Instead another function named eq_by should be created similar to this but, instead of taking a closure that takes two inputs of same type, it should take in one input, and return a generic data type which implements PartialOrd, then this data can be checked for equality.

FWIW, this variant is usually spelled *_by_key in the Rust standard library, Iterator just happens to not have eq_by_key.

For comparing two iterators, the Iterator trait has the eq/eq_by, cmp/cmp_by, and partial_cmp/partial_cmp_by method pairs, as well as ne/lt/le/gt/ge methods with no *_by or *_by_key counterparts. IMO those last five don't add much value to Iterator and perhaps shouldn't have existed in the first place. For eq, cmp, and partial_cmp, though, it seems reasonable to me to add *_by_key versions of those.

Essentially, this eq_by should be equivalent to a.iter().zip(b.iter()).all(|(ele_a, ele_b)| fn(ele_a) == fn(ele_b)) where a and b are two generic types that implement Iterator, and, fn is the function for generating argument which is checked for equality instead.

Keep in mind that this may return true for two iterators with different lengths, so it doesn't behave exactly the same as the eq method.

Keep in mind that this may return true for two iterators with different lengths, so it doesn't behave exactly the same as the eq method.

I have that in mind, that's why I said equivalent, instead of equal, however, I should've been more specific so, I'll just fix it right now.

For comparing two iterators, the Iterator trait has the eq/eq_by, cmp/cmp_by, and partial_cmp/partial_cmp_by method pairs, as well as ne/lt/le/gt/ge methods with no *_by or *_by_key counterparts. IMO those last five don't add much value to Iterator and perhaps shouldn't have existed in the first place. For eq, cmp, and partial_cmp, though, it seems reasonable to me to add *_by_key versions of those.

Well, in that case, eq_by_key is also fine, because it does convey similar meaning, just eq_by by itself, checking for relations, seems a bit misleading, though it's not catastrophically misleading, because one look at docs and you'd know what it's doing but, still, I do think that this method should be renamed to related_by, instead of eq_by