Construct an SpareCapacity, which implements Buffer.

This wraps a Vec and uses the spare capacity of the Vec as the buffer to receive data in, automatically calling set_len on the Vec to set the length to include the received elements.

This uses the existing capacity, and never allocates, so the Vec should have some non-empty spare capacity!

Examples

# fn test(input: rustix::fd::BorrowedFd) -> rustix::io::Result<()> {
use rustix::buffer::spare_capacity;
use rustix::io::{read, Errno};

let mut buf = Vec::with_capacity(1024);
match read(input, spare_capacity(&mut buf)) {
    Ok(0) => { /* end of stream */ }
    Ok(n) => { /* `buf` is now `n` bytes longer */ }
    Err(Errno::INTR) => { /* `buf` is unmodified */ }
    Err(e) => {
        return Err(e);
    }
}

# Ok(())
# }