PersistentGpuBufferable probably shouldn't be an unsafe trait

[james7132]

A good example of an unsafe trait is TrustedLen or WorldQuery, where the correct implementation of the trait is required to satisfy the safety invariants in its application, and currently PersistentGpuBufferable is not used in such a way. It is not used in any unsafe context CPU-side, and as far as I know, wgpu's validation should prevent any improperly uploading invalid inputs to the GPU.

Of the two implementations of the trait, the Arc<[u8]> implementation does not actually satisfy the safety invariants requiring proper alignment of the contents (it's min alignment is 1 and wgpu::COPY_BUFFER_ALGINMENT is currently 4 and this is not enforced in it's implementation), and both implementations do not document how they're satisfying the safety invariants.

Potential solutions here:

• Make the trait not unsafe.
• Remove the Arc<[u8]> implementation or enforce its safety invariants.
• Document the safety invariants and enforce them properly.

[JMS55]

Arc<[u8]> is used for vertex data which is always in multiples of 48 bytes, so it satisfies the alignment.

[james7132]

However, that's not enforced as a part of the implementation of the trait for Arc<[u8]>, which means the trait implementation itself is unsound according to the safety invariants of the trait.

IMO we should just replace the unsafe here with an assertion to validate the input, as it's pretty clear that this isn't a memory safety issue, as far as the Rust compiler is concerned.