Dropped variables still included in generator type

[Nemo157]

struct Foo;
impl !Send for Foo {}

let _: impl Send = || {
    let guard = Foo;
    drop(guard);
    yield;
};

(full playground) fails with

error[E0277]: `Foo` cannot be sent between threads safely
  --> src/main.rs:14:12
   |
14 |     let _: impl Send = || {
   |            ^^^^^^^^^ `Foo` cannot be sent between threads safely
   |
   = help: within `[generator@src/main.rs:14:24: 18:6 {Foo, ()}]`, the trait `std::marker::Send` is not implemented for `Foo`
   = note: required because it appears within the type `{Foo, ()}`
   = note: required because it appears within the type `[generator@src/main.rs:14:24: 18:6 {Foo, ()}]`

The guard should be dead and deallocated before the yield point so shouldn't appear in the generator type and affect the Sendness. Wrapping the guard in a new scope before the yield avoids this (included in the playground). First noticed in relation to async functions on u.rl.o.

[Aaron1011]

I'd like to work on this.

[Aaron1011]

This is going to be tricky.

The error is occurring due to the computed generator witness type including Foo. This is done during initial type checking, before any MIR has been generated.

Making this work would require type resolution to depend on the results of NLL. Specifically, the computed generator witness type would have to depend on which locals are computed to be live during mir-borrowck.

@Zoxc @nikomatsakis: Thoughts?

[Zoxc]

My plan for this is just to generate MIR for just the generator during type checking and then do the analysis on MIR. Currently that isn't very feasible given the current compiler structure.