Closure trait descriptions could be clearer.

[vi]

https://doc.rust-lang.org/reference/types/closure.html#other-traits

A closure is Send if all variables captured by non-unique immutable reference are Sync, and all values captured by unique immutable or mutable reference, copy, or move are Send.

This phrase suggests that there are three types of references:

• non-unique immutable
• unique immutable
• unique mutable

But typically only & and &mut are considered. What does "unique immutable reference" mean? Shall the phrase be made clearer and some code example of Send vs !Send closure peculiarities be demonstrated?

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IRC provided the help:

<demize> _Vi: https://doc.rust-lang.org/reference/types/closure.html#unique-immutable-borrows-in-captures

Maybe at least the phrase "unique immutable" in "Other traits" should link there?