Ignore variadic FFI test on AArch64
I've cross compiled Rust to `aarch64-linux-gnu`, and tried to run the compile-fail tests, but `variadic-ffi.rs` fails with the following error:
```
The ABI `"stdcall"` is not supported for the current target [E0570]
```
The test seems to be ignored on (32-bit) ARM, so I turned it off for AArch64 too.
Changing error message from `contains interior mutability` to `may contain interior mutability`
Fixes#40313 . I have changed the message from `contains interior mutability` to `may contain interior mutability` for the following example
```
use std::cell::Cell;
use std::panic::catch_unwind;
fn main() {
let mut x = Cell::new(22);
catch_unwind(|| { x.set(23); });
}
```
which has been added as a ui test.
Also, the message [here](https://github.com/gaurikholkar/rust/blob/master/src/librustc_mir/transform/qualify_consts.rs#L666) and it's respective `compile-fail` test have been modified.
cc @nikomatsakis @Mark-Simulacrum @eddyb
Change for-loop desugar to not borrow the iterator during the loop
This is enables the use of suspend points inside for-loops in movable generators. This is illegal in the current desugaring as `iter` is borrowed across the body.
Fix 'associate type' typo
I came across an error message mentioning an 'associate type'.
Since this is the only instance of this term in rustc (it's 'associated type' everywhere else), I think this might be a typo.
Make assignments to `Copy` union fields safe
This is an accompanying PR to PR https://github.com/rust-lang/rust/pull/42068 stabilizing FFI unions.
This was first proposed in https://github.com/rust-lang/rust/issues/32836#issuecomment-281296416, see subsequent comments as well.
Assignments to `Copy` union fields do not read any data from the union and are [equivalent](https://github.com/rust-lang/rust/issues/32836#issuecomment-281660298) to whole union assignments, which are safe, so they should be safe as well. This removes a significant number of "false positive" unsafe blocks, in code dealing with FFI unions in particular.
It desirable to make this change now, together with stabilization of FFI unions, because now it affecfts only unstable code, but later it will cause warnings/errors caused by `unused_unsafe` lint in stable code.
cc #32836
r? @nikomatsakis
add thiscall calling convention support
This support is needed for bindgen to work well on 32-bit Windows, and also enables people to begin experimenting with C++ FFI support on that platform.
Fixes#42044.
