Make the computation of coroutine_captures_by_ref_ty more sophisticated

[compiler-errors]

Currently, we treat all the by-(mut/)ref borrows of a coroutine-closure as having a "closure env" borrowed lifetime.

When we have the given code:

let x: &'a i32 = ...;
let c = async || {
    let _x = *x;
};

Then when we call:

c()
// which, because `AsyncFn` takes a `&self`, we insert an autoref:
(&c /* &'env {coroutine-closure} */)()

We will return a future whose captures contain &'env i32 instead of &'a i32, which is way more restrictive than necessary. We should be able to drop c while the future is alive since it's not actually borrowing any data originating from within the closure's captures, but since the capture has that 'env lifetime, this is not possible.

This wouldn't be true, for example, if the closure captured i32 instead of &'a i32, because the 'env lifetime is actually necessary since the data (i32) is owned by the closure.

This PR identifies two criteria where we need to take the borrow with the closure env lifetime:

1. If the closure borrows data from inside the closure's captures. This is not true if the parent capture is by-ref, OR if the parent capture is by-move and the child capture begins with a deref projection. This is the example described above.
2. If we're dealing with mutable references, since we cannot reborrow &'env mut &'a mut i32 into &'a mut i32, only &'env mut i32.

See the documentation on should_reborrow_from_env_of_parent_coroutine_closure for more info.

important: As disclaimer states on that function, luckily, if this heuristic is not correct, then the program is not unsound, since we still borrowck and validate the choices made from this function -- the only side-effect is that the user may receive unnecessary borrowck errors.

Fixes #123241

[compiler-errors]

The only thing this fails to do is assert that the parents captures are all used at least once. I'm tempted to use a coroutine for this and just use iter::from_coroutine...

[bors]

☔ The latest upstream changes (presumably #123708) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[compiler-errors]

r? oli-obk

[bors]

☔ The latest upstream changes (presumably #123725) made this pull request unmergeable. Please resolve the merge conflicts.

[oli-obk]

Nice (and great docs, thanks!)

@bors r+ rollup

[bors]

📌 Commit 599d456 has been approved by oli-obk

It is now in the queue for this repository.