You absolutely can do something like `let iter = something.iter_mut().filter(|| {...}).enumerate();` In Rust, Iterators are just need to implement a Trait (kindof like an Go or Java interface) so they can be put in normal variables just fine.
> The `mut` keyword isn't needed here because, I assume if the above assumption is correct, "iter_mut()" is returning a mutable iterator.
Every for loop implicitly calls `.into_iter()` on the thing to loop over; the docs are decent at explaining what IntoIter is [0]. By doing `&mut something` instead of just `something` you get a different IntoIter implementation which has very different semantics. Typically `(&mut something).into_iter()` would have the same behavior and might even result in the same type as `something.iter_mut()`.
[0]: https://doc.rust-lang.org/std/iter/trait.IntoIterator.html
