New lint: unused_enumerate_value

[profetia]

for (i, _) in iter.enumerate() can be replaced with for i in 0..iter.len() if the iterator implements ExactSizeIterator.

Closes #14430

changelog: [unused_enumerate_value]: new lint

[rustbot]

r? @blyxyas

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[blyxyas]

Ideally we should also check if the arbitrary type implements .len() as a method, wouldn't we? Search, while not the best regex for looking this up, we can see that it's a very common pattern to implement .len() directly on the struct, without implementing the trait.

impl ExactSizeIterator, the trait isn't very well known.

[profetia]

Emm.. I doubt it because len does not belongs to any trait in the std, so the len implemented by a user-defined type may not has the sematics of what we intended here.

[blyxyas]

In that case, we could just emit a warning if it's not coming from a ExactSizeIterator. I don't think we'll get much use if we limit the lint to just the trait.

[profetia]

Updated. Now arbitrary type is covered.

[Snowiiii]

@blyxyas does this look good now?