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