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use std::fmt::{self, Debug};
use std::thread::{self, ThreadId};
/// ThreadBound is a Sync-maker and Send-maker that allows accessing a value
/// of type T only from the original thread on which the ThreadBound was
/// constructed.
pub(crate) struct ThreadBound<T> {
value: T,
thread_id: ThreadId,
}
unsafe impl<T> Sync for ThreadBound<T> {}
// Send bound requires Copy, as otherwise Drop could run in the wrong place.
//
// Today Copy and Drop are mutually exclusive so `T: Copy` implies `T: !Drop`.
// This impl needs to be revisited if that restriction is relaxed in the future.
unsafe impl<T: Copy> Send for ThreadBound<T> {}
impl<T> ThreadBound<T> {
pub(crate) fn new(value: T) -> Self {
ThreadBound {
value,
thread_id: thread::current().id(),
}
}
pub(crate) fn get(&self) -> Option<&T> {
if thread::current().id() == self.thread_id {
Some(&self.value)
} else {
None
}
}
}
impl<T: Debug> Debug for ThreadBound<T> {
fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
match self.get() {
Some(value) => Debug::fmt(value, formatter),
None => formatter.write_str("unknown"),
}
}
}
// Copy the bytes of T, even if the currently running thread is the "wrong"
// thread. This is fine as long as the original thread is not simultaneously
// mutating this value via interior mutability, which would be a data race.
//
// Currently `T: Copy` is sufficient to guarantee that T contains no interior
// mutability, because _all_ interior mutability in Rust is built on
// std::cell::UnsafeCell, which has no Copy impl. This impl needs to be
// revisited if that restriction is relaxed in the future.
impl<T: Copy> Copy for ThreadBound<T> {}
impl<T: Copy> Clone for ThreadBound<T> {
fn clone(&self) -> Self {
*self
}
}
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