Auto merge of #40775 - estebank:variant-as-type, r=petrochenkov

Suggest using enum when a variant is used as a type

Given a file:

```rust
enum Fruit {
    Apple(i64),
    Orange(i64),
}

fn should_return_fruit() -> Apple {
    Apple(5)
}
```

Provide the following output:

```rust
error[E0412]: cannot find type `Apple` in this scope
  --> file.rs:16:29
   |
16 | fn should_return_fruit() -> Apple {
   |                             ^^^^^ not found in this scope
   |
help: there is an enum variant `Fruit::Apple`, did you mean to use `Fruit`?
  --> file.rs:12:5
   |
12 |     Apple(i64),
   |     ^^^^^^^^^^

error[E0425]: cannot find function `Apple` in this scope
  --> file.rs:17:5
   |
17 |     Apple(5)
   |     ^^^^^ not found in this scope
   |
   = help: possible candidate is found in another module, you can import it into scope:
             `use Fruit::Apple;`
```

Fix #35675.

src/librustc_resolve/lib.rs

@@ -2222,6 +2222,7 @@ fn smart_resolve_path_fragment(&mut self,
                                    -> PathResolution {
         let ns = source.namespace();
         let is_expected = &|def| source.is_expected(def);
+        let is_enum_variant = &|def| if let Def::Variant(..) = def { true } else { false };
 
         // Base error is amended with one short label and possibly some longer helps/notes.
         let report_errors = |this: &mut Self, def: Option<Def>| {
@@ -2272,6 +2273,21 @@ fn smart_resolve_path_fragment(&mut self,
             if !candidates.is_empty() {
                 // Report import candidates as help and proceed searching for labels.
                 show_candidates(&mut err, &candidates, def.is_some());
+            } else if is_expected(Def::Enum(DefId::local(CRATE_DEF_INDEX))) {
+                let enum_candidates = this.lookup_import_candidates(name, ns, is_enum_variant);
+                let mut enum_candidates = enum_candidates.iter()
+                    .map(|suggestion| import_candidate_to_paths(&suggestion)).collect::<Vec<_>>();
+                enum_candidates.sort();
+                for (sp, variant_path, enum_path) in enum_candidates {
+                    let msg = format!("there is an enum variant `{}`, did you mean to use `{}`?",
+                                      variant_path,
+                                      enum_path);
+                    if sp == DUMMY_SP {
+                        err.help(&msg);
+                    } else {
+                        err.span_help(sp, &msg);
+                    }
+                }
             }
             if path.len() == 1 && this.self_type_is_available() {
                 if let Some(candidate) = this.lookup_assoc_candidate(name, ns, is_expected) {
@@ -3424,6 +3440,22 @@ fn path_names_to_string(path: &Path) -> String {
     names_to_string(&path.segments.iter().map(|seg| seg.identifier).collect::<Vec<_>>())
 }
 
+/// Get the path for an enum and the variant from an `ImportSuggestion` for an enum variant.
+fn import_candidate_to_paths(suggestion: &ImportSuggestion) -> (Span, String, String) {
+    let variant_path = &suggestion.path;
+    let variant_path_string = path_names_to_string(variant_path);
+
+    let path_len = suggestion.path.segments.len();
+    let enum_path = ast::Path {
+        span: suggestion.path.span,
+        segments: suggestion.path.segments[0..path_len - 1].to_vec(),
+    };
+    let enum_path_string = path_names_to_string(&enum_path);
+
+    (suggestion.path.span, variant_path_string, enum_path_string)
+}
+
+
 /// When an entity with a given name is not available in scope, we search for
 /// entities with that name in all crates. This method allows outputting the
 /// results of this search in a programmer-friendly way

src/test/compile-fail/issue-35675.rs

@@ -0,0 +1,67 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+enum Fruit {
+    Apple(i64),
+    //~^ HELP there is an enum variant `Fruit::Apple`, did you mean to use `Fruit`?
+    //~| HELP there is an enum variant `Fruit::Apple`, did you mean to use `Fruit`?
+    Orange(i64),
+}
+
+fn should_return_fruit() -> Apple {
+    //~^ ERROR cannot find type `Apple` in this scope
+    //~| NOTE not found in this scope
+    Apple(5)
+    //~^ ERROR cannot find function `Apple` in this scope
+    //~| NOTE not found in this scope
+    //~| HELP possible candidate is found in another module, you can import it into scope
+}
+
+fn should_return_fruit_too() -> Fruit::Apple {
+    //~^ ERROR expected type, found variant `Fruit::Apple`
+    //~| NOTE not a type
+    Apple(5)
+    //~^ ERROR cannot find function `Apple` in this scope
+    //~| NOTE not found in this scope
+    //~| HELP possible candidate is found in another module, you can import it into scope
+}
+
+fn foo() -> Ok {
+    //~^ ERROR expected type, found variant `Ok`
+    //~| NOTE not a type
+    //~| HELP there is an enum variant
+    //~| HELP there is an enum variant
+    Ok(())
+}
+
+fn bar() -> Variant3 {
+    //~^ ERROR cannot find type `Variant3` in this scope
+    //~| NOTE not found in this scope
+}
+
+fn qux() -> Some {
+    //~^ ERROR expected type, found variant `Some`
+    //~| NOTE not a type
+    //~| HELP there is an enum variant
+    //~| HELP there is an enum variant
+    Some(1)
+}
+
+fn main() {}
+
+mod x {
+    enum Enum {
+        Variant1,
+        Variant2(),
+        Variant3(usize),
+        //~^ HELP there is an enum variant `x::Enum::Variant3`, did you mean to use `x::Enum`?
+        Variant4 {},
+    }
+}