New lint manual_split_once

[Jarcho]

This is a WIP because it still needs to recognize more patterns. Currently handles:

s.splitn(2, ' ').next();
s.splitn(2, ' ').nth(0)
s.splitn(2, ' ').nth(1);
s.splitn(2, ' ').skip(0).next();
s.splitn(2, ' ').skip(1).next();
s.splitn(2, ' ').next_tuple(); // from itertools

// as well as `unwrap()` and `?` forms

Still to do:

let mut iter = s.splitn(2, ' ');
(iter.next().unwrap(), iter.next()?)

let mut iter = s.splitn(2, ' ');
let key = iter.next().unwrap();
let value = iter.next()?;

Suggestions on other patterns to check for would be useful. I've done a search on github for uses of splitn. Still have yet to actually look through the results.

There's also the question of whether the lint shouold trigger on all uses of splitn with two values, or only on recognized usages. The former could have false positives where it couldn't be replaced, but I'm not sure how common that would be.

changelog: Add lint manual_split_once

[rust-highfive]

r? @llogiq

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[llogiq]

This already looks quite comprehensive! Great job! I think it'd be ready to merge and we can extend it with a later PR if you're OK with that.

My suggestion is informed by

• The already high code quality 👍
• The rather low risk of false positives
• The helpful suggestions as showcased by the tests

[Jarcho]

Just updated the docs as they mentioned something that currently isn't checked for. I see nothing wrong with merging it as is. It will probably be a week before I get to finishing the rest of the checks.

[llogiq]

Thank you! @bors r+

[bors]

📌 Commit 8a9c512 has been approved by llogiq

[bors]

⌛ Testing commit 8a9c512 with merge 0c8f375...

[bors]

💔 Test failed - checks-action_test

[xFrednet]

It seems like #7566 which was merged shortly before this, again uses the LocalUsedVisitor visitor

[llogiq]

Let's try that again – @bors r+

[bors]

📌 Commit aab3267 has been approved by llogiq

[bors]

⌛ Testing commit aab3267 with merge 1fc1aee...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing 1fc1aee to master...