change return type of slice_shift_char

[canndrew]

slice_shift_char splits a str into it's leading char and the remainder of the str. Currently, it returns a (Option<char>, &str) such that:

"bar".slice_shift_char() => (Some('b'), "ar")
"ar".slice_shift_char()  => (Some('a'), "r")
"r".slice_shift_char()   => (Some('r'), "")
"".slice_shift_char()    => (None,      "")

This is a little odd. Either a str can be split into both a head and a tail or it cannot. So the return type should be Option<(char, &str)>. With the current behaviour, in the case of the empty string, the str returned is meaningless - it is always the empty string.

This PR changes slice_shift_char so that:

"bar".slice_shift_char() => Some(('b', "ar"))
"ar".slice_shift_char()  => Some(('a', "r"))
"r".slice_shift_char()   => Some(('r', ""))
"".slice_shift_char()    => None

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @pcwalton (or someone else) soon.

[aturon]

@canndrew Looks like a sensible change to me!

Can you please add [breaking-change] to the bottom of the commit message? We use that to help log changes that will break downstream code. (It'd also be good to copy the text of the PR description into the commit message as well, so that people will see the full rationale.)

Otherwise, with that and a rebase, I'd be happy to take this.

[canndrew]

All done. Should be building again now.