Tracking Issue for thread spawn hooks

Feature gate: #![feature(thread_spawn_hook)]

This is a tracking issue for thread spawn hooks as proposed in https://github.com/rust-lang/rfcs/pull/3642

Public API

// std::thread:

pub fn add_spawn_hook<F, G>(hook: F)
where
    F: 'static + Send + Sync + Fn(&Thread) -> G,
    G: 'static + Send + FnOnce();

impl Builder {
    pub fn no_hooks(mut self) -> Builder;
}

Steps / History

• RFC: https://github.com/rust-lang/rfcs/pull/3642
• Implementation: https://github.com/rust-lang/rust/pull/125405
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• Should the return value of the hook be an Option, for when the hook does not require any code to be run in the child?
• Should the hook be able to access/configure more information about the child thread? E.g. set its stack size.
• What should we do when threads are spawned after TLS destruction? (E.g. in an atexit() handler.) Panic? (https://github.com/rust-lang/rust/issues/138696) Or skip them? (https://github.com/rust-lang/rust/pull/138702)
• Giving the hook access to &Thread before the thread has finished initializing means it can't observe the thread ID in most cases. Is that the right behavior?

Start by reading RFC 3642 and the implementation referenced in PR 125405, then review the unchecked FCP and stabilization steps. The unresolved questions about hook return values, child-thread configuration, TLS destruction, and thread IDs must be resolved before the feature can be stabilized.