Never worked in Rust though so wondered if the iterator api had some weird optional notion of size that could be utilised throughout the chain.
Fwiw, this does exist: [Iterator::size_hint] (https://doc.rust-lang.org/std/iter/trait.Iterator.html#metho...)
[0]: https://github.com/rust-lang/rust/blob/master/tests/codegen/...
[1]: https://github.com/rust-lang/rust/blob/d4025ee454169fbd22f57...
Um, yes it is, extensively?
SpecFromIterNested is a specialization trait for alloc::vec::Vec's FromIterator which handles both the TrustedLen and ordinary Iterator scenarios
For an ordinary Iterator, it calls next() once to check this Iterator isn't done, if it's done, we can just give back a Vec::new() since that's exactly what was needed. Otherwise, it then consults the hint's low estimate, and it pre-allocates enough capacity on that basis, unless it's lower than Vec's own guess of the minimum worthwhile initial capacity.
For Iterators which impl TrustedLen (ie promise they know exactly how many items they yield) it instead checks the upper end of the hint, to see if it's None, if it is the iterator knows it's too big to store in memory, we should panic. Otherwise though we can Vec::with_capacity
let v: Vec<_> = (0..23456).collect();
Sorry, that was a mistake on my part. I did not see it explicitly in any of the code. Thank you for pointing out in detail where the stdlib uses this.