Support "while" and "while let"

[sanmai-NL]

I believe

@while let ...

isn't supported currently (0.12.0). If not, it would be a welcome addition besides @for and @if let.

[lambda-fairy]

I don't think plain @while is supported either.

I'll be happy to accept a PR for this. Should be easy to adapt from the code for @if.

The relevant parts of the code are in maud_macros/src/parse.rs and maud_macros/src/render.rs.

[lambda-fairy]

Don't worry about how I removed the "help wanted" label -- I still want help on this issue!

I just realized that since all issues here can be marked as "help wanted", the label itself is redundant 😄

[sanmai-NL]

Great crate @lfairy, by the way! Keep it up. I appreciate the request for contribution, but I'm a Rust beginner and haven't even had time so far to do much with Rust. I'll have to see whether I can contribute to Maud at all.

I have been reconsidering my request in the meantime. I may not have this down correctly, but I think in all Rust versions up to today (< 1.15) while loops are r-values but must evaluate to (). Some have petitioned for this to be changed, though. Anyway, that means that the obvious @while let statement could not evaluate to e.g. a maud::Markup value. That means that unlike @if and @for, it is useless to have @while let inside html! { ... }. The alternative would be to perform the while let statement before the html! { ... } and then use e.g. @for to pull out the maud::Markup values. What are your thoughts?

[lambda-fairy]

@sanmai-NL I can see where you're coming from, but the fact that these Rust control structures return () is a red herring!

Maud doesn't care about what these control structures return. Instead, the body of an @if or @for pushes to an implicit buffer, and it is this buffer that is used as the resulting Markup. In other words: the structure itself always evaluates to () anyway; it's their side effects that we care about. So the RFC isn't relevant to this issue.

You can convince yourself that this is the case by playing around with html_debug!. @while should be expanded in much the same way as @if and @for are now.

[utkarshkukreti]

@sanmai-NL if you want, I'd be happy to submit a PR for this.