ThreadAlone<T>

This is a fork of dtolnay's threadbound crate, which allows implementing Send on non-Copy types.

The reason that the original crate does not allow it is, if a ThreadBound object dropped on another thread from where it was created, it cannot be handled in any way. Instead, this crate aborts if that happens: so be very cautious when using this crate on multi-threaded environemnt.

ThreadBound<T>

ThreadBound is a wrapper that binds a value to its original thread. The wrapper gets to be Sync and Send but only the original thread on which the ThreadBound was constructed can retrieve the underlying value.

[dependencies]
threadbound = "0.1"

Version requirement: rustc 1.31+

Example

extern crate threadbound;

use std::marker::PhantomData;
use std::rc::Rc;
use std::sync::Arc;
use threadbound::ThreadBound;

// Neither Send nor Sync. Maybe the index points into a
// thread-local interner.
#[derive(Copy, Clone)]
struct Span {
    index: u32,
    marker: PhantomData<Rc<()>>,
}

// Error types are always supposed to be Send and Sync.
// We can use ThreadBound to make it so.
struct Error {
    span: ThreadBound<Span>,
    message: String,
}

fn main() {
    let err = Error {
        span: ThreadBound::new(Span {
            index: 99,
            marker: PhantomData,
        }),
        message: "fearless concurrency".to_owned(),
    };

    // Original thread can see the contents.
    assert_eq!(err.span.get_ref().unwrap().index, 99);

    let err = Arc::new(err);
    let err2 = err.clone();
    std::thread::spawn(move || {
        // Other threads cannot get access. Maybe they use
        // a default value or a different codepath.
        assert!(err2.span.get_ref().is_none());
    });

    // Original thread can still see the contents.
    assert_eq!(err.span.get_ref().unwrap().index, 99);
}

License