Add SingleBox::leak to provide access to a &'static mut

[jannic]

The use case for the added function is to provide a &'static mut.
So this

    static mut BUFFER: [u8; 1024] = [0u8; 1024];
    let buf: &mut [u8; 1024] = unsafe { &mut BUFFER };

can be replaced by

   static BUFFER: AllocSingle<[u8; 1024]> = AllocSingle::new();
   let buf: &mut [u8; 1024] = BUFFER.alloc([0u8; 1024]).unwrap().leak();

This avoids using unsafe in user code. In case the second line is accidentally run twice, it'll cause a panic instead of UB. (And even the panic could be avoided by replacing unwrap by appropriate error handling.)

Actual situation where this would be useful: rp-rs/rp-hal#864

Please let me know if there's an easier way to do the same!

[thejpster]

Won't your example stack allocate the array in order to move it into the 'heap' allocation?

[jannic]

Won't your example stack allocate the array in order to move it into the 'heap' allocation?

Yes, of course. Could be avoided by using alloc_const_val.

[jamesmunns]

Do we need this extra 'b lifetime? If 'a is 'static already, won't returning &'a mut T already get you the static lifetime?

[jannic]

Not 100% sure!
If 'a is 'static and we want a &'static mut T, then sure, that would work.
But what if we actually need a shorter lifetime? I guess it's still fine because &'a T is covariant in 'a, https://doc.rust-lang.org/reference/subtyping.html#variance, so any &'a T is a subtype of &'static T for all lifetimes 'a?