Improving an error message: "cannot bind by-move into a pattern guard"

[catamorphism]

Based on an IRC conversation just now, I can think of two ways to improve this error message:

1. Because pattern guards use the "if" keyword, someone may have written one by accident. Then, if they see this error message, they're going to ask "what's a pattern guard"? The error could explain this (it's a little hard to figure out right now since the entire pattern gets underlined, not the guard).
2. Probably more importantly, the error should suggest what to do to fix it.

The error in question is in rustc::middle::check_match.

[emberian]

Triage bump

[thestinger]

Still an issue.

[flaper87]

visiting for triage

[dhardy]

Re: 2 — how to solve this problem?

Matching on a reference (match &obj {...}) works until you want to move the object, which was presumably the point of making it movable in the first case.

Can match substitute variables bound by the pattern (p in this case) with a reference while evaluating the guard and release the reference again before the block? Example of problem:

for entry in try!(fs::readdir(some_path)).move_iter(){
    match &entry {
        p if /* p a borrowing reference here */ p.is_file() => {
             /* p owned, reference released here */ ...
        }, ...
    }
}

The best solution I have for now is to use a temporary control variable and match twice.

[steveklabnik]

I'm pulling a massive triage effort to get us ready for 1.0. As part of this, I'm moving stuff that's wishlist-like to the RFCs repo, as that's where major new things should get discussed/prioritized.

This issue has been moved to the RFCs repo: rust-lang/rfcs#684