Add Itertools::intersperse_with

[gin-ahirsch]

Pretty self-explanatory. Useful for non-clonable elements or otherwise stateful interspersing.
intersperse() could be implemented via intersperse_with(), allowing the change to be more minimal. I haven't done it currently, but if you don't see any downsides I'd amend this change to do self.intersperse_with(|| element) for intersperse() instead.

[FauxFaux]

I came here to write exactly this.

I planned to:

• make the existing intersperse implementation call this one. Copy-pasting the code is presumably more efficient, though.
• not use FnMut, but Fn. I guess FnMut is more general.

[phimuemue]

make the existing intersperse implementation call this one.

I second this suggestion. We do it in other places, too, and my most recent tests indicate that the compiler is actually smart enough to inline things properly (at least with --release).

As a side note: Is possibly even Interleave similar enough so that it could share code with Intersperse?

[gin-ahirsch]

It may not actually be possible to implement intersperse() through intersperse_with():

error[E0308]: mismatched types
   --> src/lib.rs:401:31
    |
396 |     fn intersperse<F>(self, element: Self::Item) -> IntersperseWith<Self, F>
    |                    - this type parameter
...
401 |         self.intersperse_with(|| element)
    |                               ^^^^^^^^^^ expected type parameter `F`, found closure

Basically the same underlying issue as why #377 can't just return with a new closure - requiring impl Trait in return position.
The same will be true for interleave().

[gin-ahirsch]

Actually the issue is not just impl Trait in return position, but impl Trait in return position for traits. I'm not sure this is even on the map for Rust, as a Trait's interface is expected to be "fixed".
I tried using inherent trait methods (impl dyn Itertools), but that doesn't like methods with generic type parameters that don't have the restriction of Self: Sized:

error[E0038]: the trait `Itertools` cannot be made into an object
    --> src/lib.rs:2590:9
     |
1394 |     fn find_position<P>(&mut self, mut pred: P) -> Option<(usize, Self::Item)>
     |        ------------- method `find_position` has generic type parameters
...
2590 | impl<Item> dyn Itertools<Item=Item> {
     |            ^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Itertools` cannot be made into an object

I'm not sure why that would be problematic if not invoked from the inherent methods, but it seems that it is.
Even if that would be solved somehow, we cannot invoke generic methods at all though:

error: the `intersperse_with` method cannot be invoked on a trait object
    --> src/lib.rs:2467:14
     |
2467 |         self.intersperse_with(|| element)
     |              ^^^^^^^^^^^^^^^^

It's interesting that the method can accept self by value though, considering its type would be dyn Itertools.

[jswrenn]

This looks good to me. Could you resolve the merge conflict?

[jswrenn]

Thanks!

bors r+

[bors]

Build succeeded:

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