Does the `map` call serve any purpose here?

Code:

pub fn iter(&self) -> impl Iterator<Item = (&usize, &T)> + '_ {
    self.values.iter().map(|(key, value)| (key, value))
}

Quoted from here, it seems following is enough:

pub fn iter(&self) -> impl Iterator<Item = (&usize, &T)> + '_ {
    self.values.iter()
}

Any difference?

>Solution :

Yes, the former returns a type that implements Iterator<Item = (&usize, &T)> + '_ while the latter returns a type that implements Iterator<Item = &(usize, T)> + '_.

Here a minimal example:

pub fn iter1(v: &[(usize, i32)]) -> impl Iterator<Item = (&usize, &i32)> + '_ {
    v.iter().map(|(key, value)| (key, value))
}

pub fn iter2(v: &[(usize, i32)]) -> impl Iterator<Item = (&usize, &i32)> + '_ {
    v.iter()
}

fails to compile with this error message:

error[E0271]: expected `std::slice::Iter<'_, (usize, i32)>` to be an iterator that yields `(&usize, &i32)`, but it yields `&(usize, i32)`
 --> src/lib.rs:5:37
  |
5 | pub fn iter2(v: &[(usize, i32)]) -> impl Iterator<Item = (&usize, &i32)> + '_ {
  |                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `&(usize, i32)`, found tuple
6 |     v.iter()
  |     -------- return type was inferred to be `std::slice::Iter<'_, (usize, i32)>` here
  |
  = note: expected reference `&(usize, i32)`
                 found tuple `(&usize, &i32)`

For more information about this error, try `rustc --explain E0271`.
error: could not compile `playground` due to previous error

Playground.

The former works thanks to ergonomics build into Rust’s pattern syntax called binding modes:

When a reference value is matched by a non-reference pattern, it will be automatically treated as a ref or ref mut binding.

So it is basically syntactic sugar for v.iter().map(|&(ref key, ref value)| (key, value)), which makes it clearer why the former works, but the latter doesn’t.