Why is the lower bound on Iterator::size_hint useful?

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I get that the upper bound is used for preallocating. Cant imagine why someone would want to know that the smallest it could possibly be is.

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Actually in the current implementation the lower bound is used for preallocating. I don't know uses of the upper bound.

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It's not. The upper bound can be infinite or unknown, neither of which can be used for pre-allocation.

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Right. I assumed in the case of some.

Although i can also see hypothetically now why you may want both...

Take upper bound if some, else use lower bound

But as @SkiFire13 pointed out, the upper bound is not used this way, so do you know of a purpose (in std at least) for upper bound?

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No: Is .size_hint().1 ever used? - libs - Rust Internals

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