warn on `iter.enumerate.map(|(_, pat)| expr)`

[KisaragiEffective]

What it does

emits warning on iterator.enumerate().map(|(_unused_index, elem)| expr).

Advantage

• Removes counter, improving performance.

Drawbacks

None

Example

https://github.com/KisaragiEffective/origlang/blob/fa487aab90c7bbe1b0b4ebaedfa33b847985a1ba/package/origlang-compiler/src/type_check.rs#L270:

iter.enumerate().map(|(_i, elem)| { whatever })

Could be written as:

iter.map(|elem| { whatever })

[Ethiraric]

@rustbot claim

[y21]

Looks very similar to https://rust-lang.github.io/rust-clippy/master/index.html#/unused_enumerate_index , we can probably just extend that lint?

[Ethiraric]

Indeed! Thanks for the pointer!

[Ethiraric]

I don't think I can work on the lint directly as it is called from clippy_lints/src/loops/mod.rs, which calls all lints that operate on loops.

I know not enough about the clippy architecture and performance impact to know which would be best to implement it. What I can think of is to either:

1. Move the lint out of the loops mod as a standalone mod
2. Create another LateLintPass in a standalone mod that would emit UNUSED_ENUMERATE_INDEX just for the case described in this issue

[y21]

You could also create methods/unused_enumerate_index.rs, look for the iterator adapter pattern here and emit crate::loops::UNUSED_ENUMERATE_INDEX, that would likely be the simplest option.

Sounds similar to what you had in mind for option 2, but I don't think we need an entirely new, uncategorized lint pass. Unless you also have other patterns in mind that aren't just method calls

[Ethiraric]

I could have looked a bit further and find the methods directory myself, which is what I was looking for, but I didn't. Thanks again for helping!

I didn't have anything in mind that would warrant a new lint pass, but that was the only other way I thought of. My knowledge of this codebase grows with time, but is still very vague.