[pornel]

`iterator.map(…)` must return a new wrapper type, because it needs to store the closure somewhere, so it returns

    struct Map { closure, original_iterator }

This fact could have been hidden by making these methods return `impl Iterator`, but it would be strictly less useful/performant in case you needed to store the iterator somewhere, because then you couldn't name the actual type, and would need to work with an abstract one.

But I do agree that the way documentation presents traits and their implementations can be overwhelming, especially the Iterator which is a pretty large trait.

[estebank]

> because then you couldn't name the actual type

I'm sure you know this, but `type Alias = impl Trait;` is on its way which would make naming those possible.

[tialaramex]

Although that's true, and potentially useful as a feature (not sure I want it, but pretty sure I wouldn't be angry if it was added)..

I don't think that's what your parent was talking about when they say "couldn't name the actual type".

They're saying, suppose I have a function that says it return an Iterator and I realise ah, it's not really returning an Iterator, but some subtype of Iterator - or, in Rust where subtypes don't exist, a type which merely implements Iterator. I now can't talk about the actual type being returned, because that's hidden from me, and it needn't be.

[sanxiyn]

For posterity, this is Rust RFC 2515 type_alias_impl_trait.