try-blocks and label-break-value do not work together.

[jonhoo]

I've been playing around with label-break-value today, and it comes in super handy in async blocks. However, it seems to run into issues when combined with the try_blocks feature.

I tried this code:

#![feature(try_blocks, label_break_value)]

fn main() {
    let _: Result<(), ()> = try {
        'foo: {
            Err(())?;
            break 'foo;
        }
    };
}

I expected to see this happen: the code should compile just fine.

Instead, this happened:

error[E0695]: unlabeled `break` inside of a labeled block
 --> src/main.rs:6:20
  |
6 |             Err(())?;
  |                    ^ `break` statements that would diverge to or through a labeled block need to bear a label

error: aborting due to previous error

This is a continuation of the discussion here. As @ciphergoth pointed out, this code (following the syntactic sugar from the label-break-value RFC) compiles:

#![feature(try_blocks, label_break_value)]

fn main() {
    let _: Result<(), ()> = try {
        'foo: loop {
            break {
                Err(())?;
                break 'foo;
            }
        }
    };
}

Meta

rustc --version --verbose:

rustc 1.45.0-nightly (a74d1862d 2020-05-14)

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

[nikomatsakis]

So the error is generated here

rust/src/librustc_passes/loops.rs

Lines 75 to 78 in 7f940ef

	if self.require_label_in_labeled_block(e.span, &label, "break") {
	// If we emitted an error about an unlabeled break in a labeled
	// block, we don't need any further checking for this break any more
	return;

if I had to guess, I would assume that this function

rust/src/librustc_passes/loops.rs

Lines 200 to 205 in 7f940ef

	fn require_label_in_labeled_block(
	&mut self,
	span: Span,
	label: &Destination,
	cf_type: &str,
	) -> bool {

ought to be modified to check whether the span.is_desugaring(QuestionMark) and, if so, not to emit the error. (See is_desugaring rust docs for a bit more info.)

[nikomatsakis]

Tagged as E-mentor since there are some instructions above.

[samrat]

@rustbot claim