Auto traits with closures

[arora-aman]

One way to use AssertUnwindSafe is to do something like test/mir_calls_to_shim.

fn assert_panics<F>(f: F) where F: FnOnce() {
    let f = panic::AssertUnwindSafe(f);
    let result = panic::catch_unwind(move || {
        f.0()
    });
    if let Ok(..) = result {
        panic!("diverging function returned");
    }
}

This would not work once capture_disjoint_fields is enabled since f.0 is captured, essentially rendering the wrapper redundant.

[nikomatsakis]

This is quite interesting, and similar to the problem where we capture subfields that are not Clone or entities which themselves are Clone

[arora-aman]

Similar issue with Send and Sync
https://github.com/rust-lang/rust/blob/de88bf148b122b27ce48a4a6679c41c834d33019/compiler/rustc_interface/src/util.rs#L121-L126

Here run.0 isn't Send and Sync anymore.

[arora-aman]

https://doc.rust-lang.org/nightly/nightly-rustc/rustc_trait_selection/infer/struct.InferCtxt.html#method.predicate_may_hold

[arora-aman]

example code: https://github.com/rust-lang/rust/blob/0148b971c921a0831fbf3357e5936eec724e3566/compiler/rustc_trait_selection/src/autoderef.rs#L127-L138