manual_split_once: lint manual iteration of SplitN

[Alexendoo]

changelog: manual_split_once: lint manual iteration of SplitN

Now lints:

let mut iter = "a.b.c".splitn(2, '.');
let first = iter.next().unwrap();
let second = iter.next().unwrap();

let mut iter = "a.b.c".splitn(2, '.');
let first = iter.next()?;
let second = iter.next()?;

let mut iter = "a.b.c".rsplitn(2, '.');
let first = iter.next().unwrap();
let second = iter.next().unwrap();

let mut iter = "a.b.c".rsplitn(2, '.');
let first = iter.next()?;
let second = iter.next()?;

It suggests (minus leftover whitespace):

let (first, second) = "a.b.c".split_once('.').unwrap();

let (first, second) = "a.b.c".split_once('.')?;

let (second, first) = "a.b.c".rsplit_once('.').unwrap();

let (second, first) = "a.b.c".rsplit_once('.')?;

Currently only lints if the statements are next to each other, as detecting the various kinds of shadowing was tricky, so the following won't lint

let mut iter = "a.b.c".splitn(2, '.');
let something_else = 1;
let first = iter.next()?;
let second = iter.next()?;

[rust-highfive]

r? @xFrednet

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[dswij]

LGTM! Just a question and a suggestion to add the limitations to docs, otherwise the changes look great.

[xFrednet]

This version already looks good to me. I like the suggestion to add a limitations section to the docs, and then we can merge this. 👍

Thank you very much @dswij for the review!

[xFrednet]

Looks good to me, thank you for the update! 👍

@bors r=dswij,xFrednet

[bors]

📌 Commit 4424aa4 has been approved by dswij,xFrednet

[bors]

⌛ Testing commit 4424aa4 with merge ed22428...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: dswij,xFrednet
Pushing ed22428 to master...