Tracking issue for Vec::resize_with and resize_default

[joshlf]

Currently, Vec has a resize method which takes a new parameter so that if the resize involves growing the vector, the new parameter is cloned into each newly-created cell in the vector. T must implement Clone in order for this method to be available.

It would be useful to add a resize_default method that instead requires that T: Default, and calls T::default() to fill the newly-created cells. Not only would this be ergonomic, but for certain implementations of T::default, it might allow the compiler to make better optimizations.

[clarfonthey]

This would be an interesting addition for types which support Default but not Clone. I'll try and make a PR for it.

In the future, just FYI, feature requests should go in the RFCs repository.

[joshlf]

Ah OK, noted. That's even for feature requests like this one that don't deserve an RFC?

[clarfonthey]

Yep! The general rule is either PR here or RFC elsewhere.

Also, @joshlf would you mind renaming this issue to "Tracking issue for Vec::resize_default" ?

And @nikomatsakis would you mind tagging it?

[joshlf]

Gotcha; thanks!

[Mark-Simulacrum]

@rust-lang/libs Nominating for stabilization. The only potential concern is that we can generalize this to take a closure (see below). I don't know that it'd be all that useful, though, so I'm inclined to say that we shouldn't do so, and Clone and Default are the two "producing" traits in std, so standardizing around them seems logical.

fn resize_with(&mut self, new_len: usize, f: F)
where F: FnMut() -> T

[clarfonthey]

@Mark-Simulacrum I'd be willing to make a PR that adds resize_with under a separate feature flag. I think that all three are useful; resize_default would simply be documented as equivalent to resize_with(Default::default)

[sfackler]

I might lean more towards just having resize_with, for now at least. It's strictly more powerful and doesn't seem all that more verbose than resize_default?