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A comment on Rust – A hard decision pays off. Parent comment

Re:Rust – A hard decision pays off by tialaramex on Wednesday August 24, 2022 @02:08PM (#32583425)
Mere hints aren't enough for collect()

It will only care about your hints if you implement the unsafe trait TrustedLen (which promises those hints are correct)

Take is indeed TrustedLen if the Iterator it's taking from is TrustedLen (as in this case it can be sure of the size hint)

If you get to ~100 via some means other than take()ing 100 of them, chances are you don't have TrustedLen as a result.

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Re:Rust – A hard decision pays off by Alexendoo on Wednesday August 24, 2022 @02:55PM (#32584077)
TrustedLen will mean it can safely take the upper bound, but Vec for example will still use the lower bound of the hint when something isn't TrustedLen

https://doc.rust-lang.org/src/alloc/vec/spec_from_iter_neste...

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Re:Rust – A hard decision pays off by tialaramex on Wednesday August 24, 2022 @03:14PM (#32584353)
Ooh nice, I hadn't seen that. OK, so there are probably places where I assumed the iterator won't do as nice a job of collect() as I did manually but I was wrong.

I wonder how the hell I didn't see that when I was tracking down how collect() ends up caring about TrustedLen.

Thanks.

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