Unify errors for Unit pattern on Tuple and Struct variants

[djrenren]

Code Example (playground):

enum MyEnum {
    Tuple(i32),
    Struct { s: i32 },
}

fn foo(en: MyEnum) {
    match en {
        MyEnum::Tuple => "",
        MyEnum::Struct => "",
    };
}

Currently we provide a "do you mean..." for the Struct but not for the Tuple. We probably should provide one for both.

[Phosphorus15]

Consider fixing #63988 Btw (which is a similar enhancement)

[almielczarek]

I'd like to work on this if it is still available.

[estebank]

@almielczarek go ahead!

Ideally, you could turn the output into

error[E0532]: expected unit struct/variant or constant, found tuple variant `MyEnum::Tuple`
 --> src/lib.rs:8:9
  |
8 |         MyEnum::Tuple => "",
  |         ^^^^^^^^^^^^^ help: missing arguments: `MyEnum::Tuple(_)`

error[E0532]: expected unit struct/variant or constant, found struct variant `MyEnum::Struct`
 --> src/lib.rs:9:9
  |
9 |         MyEnum::Struct => "",
  |         ^^^^^^^^^^^^^^ help: missing struct fields: `MyEnum::Struct { s }`

but just changing the tuple case would be enough.

[guanqun]

@estebank I created a PR, could you please help review. I took a look at your suggestion of updating all the errors to help: missing struct fields.., that seemed too many changes, not sure if it was necessary, so I just added the tuple case.