Fix AnimationTargetId collisions

[greeble-dev]

Objective

Fixes #22842. TLDR: AnimationTargetId::from_iter wrongly returns the same id for ["ab"] and ["a", "b"].

Solution

As suggested in the issue, add the length of each string to the hash. The original suggestion used to_be_bytes, but I used to_le_bytes on the assumption that most users are targeting LE and this will be more efficient.

• There's probably a way to break it with maliciously crafted names, but I can't imagine a case where that's a problem.
• I considered hashing 0xffu8 after each string instead of the length.
  • That would be more efficient, and should be safe since it's not a valid UTF8 character.
  • But I wasn't quite confident enough.
• I also refactored AnimationTargetId::from_names to reuse AnimationTargetId::from_iter.

Testing

cargo test -p bevy_animation
cargo run --example animated_mesh
cargo run --example animated_mesh_events
cargo run --example animation_masks

[greeble-dev]

This test might not be justified. I originally added it because from_iter and from_names had different implementations. But later on I refactored them to share the same implementation.

[hukasu]

instead of 0xffu8, maybe any other control character since they would cause problems in actual strings?

[greeble-dev]

instead of 0xffu8, maybe any other control character since they would cause problems in actual strings?

I think that would be less safe, although only slightly. It's difficult to construct a str with an invalid character (i.e. str::from_utf8(&[0xffu8]) is an error), but it can contain control characters. Kinda unlikely anyone would make a joint name with a control character, but I've seen stranger things.