implement Iterator::{exactly_one, collect_array}

[jdonszelmann]

As per #149266

[rustbot]

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[jdonszelmann]

Implementing exactly_one in terms of collect_array seems to generate the exact same code as a dedicated implementation:
https://godbolt.org/z/Tc6P5MhrE

[jdonszelmann]

As you can see from the 2nd commit and the CI failures, since exactly_one conflicts with itertools we get some FCWs. How do we usually deal with those? I doubt this is the first iterator method that also already existed in itertools.

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer