Hint on missing tuple parens in pattern

[cramertj]

It'd be nice to smartly hint about missing tuple parentheses in patterns, especially in positions where parentheses nesting is required.

Example of incorrect code:

fn main() {
    match Some((1, 2)) {
        Some(x, y) => {}
        None => {}
    }
}

The error:

error[E0023]: this pattern has 2 fields, but the corresponding tuple variant has 1 field
 --> src/main.rs:3:9
  |
3 |         Some(x, y) => {}
  |         ^^^^^^^^^^ expected 1 field, found 2

This error is confusing, especially since Some is a "tuple pattern" which is confusable with its tuple field. An error that suggests missing tuple parentheses would be helpful.

This issue has been assigned to @sam09 via this comment.

[Centril]

I think the right approach here would be to check if the scrutinee type (expected type in this context) has a single field of a tuple type with as many fields as the tuple struct pattern has.

Place to change:

rust/src/librustc_typeck/check/pat.rs

Lines 678 to 688 in 53df91a

	let subpats_ending = if subpats.len() == 1 { "" } else { "s" };
	let fields_ending = if variant.fields.len() == 1 { "" } else { "s" };
	struct_span_err!(tcx.sess, pat.span, E0023,
	"this pattern has {} field{}, but the corresponding {} has {} field{}",
	subpats.len(), subpats_ending, res.descr(),
	variant.fields.len(), fields_ending)
	.span_label(pat.span, format!("expected {} field{}, found {}",
	variant.fields.len(), fields_ending, subpats.len()))
	.emit();
	on_error();
	return tcx.types.err;

If you tackle this, first please also refactor the snippet highlighted above into a diagnostics-only method and then do your changes in there.

[sam09]

Can I pick up this one? Doesn't look like anybody working on this.

[zackmdavis]

@sam09 Please do.

@rustbot assign @sam09

[sam09]

I think the right approach here would be to check if the scrutinee type (expected type in this context) has a single field of a tuple type with as many fields as the tuple struct pattern has.

variant.fields.len() gives me the number of fields of expected type, which should be 1 here. I have a condition, varaints.fields.len()==1. However I can't see how many fields varaints.field[0] has, when trying to compare to the fields the tuple struct pattern has (subpats.len() in this case)

Maybe I misunderstood it, or am missing something here

[sam09]

This #64161 seems to have changed some things here. Is this issue still relevant?

[Centril]

cc @estebank

Ostensibly it is still relevant as we could emit a specific suggestion in this case.

[estebank]

Yes, that PR only adds a bit of context that should generally be useful, but this is about adding a structured suggestion for this case in particular. This is more focused and more useful for this case in particular :)

Looking forward to it!