `self::Enum`?
+
+    fn item() {}
+}