Can't use label for while loop

[chris-morgan]

Say!
I like labelled breaks/continues!
I do! I like them, Sam-I-am!
And I would use them in a boat!
And I would use them with a goat...
And I will use them in the rain.
And in the dark. And on a train.
And in a car. And in a tree.
They are so good, so good, you see!

So I will use them in a box.
And I will use them with a fox.
And I will use them in a house.
And I will use them with a mouse.
And I will use them here and there.
Say! I will use them ANYWHERE!

I will use them with a for loop.
And I will use with a loop loop.
Say! I will use them ANYWHERE!
… except in a while loop.

Because they're not supported there.

[chris-morgan]

Code examples:

fn main() {
    'eggs: loop {
        'ham: for _ in range(0, 0) {
            if 1 + 1 == 2 {
                break 'eggs;
            } else {
                continue 'ham;
            }
        }
    }
}

Compiles fine.

But with a while loop:

fn main() {
    'eggs: while 1 + 1 == 2 {
        break 'eggs;
    }
}

It doesn't compile:

green.rs:2:12: 2:17 error: expected `for` or `loop` after a label
green.rs:2     'eggs: while 1 + 1 == 2 {
                      ^~~~~

[edwardw]

Should it? The AST node definition for while loop is ExprWhile(@Expr, P<Block>); there is no label there.

[huonw]

That's only because while doesn't have support for labelled breaks, a label would only be added if/when the compiler itself supported codegen for labelled whiles.

[pnkfelix]

nominating.

I believe that, if necessary, we can add support for labelled whiles after 1.0 backwards compatibly. But I have to admit, it is super-surprising that loop { } supports it while while cond { } does not.

Therefore, I am recommending that we tag this as "P-high", but not add it to the 1.0 milestone.

[flaper87]

Sounds like we should support this.

@pnkfelix +1 for your tagging recommendation.

[pnkfelix]

P-high, but not 1.0 (we can add it backwards compatibly).

[Pythoner6]

I've done some work on this (initial commit: Pythoner6@7e9a2fe). Once I have time to rebase and re-run the tests, I'll be submitting a pull request.