Handle closures

[Munksgaard]

There are two issues here: linearity needs to be checked inside of closures, and we need to handle moved variables

[Manishearth]

I believe the ctxt has some data on closure capture clauses in a map.

[Manishearth]

http://doc.rust-lang.org/rustc/middle/ty/type.UpvarCaptureMap.html, I think?

[Munksgaard]

http://doc.rust-lang.org/rustc/middle/ty/fn.closure_upvars.html, maybe?

[Manishearth]

Oh, forgot that middle::ty has a helper funciton for everything you'll ever need ;)

[Munksgaard]

It occured to me that you could actually just check the def_map to see whether or not something is an upvar. The result is b1782ad. However, I realized that we actually have a bigger problem: we cannot guarantee that a closure will ever be run!

Consider the following code, which is basically just a stripped version of the test from b1782ad:

let x = Foo;
let cl = || {
    close(x);
};

cl is a closure that has captured x and will correctly close it, but only if it is actually run. I don't think we can actually tell whether or not that is the case.

[Manishearth]

There's also an upvar capture map somewhere in the tcx.

We should assume that closures that capture droppy things by move will be run once.

[Munksgaard]

I don't like that assumption, but I'm also not sure that there's actually anything else we can do.

[Manishearth]

I think it's reasonable.

Note that a closure capturing by move will be FnOnce unless it's capturing by move via move || (in which case it may or may not be. So those closures are run either zero times or once.

Since we don't know when it's going to happen, we assume that "once" is bad.

Alternatively we can track the closure dependency and when it gets run but that sounds complicated.

[Munksgaard]

What do you mean "once" is bad?

But yes, it would be very nasty to track the closure.

[Manishearth]

@Munksgaard I mean that we disallow closures of this form.

Instantiating a closure that takes an upvar by-move is a move. This is similar to let x = foo(chan) in an abstract sense.