rust - How to reverse after zip two chains -

i have following code not compile.

fn main() {   let =     "123"     .chars()     .chain("4566".chars())     .zip(         "bbb"         .chars()         .chain("yyy".chars()))     .rev()     .map(|x, y| y)     .collect::<string>();   println!("hello, world! {}", a); } 

got error following:

src/main.rs:37:10: 37:15 error: trait `core::iter::exactsizeiterator` not implemented type `core::iter::chain<core::str::chars<'_>, core::str::chars<'_>>` [e0277] src/main.rs:37         .rev()                         ^~~~~ src/main.rs:37:10: 37:15 error: trait `core::iter::exactsizeiterator` not implemented type `core::iter::chain<core::str::chars<'_>, core::str::chars<'_>>` [e0277] src/main.rs:37         .rev()                         ^~~~~ src/main.rs:38:10: 38:23 error: type `core::iter::rev<core::iter::zip<core::iter::chain<core::str::chars<'_>, core::str::chars<'_>>, core::iter::chain<core::str::chars<'_>, core::str::chars<'_>>>>` not implement method in scope named `map` src/main.rs:38         .map(|x, y| y) 

my understanding rev() method defined in iterator implements trait doubleendediterator

fn rev(self) -> rev<self> self: doubleendediterator { ... } 

also zip implements trait:

impl<a, b> doubleendediterator zip<a, b> b: doubleendediterator + exactsizeiterator, a: doubleendediterator + exactsizeiterator 

so problem chain doesn't implement exactsizeiterator. how work around this?

i tried add .take() both chains convert type take implements exactsizeiterator, take doesn't implement doubleendediterator.

note simplified example. in reality cannot reverse both chain first , zip.

you're looking following impl (spoiler: doesn't exist):

impl<a, b> exactsizeiterator chain<a, b>   a: exactsizeiterator, b: exactsizeiterator { ... } 

an exactsizeiterator must implement 1 method, len(&self). idea behind hypothetical implementation sum both lengths chain_a_b.len() == a.len() + b.len().

the reason doesn't exist rust cannot guarantee addition (usize + usize) not overflow. forbids it. sounds little strict, that's status quo now, unfortunately.

worse: if impl existed, run fact chars not exactsizeiterator, still not work.

an alternative (probably not one) collect chains vector. bad because of memory allocation, if isn't perf bottleneck worth tradeoff.