Use &mut [IoSlice] for request sending

[psychon]

XCBConnection creates a copy of the passed in slice of request buffers. So far, RustConnection does not, but instead switches between write_vectored and plain write for a buffer:

x11rb/src/rust_connection/mod.rs

Lines 682 to 683 in 314242b

	let remaining = &bufs[0][count..];
	write.write_all(remaining, fds)?;

I think it should be possible to get rid of that write part by using a mutable slice for request sending. That way, partial writes can be done by replacing the respective IoSlice.

[psychon]

Changing the public API from &[] to &mut [] is easy. Using this change in RustConnection::write_all_vectored then runs into lifetime problems. Oh and compute_length_field also ended up with lifetime problems. Someone who understands covariance and invariance may give this a shot.

Part of the problem is that I do not see how to implement https://doc.rust-lang.org/std/io/struct.IoSlice.html#method.advance on stable without unsafe code. When you have a &'b IoSlice<'a>, you can only get an &'b [u8], but would need an &'a [u8] to implement advance().

[notgull]

I created the advance crate as an "unsafe-quarantine microcrate" to allow for usage of the advance function in stable environments. Using that, we could probably resolve this issue.

[eduardosm]

IoSlice::advance will be stabilized in Rust 1.81