Keep track of initialized bytes in read_to_end

[Darksonn]

This keeps track of the number of initialized bytes inside read_to_end.

Note that old implementation is actually unsound as replacing the buffer inside ReadBuf does not require unsafe, and it does not check whether the pointer has been changed.

// This should no longer even be possible in safe Rust. An implementor
// would need to have unsafely *replaced* the buffer inside `ReadBuf`,
// which... yolo?
assert!(new_len <= buf.capacity());

[carllerche]

We probably want to wait for #3428 to get merged into master before merging this.

[carllerche]

Thanks

[vitalyd]

@Darksonn a small suggestion for VecWithInitialized<V>: use a sealed trait to constrain V to be a Vec<u8> or &mut Vec<u8>. As it's written now, VecWithInitialized::new is unsafe because it requires V to be one of those 2 types, but the generic is constrained to just V: AsMut<Vec<u8>>.

Perhaps a formulation like the following would be better:

mod private {
   pub trait VecU8: AsMut<Vec<u8>> {} // better name welcome

   impl VecU8 for Vec<u8> {}
   impl VecU8 for &mut Vec<u8> {}
}

pub struct VecWithInitialized<V: VecU8> { ... }

Just an idea in passing ... 😄

[Darksonn]

Feel free to submit a PR with that!

[vitalyd]

Ok, let me try to find a bit of time to do that 😁. Mostly wanted to run that by you in case you considered and rejected the idea for some reason.