VertexStream::new_with_stride() pointer parameter leads to easy segfault

[JMS55]

What I tried doing (and got a segfault with):

VertexStream::new_with_stride::<Vec3, _>(
    &vertex_buffer, // vertex_buffer is a Vec<u8>
    vertex_stride,
)

What you actually need to do:

VertexStream::new_with_stride::<Vec3, _>(
    vertex_buffer.as_ptr(),
    vertex_stride,
)

I suggest changing the function to not take a raw pointer.

The whole T vs VertexType thing was also confusing to me, it took me a while to figure out how to use the function. I'd prefer simply passing the size myself rather than it using size_of::<T>().

[Uriopass]

What would you suggest as a proper API?
This would lead to a breaking change too, which is fine but the next release is probably not anytime soon.
I think this function should be marked unsafe too, it seems easy to cause UB.

[JMS55]

Probably take an &[T] instead of a raw pointer.

[Uriopass]

but since T are stored with strides: eg a slice of [T, U, T, U, T, U]
I think that would be UB to make the slice

My idea would be to simply take *const u8 and acknowledge the unsafeness. I'm not sure how stride can be encoded safely and VertexStream isn't used in many functions

[JMS55]

Oh, you're right :/. Would &[u8] be an issue? Or *const u8 would probably be fine, but I want something that makes passing &Vec<u8> (pointer to the vec structure itself on the stack, not the vec's allocated memory) impossible.

[Uriopass]

&[u8] could be a good idea! I guess users would use bytemuck which would solve a few problems with getting this right along with a nice lifetime

Also starts to look a lot like a static VertexDataAdapter

[Uriopass]

Opened #60 if you want to take a look

[Uriopass]

@JMS55 how are you using the stream? Now that I'm reading the API more carefully I'm not sure if it's possible to have an offset inside a vertex struct using a slice. I'm also not sure how meshopt knows where the stream ends and I don't understand the "stream" name since it's finite really.

[JMS55]

I use it to get a position-only stream of the vertices https://github.com/bevyengine/bevy/blob/main/crates/bevy_pbr/src/meshlet/from_mesh.rs#L98-L101.

vertex_buffer is a Vec<u8>.

[Uriopass]

This only works because position is the first element :/ I'm not sure how it'd look if you only wanted scale or rotation for example
I think this should be VertexDataAdapter but at the same time meshopt is made with this stream thing.. Not sure if it's better to encapsulate or just leave it raw without the footgunny size_of