Context: I'm preparing a presentation about error handling in Rust and it looks like a crate containing write_err! and input_string would make sense. In a way it increases our maintenance burden but it also makes some things easier by allowing us to have InputString in public API. It's also so trivial that it could be stabilized ~immediately which could give us more experience with the process. (BTW I have a recent experienced: yesterday I've stabilized my first crate! Also see MSRV section for some fun.)

To be clear: this doesn't propose to move all error stuff, in particular, it won't move the ParseIntError which is unrelated to no-std support.