Add a const-constructors for sync primitives

[mental32]

Motivation

#2756 describes that the use of const-constructors with sync primitives and that there are currently no const-constructors for them.

Closes #2756

Solution

I've added a const_new public function to:

• loom::std::atomic_usize::AtomicUsize
• loom::std::parking_lot::Mutex
• sync::batch_semaphore::Semaphore
• sync::mutex::Mutex
• util::linked_list::LinkedList

All of the const_new constructors have the following cfg:

    #[cfg(all(
        feature = "parking_lot", 
        not(all(loom, test)),
    ))]

As specified by the concern in this comment

Disclaimer

I have not modified or added any new tests as I'm not sure what needs to be done on the testing side for these changes. also since the cfg specifies that the constructors should not be present when cfg(test)

[mental32]

There are some CI tests that I don't believe will work, due to a using feature(const_fn) this feature is however needed since if I try to build it on stable without it I get the following error:

   Compiling tokio v0.3.0 (/home/mental/Desktop/open_projects/tokio/tokio)
error[E0723]: trait bounds other than `Sized` on const fn parameters are unstable
  --> tokio/src/util/linked_list.rs:69:6
   |
69 | impl<T: Link> LinkedList<T> {
   |      ^
   |
   = note: see issue #57563 <https://github.com/rust-lang/rust/issues/57563> for more information
   = help: add `#![feature(const_fn)]` to the crate attributes to enable

error: aborting due to previous error

For more information about this error, try `rustc --explain E0723`.
error: could not compile `tokio`.

To learn more, run the command again with --verbose.

I'm not sure how to specify the cfg this PR so that these changes are only available on nightly.

[mental32]

I'm not sure how to specify the cfg this PR so that these changes are only available on nightly.

never mind, figured it out 😅

[hawkw]

I think we'd really prefer not to add a nightly feature flag to Tokio. The problem with using a feature flag for nightly-only code is that another dependency could enable the feature flag, resulting it being enabled in the downstream application's build regardless of whether or not the nightly compiler is actually being used. This would result in a broken build.

For stuff like this, I think we'd prefer to add a RUSTFLAGS cfg. Unlike a feature flag, these must be passed as an environment variable at build-time, and cannot be set in Cargo.toml, so only the top level build can enable them. However, I'd rather not require nightly at all, and I think if we're willing to make a somewhat more substantial change, it's possible to do this without needing nightly.

I'd like to hear what other maintainers think about this, but that's my perspective.

[mental32]

I'm not married to using a nightly feature flag or nightly at all for that matter so currently I'm open to any kind change :)

As for moving forward with this, I'm not quite sure what the next steps are.. Currently I'm watching this thread waiting for other maintainers opinions before doing anything so I feel like this is just blocked still atm.

[Darksonn]

Can you open a new PR that makes the change we described to LinkedList? Then we'll merge that and continue here.

[hawkw]

@mental32 btw, feel free to ping me if you get stuck on the LinkedList change. Thanks!

[mental32]

Sure thing! It may take a little while however so bear with me :)

[mental32]

All ready 👍 only blocked on #2805 for the moment

[Darksonn]

You should be able to merge in master now with the new LinkedList. Please prefer merges to rebases.

[mental32]

I think everything's good to go here 😄

[Darksonn]

I will merge it once CI passes.