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/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 {},
+    }
+}