match arm bindings have weird lifetimes

[arielb1]

I should have opened this issue a long while ago, but now I have a place in the code I want to link it to.

Match bindings have weird lifetime, especially when guards are involved: instead of the destructors executing when the bindings go out of scope, all of the destructors of all match bindings are run just after the match ends.

e.g. I think this causes unnecessary drop elaboration and etc. load on big functions with many-armed matches.

One very weird case is this:

    fn main() {
         match 0 {
             a | a
             if { println!("a={}", a); false } => {}
             _ => {}
         }
     }

Here, the guard is executed twice, and a is only storage-killed after the match ends, so we have storage-live -> storage-live -> storage-dead, which might annoy some things.

FIXME: remember more weird cases and clean them up.

[arielb1]

I think that it will be hard to fix this in a way AST borrowck and region inference will understand, but afterwards this is fair game.

[nikomatsakis]

I think that this fits into what @pnkfelix is doing in overhauling how we handle match arms?

[nikomatsakis]

I guess this is just a matter of generating better code...?

[pnkfelix]

Re-triaging for #56754. NLL-complete. P-medium. Assigning to self.