Just remembered you can also do vec![value; length], and not just vec![value1, value2, value3...] to create a Vec.

[steffo@a.junimo.party]

Just remembered you can also do vec![value; length], and not just vec![value1, value2, value3...] to create a Vec.

https://doc.rust-lang.org/std/macro.vec.html

#Rust

[steffo@a.junimo.party]

Cool thing is that it repeatedly uses Clone on the given value instead of Copy, so you can give it pretty much everything as input!

#Rust

[wolvericcatkin@tech.lgbt]

@steffo Also of note: std::iter::repeat_n, and Iterator::take and Iterator::collect...

[steffo@a.junimo.party]

@WolvericCatkin oh yeah i knew about those, i had just completely forgotten about the vec! macro lol

[wolvericcatkin@tech.lgbt]

@steffo Tbh, reading the methods vec! ends up expanding out to, they seem like a bit of a mess... repeat_n may be the better option from a technical standpoint...

[steffo@a.junimo.party]

@WolvericCatkin

don't you lose information about capacity that way, though?

iirc iterators can only provide a size_hint, not give guarantees about their size

does the implementation of collect<Vec<_>> take them into account?

[wolvericcatkin@tech.lgbt]

@steffo std::iter::repeat_n has a guaranteed size, because it has a set number of iterators, and is cloning the item until the last iteration, so it's also ExactLenIterator.

Depending on the provider, Vec::from_iterator can take advantage of Iterator::size_hint.

[wolvericcatkin@tech.lgbt]

@steffo This file:
https://doc.rust-lang.org/src/alloc/vec/spec_from_iter_nested.rs.html

[steffo@a.junimo.party]

@WolvericCatkin oh that's very nice then!

i'll admit i haven't looked very much into the internals of Iterator and have been treating them pretty much as opaque types

[wolvericcatkin@tech.lgbt]

@steffo vec! ends up expanding out into a function that uses Vec::extend_with, which explicitly notes it has to compensate for compiler bugs, but otherwise has quite similar implementation...

[wolvericcatkin@tech.lgbt]

@steffo Bruh- ok, apparently looking at where Vec::from_iterator ends up, it does end up running into the same bug as vec! eventually, just in a different method... Iterator handling with Vec is so frustratingly indirected...

It does still pre-allocate with Iterator::size_hint though.

[steffo@a.junimo.party]

@WolvericCatkin hahahaha, seems like there's no way around it