Replace once_cell crate from librustc_datastructures with an (unstable) impl from std

[matklad]

After #75648 is merged, usage of once_cell in this module:

https://github.com/rust-lang/rust/blob/5b04bbfcbb79ed7325ea2b580458a80d95da6bbb/src/librustc_data_structures/sync.rs

should be replaced by std::[OnceCell|SyncOnceCell]. The dependency from corresponding Cargo.toml can be removed afterwards.

It might make sense to request a perf run, as crates.io version uses parking_lot, while the standard one uses std mutex, but I expect the difference to not matter -- fast path is equivalent for both.

[marmeladema]

@bors claim

[matklad]

#75648 is done!

Though, I am not unsure if we maybe should wait until it tickles down to beta? Not sure how bootstraping works with libstd, I guess, we can just try and see if the build fails.

[Mark-Simulacrum]

The compiler always builds with the in-tree standard library, so cfg(bootstrap) should never be necessary (beyond lints, and other compiler-implemented rather than library-implemented) details.

[matklad]

ping @marmeladema, this is ready to be implemented.

[marmeladema]

I'll start soon! Sorry I got a bit overwhelmed with work. If in the next couple of days I haven't replied feel free to re-assign.

[marmeladema]

On it!

[marmeladema]

@matklad once this is merged, should other sub-crates that depend on once_cell also be updated? Like librustc_interface for example. Also, what about lazy_static? Some usage can probably be replaced by Lazy.

[matklad]

We definitely should actively remove once_cell crate from all rustc’s crates, because having two difference sources of the same data structure feels bad.

Replacing lazy_static with std::lazy would also be good!

[marmeladema]

We definitely should actively remove once_cell crate from all rustc’s crates, because having two difference sources of the same data structure feels bad.

Replacing lazy_static with std::lazy would also be good!

I'll try to take of it once this issue is fixed 👍