Rust: Tracking issue for RFC 2294, "if let guard"

[Centril]

This is a tracking issue for the RFC "if let guard" (rust-lang/rfcs#2294).

Steps:

• [ ] Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
• [ ] Adjust documentation (see instructions on forge)
• [ ] Stabilization PR (see instructions on forge)

Unresolved questions:

• [ ] Work out the exact semantics / borrowck (cc @nikomatsakis, @pnkfelix)

[imjacobclark]

:wave: After a conversation about this change with @varkor I'm starting to take a look at this with a view to implementing it!

[alexreg]

@imjacobclark Hey. Any progress over the past few months? :-)

[ebkalderon]

Just ran into a case where this ability would be useful. I look forward to this feature landing someday!

[Vincent-liuwingsang]

+1 This would be a nice feature to have!

[Silhalnor]

Am currently learning Rust and thought it was unintuitive that I couldn't write this syntax. (Thought I must be mis-writing it somehow.) Lo, here it is in progress! Woo~

[Boscop]

Any update on this? :)

[LeSeulArtichaut]

I was willing to take a look at this, but I didn't have time to... I'll try to free some time for this ASAP!

[LeSeulArtichaut]

Currently looking into this, though I'm not sure I will be able to implement all of it