Prevent double boxing in `boxed`

[stepancheg]

future.boxed() creates a new box even if future itself is already a Box<Future>.

This can be inconvenient in this use case:

type SomeFuture = ...;
fn make_future() -> SomeFuture { ... }

fn accept_future<F : Future>(f: F) {
    f.boxed() ...
}

accept_future(make_future());

If SomeFuture is an alias for Box<Future> then accept_future makes a Box<Box<Future>>. Which could be worked around (e. g. by creating another function like

fn accept_future_boxed(future: Box<Future>) { ... }

but that would be inconvenient, and still won't prevent accidental wrapping futures in double boxes.

boxed could implemented like this:

fn boxed<F : Future + Sized + 'static>(f: F) -> Box<Future> {
    if TypeId::of::<F>() == TypeId::of::<Box<Future>>() {
        assert!(mem::size_of::<F>() == mem::size_of::<Box<Future>>());
        unsafe {
            let mut r: Box<Future> = mem::uninitialized();
            ptr::copy_nonoverlapping(&f as *const F, &mut r as *mut Box<Future> as *mut u8 as *mut F, 1);
            mem::forget(f);
            r
        }
    } else {
        Box::new(f)
    }
}

PR #512 implements boxed in Future and Stream this way.

[3Hren]

This possibly can be implemented without RTTI using something like Boxed trait using specializations.

[stepancheg]

This possibly can be implemented without RTTI using something like Boxed trait using specializations.

First, TypeId::of is not RTTI, because function parameter is a type, not a value. It is compile time type indentification, and condition is optimized away by compiler.

Second, if you know how to implement it with Boxed trait, please show me how. I tried, and failed.

[3Hren]

First, TypeId::of is not RTTI, because function parameter is a type, not a value. It is compile time type indentification, and condition is optimized away by compiler.

I'm not sure (but likely so). Docs/ASM proof?

Second, if you know how to implement it with Boxed trait, please show me how. I tried, and failed.

If I understood the issue correctly, here is the proof of concept: #513.

[3Hren]

Your tests from #512 passes. Decided not to copy/paste them nevertheless.

[stepancheg]

I'm not sure (but likely so). Docs/ASM proof?

https://godbolt.org/g/JdxAes

If I understood the issue correctly, here is the proof of concept: #513.

Thanks. It doesn't work in stable though.