Conditional function parameters dependent on trait bound

language design
1

Sometimes I would like to change the type of a function parameter or struct parameter depending on whether it satisfies a trait bound or not.

unsafe fn myMultiThreadedFunction<T> (sharedResource: (if T: Sync <Arc<MyAtomicTrait<T>>> else Arc<Mutex<T>>) {
    // snip
}

This is obviously just example syntax. This might include some unsafe Rust, though, as it has a similar basis to unions, with types not being available at compile time.

Do you think this should be implemented?

2

«or not» means what you're asking for is specialization, so it brings in all the usual problems and known unsoundness of that feature.

1 Like
3

This would be possible with (full) specialization, e.g. with something like this:

trait SyncSwitch {
    type Shared;
}

default impl<T> SyncSwitch for T {
    type Shared = Arc<Mutex<T>>;
}

impl<T: Sync> SyncSwitch for T {
    type Shared = Arc<MyAtomicTrait<T>>;
}

unsafe fn myMultiThreadedFunction<T> (sharedResource: <T as SyncSwitch>::Shared) {
    // snip
}

But specialization is currently unstable and incomplete, so it isn't possible right now. And because of the soundness problems with specialization it won't become possible in the near future.

4

I see, thanks for the response!