… sufficiently aligned

Co-Authored-By: Vadim Petrochenkov <[email protected]>

Miri tests: use xargo to build separate libstd

This uses `cargo miri setup` to prepare the libstd that is used for testing Miri, instead of adjusting the entire bootstrap process to make sure the libstd that already gets built is fit for Miri.

The issue with our current approach is that with `test-miri = true`, libstd and the test suite get built with `--cfg miri`, which e.g. means hashbrown uses no SIMD, and not all things are tested. Such global side-effects seem like footguns waiting to go off.

On the other hand, the new approach means we install xargo as a side-effect of doing `./x.py test src/tools/miri`, which might be surprising, and we also both have to build xargo and another libstd which costs some extra time. Not sure if the tools builders have enough time budget for that. Maybe there is a way to cache xargo?

We have to first first land rust-lang/miri#870 in Miri and then update this PR to include that change (also to get CI to test Miri before bors), but I wanted to get the review started here.

Cc @oli-obk (for Miri) @alexcrichton (for CI) @Mark-Simulacrum (for bootstrap)

Fixes rust-lang#61833, fixes rust-lang#63219

Don't recommend `extern crate` syntax

`extern crate` syntax is not a good recommendation any more, so I've changed it to just print a suggested crate name.

gitignore: add comment explaining policy

Based on rust-lang#63307 (comment), I added a comment what I think should be gitignored and what not. This is just a proposal, obviously.  Also see rust-lang#53768 for some more discussion.

The summary is that if there are junk files that you create locally and are fine leaving around (such as `mir_dump`), git has the option for you to add them to `.git/info/exclude`. Others might prefer to keep their working dir clean of those same junk files, so we shouldn't just ignore them for everyone.

I then also cleaned up a few more things, but there were many things that I had no idea where they came from so I didn't touch them.

move of packed fields might or might not occur when they actually are sufficiently aligned

See taiki-e/pin-project#34, where it was pointed out that we actually don't move fields of 1-aligned types when dropping a packed struct -- but e.g. in a `packed(2)` struct, we don't do something similar for 2-aligned types. The code for that is [here](https://github.com/rust-lang/rust/blob/db7c773a6be2f050d1d1504763819ea3916f5428/src/librustc_mir/util/alignment.rs#L7).

reduce visibility

r? @petrochenkov

…sts, r=cramertj

Test interaction between `async { ... }` and `?`, `return`, and `break`

Per the second checkbox in rust-lang#62121 (comment), test that `async { .. }` blocks:
1. do not allow `break` expressions.
2. get targeted by `return` and not the parent function.
3. get targeted by `?` and not the parent function.

Works towards resolving blockers in rust-lang#63209.

r? @cramertj