Inconsistent naming of mut methods (get_mut vs. mut_iter)

[mbrubeck]

The Rust stdlib sometimes uses prefix mut_ and other times suffix _mut. This is a really minor papercut, but it does cause me to mistype names on a regular basis.

For example, we have:

find_mut
get_mut
front_mut
back_mut
borrow_mut

...but also:

mut_iter
mut_slice
mut_last
mut_split_at

[brson]

Another conventions issue. Nominating.

[brson]

P-backcompat libs, 1.0

[lilyball]

find_mut, get_mut, and borrow_mut to me seem fine. Those are verbs, not nouns. mut_iter, mut_slice, etc are nouns. So the former is saying "do this thing, mutably". The latter is saying "return this mutable thing".

front_mut and back_mut seem like the latter, which suggests they should be mut_front and mut_back.

mut_split_at is an interesting case, as "split" is both verb and noun. I think it's correct as-is. split_mut_at and split_at_mut both suggest there's a non-mutable split_at, which there isn't. I think it's ok to read this as "return a mutable split at (index)". There's also a parallel between mutable split and mutable slice.

[aturon]

The guidelines now have a convention for this placement. I will be rolling them out in a patch soon.