See if removing a vec allocation in animation code improves performance

[alice-i-cecile]

          do we need to make a whole new vec or can we just add/remove elements until the len is the same?

Originally posted by @rodolphito in #11306 (comment)

This wasn't part of my PR, so I didn't change it there :)

[nicopap]

To quote myself

It's possible to do a clear() followed by an extend(…). The Vec::new here doesn't cause an allocation until something is pushed to it.

[nicopap]

I think I might have been misunderstood. the code in question is:

        animation.path_cache = vec![Vec::new(); animation_clip.paths.len()];

In this, we have to Vec constructors (1) vec![…], (2) Vec::new().

(2) doesn't allocate, while (1) will probably.

Replacing the code by:

        let new_len = animation_clip.paths.len();
        animation.path_cache.clear();
        animation.path_cache.extend((0..new_len).map(|_| Vec::new()));

Will likely result in better code generation.

A more "rusty" approach would use ….extend(iter::repeat_with(Vec::new).take(new_len)). While this does still eliminate the one allocation, it generally results in worse code generation. In this case Vec::new() may be translated into a bzero().

[nicopap]

I'm an idiot, I misunderstood what the comment referred to. I've a PR coming where allocation is minimized as much as possible. What I suggest isn't the optimal solution.