diagnostic could be better whe using "..." instead of ".." when destructuring enum

[matthiaskrgr]

enum MyEnum {
    A(i32),
    B(u32),
}

fn use_enum(e: MyEnum) {
    if let MyEnum::A(...) = e {
        println!("got enum")
    };
}

The problem here is that we have if let MyEnum::A(...) = e instead of if let MyEnum::A(..) = e

The compiler however points towards ) in the error message instead of the superfluous .:

 error: unexpected token: `)`
 --> src/main.rs:7:25
  |
7 |     if let MyEnum::A(...) = e {
  |                         ^

error: aborting due to previous error

Fun fact: when using four dots MyEnum::A(....), it correctly points at the last dot :)

error: unexpected token: `.`
 --> src/main.rs:7:25
  |
7 |     if let MyEnum::A(....) = e {
  |                         ^

error: aborting due to previous error

This issue has been assigned to @rakshith-ravi via this comment.

[rakshith-ravi]

@matthiaskrgr seems like a simple issue to fix. If I were to take a shot at fixing this, where would I start?

[Centril]

Mentoring instructions:

Add a branch after this one...

rust/src/librustc_parse/parser/pat.rs

Lines 294 to 297 in cdb50c6

	} else if self.check(&token::DotDot) && !self.is_pat_range_end_start(1) {
	// A rest pattern `..`.
	self.bump(); // `..`
	PatKind::Rest

...but instead of self.check(&token::DotDot), check that self.token.kind == token::DotDotDot.

This should be interpreted as a ::Rest as in the previous branch, but an error should also be emitted. (Add the code for error emission in a separate method, with an appropriate .span_suggestion(...), where the span of the ... is replaced with ... See fn recover_intersection_pat for an example on emitting an error with a suggestion.)

[rakshith-ravi]

Sure thing! Will get to it right after work. Thanks for the pointers.

[Centril]

@rustbot assign @rakshith-ravi