Summary

Introduce a new while let PAT = EXPR { BODY } construct. This allows for using a refutable pattern match (with optional variable binding) as the condition of a loop.

Motivation

Just as if let was inspired by Swift, it turns out Swift supports while let as well. This was not discovered until much too late to include it in the if let RFC. It turns out that this sort of looping is actually useful on occasion. For example, the desugaring for loop is actually a variant on this; if while let existed it could have been implemented to map for PAT in EXPR { BODY } to

// the match here is so `for` can accept an rvalue for the iterator,
// and was used in the "real" desugaring version.
match &mut EXPR {
    i => {
        while let Some(PAT) = i.next() {
            BODY
        }
    }
}

(note that the non-desugared form of for is no longer equivalent).

More generally, this construct can be used any time looping + pattern-matching is desired.

This also makes the language a bit more consistent; right now, any condition that can be used with if can be used with while. The new if let adds a form of if that doesn’t map to while. Supporting while let restores the equivalence of these two control-flow constructs.

Detailed design

while let operates similarly to if let, in that it desugars to existing syntax. Specifically, the syntax

['ident:] while let PAT = EXPR {
    BODY
}

desugars to

['ident:] loop {
    match EXPR {
        PAT => BODY,
        _ => break
    }
}

Just as with if let, an irrefutable pattern given to while let is considered an error. This is largely an artifact of the fact that the desugared match ends up with an unreachable pattern, and is not actually a goal of this syntax. The error may be suppressed in the future, which would be a backwards-compatible change.

Just as with if let, while let will be introduced under a feature gate (named while_let).

Drawbacks

Yet another addition to the grammar. Unlike if let, it’s not obvious how useful this syntax will be.

Alternatives

As with if let, this could plausibly be done with a macro, but it would be ugly and produce bad error messages.

while let could be extended to support alternative patterns, just as match arms do. This is not part of the main proposal for the same reason it was left out of if let, which is that a) it looks weird, and b) it’s a bit of an odd coupling with the let keyword as alternatives like this aren’t going to be introducing variable bindings. However, it would make while let more general and able to replace more instances of loop { match { ... } } than is possible with the main design.

Unresolved questions