fn ok_or_errno(rc: c_int) -> Result<()>Expand description
The termios calls’ contract: 0, or -1 and the reason is in errno.
Read here rather than at the call site because nothing runs in between —
rc is already a value by the time this is entered, and Rust code cannot
touch errno — so the read is as prompt as an inline one, and the two call
sites in enter stop being four lines each.