Propagate ExactSizeIterator up from Chain's iterators

[Aceeri]

Wasn't entirely sure if this was intentional but noticed that Chain does not implement ExactSizeIterator if their internal iterators do.

I leave the implementation up to the existing size_hint.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @joshtriplett (or someone else) soon.

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[clarfonthey]

I'm in favour of this, but it's worth pointing out the discussion in #34433 for why this wasn't merged before.

[Aceeri]

Ah hmm... didn't expect this rabbit hole to go so far.

[rust-log-analyzer]

The job x86_64-gnu-llvm-13 failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling unwind v0.0.0 (/checkout/library/unwind)
error: implementation has missing stability attribute
   --> library/core/src/iter/adapters/chain.rs:285:1
    |
285 | / impl<A, B> ExactSizeIterator for Chain<A, B>
286 | | where
287 | |     A: ExactSizeIterator,
288 | |     B: ExactSizeIterator,
289 | | {
    | |_^


error[E0271]: type mismatch resolving `<B as Iterator>::Item == <A as Iterator>::Item`
    |
    |
285 | impl<A, B> ExactSizeIterator for Chain<A, B>
    |      -  -  ^^^^^^^^^^^^^^^^^ expected type parameter `A`, found type parameter `B`
    |      |  found type parameter
    |      expected type parameter
    |
    = note: expected associated type `<A as Iterator>::Item`
    = note: expected associated type `<A as Iterator>::Item`
               found associated type `<B as Iterator>::Item`
    = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
    = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
note: required because of the requirements on the impl of `Iterator` for `Chain<A, B>`
    |
    |
41  | impl<A, B> Iterator for Chain<A, B>
note: required by a bound in `ExactSizeIterator`
   --> library/core/src/iter/traits/exact_size.rs:76:30
    |
76  | pub trait ExactSizeIterator: Iterator {