Add AsyncWriteExt::write_all_vectored

[davidpdrsn]

In #3678 we added AsyncWriteExt::write_vectored. It would be nice to also provide AsyncWriteExt::write_all_vectored that writes all the data similarly to std::io::Write::write_all_vectored.

[davidpdrsn]

This should wait for std::io::Write to stabilize as we want to mirror the API.

The implementation will probably also use on IoSlice::advance which is also unstable.

[Tnze]

Is there any workaround? How could we manually use write_vectored to ensure write all data?

[Darksonn]

Manually? You can call it in a loop.

[Tnze]

Manually? You can call it in a loop.

Actually, no. Because if the first call doesn't finish the work, you can't write starting at the latest position in the second call.

[Darksonn]

You have to update the slice to skip the bytes at the start. You can use IoSlice::advance_slices to do that. The method is unstable, but you can copy its implementation to do it on stable Rust.

[Arnavion]

IoSlice{,Mut}::advance{,_slices} are now stable in 1.81