Pattern guard can consume value that is being matched

Following compiles, and crashes when trying to print moved from value:

fn main() {
    let a = Some("...".to_owned());
    let b = match a {
        Some(_) if { drop(a); false } => None,
        x                             => x,
    };
    println!("{:?}", b);
}

[apasel422]

CC rust-lang/rfcs#107 (comment), #27282.

[alexcrichton]

triage: T-lang I-nominated

I thought we fixed this :(

[Aatch]

@alexcrichton unfortunately no. I fixed a very similar issue involving multiple guards, but not this case.

I did try to fix it, but it essentially requires SEME to work. This is because preventing this case introduces edges between successive patterns in the CFG and consequentially causes code like this:

match foo {
  (ref a, 0) => (),
  (ref a, 1) => ()
  _ => ()
}

to be rejected because the borrow in the first pattern overlaps the borrow in the second pattern. The reason why this has to be this way is because both move checking and borrow checking are done using the CFG, right now the CFG for a match "visits" all the arms in parallel, with pattern guards chaining into each other where appropriate. Improving it further requires chaining from the guard to the next pattern instead, but also requires indicating that the borrow in the pattern ends at both the "else" branch of the guard and the end of the block for the arm. In other words, we need regions with multiple exits aka SEME.

[arielb1]

Can't we just borrow the discriminant in all guards separately (cc @pnkfelix)? Actually, that might cause conflicts with ref mut borrows, but that conflict can be ignored (actually, we may want to mark patterns as aliasable in guards).

This issue won't be really fixed with MIR if we are talking about bypassing-exhaustiveness rather than

[Aatch]

@arielb1 did you accidentally comment on the wrong issue? I'm not sure how discriminants and exhaustiveness come in here.

[nikomatsakis]

triage: P-high

[nikomatsakis]

I think this is one of those "fixed by moving region/borrowck to MIR"

[brson]

@pnkfelix Is this still on your radar? It's been assigned a while.