Make AtomicBool the same size as bool

[Amanieu]

Reopening #32365

This allows AtomicBool to be transmuted to a bool, which makes it more consistent with the other atomic types. Note that this now guarantees that the atomic type will always contain a valid bool value, which wasn't the case before (due to fetch_nand).

r? @alexcrichton

[alexcrichton]

@bors: r+ 915fa57

Thanks!

[alexcrichton]

Note that the libs team discussed this change during triage the other day and the conclusion was that we're amenable to this with the other recent changes to std::sync::atomic

[bors]

⌛️ Testing commit 915fa57 with merge 8492b6a...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild

[ubsan]

@Amanieu Is this a guarantee? Like, on all platforms, all compilers, AtomicBool can be transmuted to bool? Or just specific platforms?

[Amanieu]

As it is currently implemented, a transmute will work fine. However I am not sure whether we should make this guarantee to users. Note that currently, atomic-rs relies on AtomicU{8,16,32,64} being transmutable to & from their respective integer type.

[alexcrichton]

When discussed with the libs team, the conclusion was that AtomicBool will not be able to be transmuted to bool, or rather it's not a guarantee that's provided.

[durka]

This was a breaking change because people (edit: people such as @aturon) were previously transmuting usize to AtomicBool (see #33724).