regression: error[E0512]: cannot transmute between types of different sizes, or dependently-sized types (`Thread`)

[cuviper]

1. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/DoumanAsh.nng-c/log.txt
2. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/reg/thread-waker-1.1.0/log.txt
3. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/reg/utils-atomics-1.1.2/log.txt

They are all variants of transmuting Thread, including the Lock(Thread) in the 3rd one:

[INFO] [stdout] error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
[INFO] [stdout]   --> src/locks.rs:42:33
[INFO] [stdout]    |
[INFO] [stdout] 42 |                 return unsafe { core::mem::transmute(self) }
[INFO] [stdout]    |                                 ^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout]    |
[INFO] [stdout]    = note: source type: `Lock` (128 bits)
[INFO] [stdout]    = note: target type: `*mut ()` (64 bits)
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
[INFO] [stdout]   --> src/locks.rs:54:29
[INFO] [stdout]    |
[INFO] [stdout] 54 |                 return Self(core::mem::transmute(raw))
[INFO] [stdout]    |                             ^^^^^^^^^^^^^^^^^^^^
[INFO] [stdout]    |
[INFO] [stdout]    = note: source type: `*mut ()` (64 bits)
[INFO] [stdout]    = note: target type: `Thread` (128 bits)

I'm sure the answer is "don't do that!" -- but at least we can have an issue for reference.

Version it worked on

It most recently worked on: 1.83.0

Version with regression

Using rustc 1.84.0-beta.4 in crater #134138.

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged

[compiler-errors]

This bisects to #123550.

The regressed crates were probably relying on specific layout guarantees about the Thread struct which are no longer upheld after the internal representation of Thread changed (namely, that the size of Thread is no longer 64 bits).

[cuviper]

I don't think it was ever a guarantee, just Hyrum's Law in action.

[compiler-errors]

sorry, when I wrote guarantee I really meant assumptions. False guarantees :)

[cuviper]

The may crate had the courtesy to assert it first:

1. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/Alex-q-z.kvcachesim/log.txt
2. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/AnfriH.MechSync/log.txt
3. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/Netnix-SA.moquist/log.txt
4. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/Vitalick.rust-prj/log.txt
5. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/asagikaki.thaqqa/log.txt
6. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/jonataslaw.server_nano/log.txt
7. https://crater-reports.s3.amazonaws.com/beta-1.84.0-4-retry2/beta-2024-12-08/gh/natanaeladit.may/log.txt

[INFO] [stdout] error[E0080]: evaluation of constant value failed
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-6f17d22bba15001f/may-0.3.45/src/sync/spsc.rs:72:15
[INFO] [stdout]    |
[INFO] [stdout] 72 | const _: () = assert!(std::mem::size_of::<Thread>() == std::mem::size_of::<usize>());
[INFO] [stdout]    |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'assertion failed: std::mem::size_of::<Thread>() == std::mem::size_of::<usize>()', /opt/rustwide/cargo-home/registry/src/index.crates.io-6f17d22bba15001f/may-0.3.45/src/sync/spsc.rs:72:15
[INFO] [stdout]    |
[INFO] [stdout]    = note: this error originates in the macro `assert` (in Nightly builds, run with -Z macro-backtrace for more info)
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
[INFO] [stdout]   --> /opt/rustwide/cargo-home/registry/src/index.crates.io-6f17d22bba15001f/may-0.3.45/src/sync/spsc.rs:86:35
[INFO] [stdout]    |
[INFO] [stdout] 86 |         let mut handle = unsafe { std::mem::transmute(thread) };
[INFO] [stdout]    |                                   ^^^^^^^^^^^^^^^^^^^
[INFO] [stdout]    |
[INFO] [stdout]    = note: source type: `Thread` (128 bits)
[INFO] [stdout]    = note: target type: `NonZero<usize>` (64 bits)

That appears to be fixed in may v0.3.47: Xudong-Huang/may@e965379

[Mark-Simulacrum]

cc #132619 (another report of the same problem)

It looks like this is almost ready to be fixed by landing #132654, so a future release will probably "fix" these crates again -- but since this isn't a stable implementation detail it feels reasonable to let Thread change in size back and forth a few times.

[Mark-Simulacrum]

I'm going to go ahead and close this as won't fix; I don't think it merits compat notes that are something like "standard library types may change in size". The breakage seems fairly limited too.