wgpu-hal dynamic dispatch, towards fewer generics in wgpu-core

[Wumpf]

Connections

• Towards (or the whole story? TBD!) of Make wgpu-core less generic (brainstorm) #5124

Description

Still heavy work in progress!

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This latest attempt to degenerification of wgpu core tries to be as straight forward as possible:

• introduce dynamic base types that cast to concrete types in HAL (DynDevice, DynCommandEncoder etc.)
• wherever wgpu-core passes around &A::CommandEncoder (and similar) instead pass around & dyn DynCommandEncoder
• start using a lot of Box<&dyn DynBuffer> (and similar)
• drop <A: hal::API> from all the wgpu-core types
• profit!

Previous more complex attempts tried to avoid boxing as much as possible, I'm deliberately doing here the opposite, well aware that this may cost us! But nobody actually measured how much yet and neither have I, so the jury is still out on this.

The hope is that this approach is simple enough that we can pull it off and cheap enough that we can worry about optimizing things later (it's not too hard to come up with ideas on how to tackle ensuing costs, we just have to know what to optimize first!).

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I'm keeping this in a fairly landable state at every step (minus Metal & web things which I'm not testing all the time) and try to keep steps granular, BUT....

• there's still quite a bit of experimentation going on where I have to go back and change some things
  • I may or may not clean up the commit history to eliminate a few things where I went in circles.
• this overall approach has the risk of degrading performance more than we're willing to pay (we are willing to pay a certain amount, since status quo is not great for all the reasons described in Make wgpu-core less generic (brainstorm) #5124), but it's hard to tell if it's bad or not without a full vertical slice
  • I may end up getting this almost to completion and then re-open a series of smaller, more reviewable PRs to land it

Testing
Hopefully covered by existing tests ;-)

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[Wumpf]

closing in favor of 3 part pr series coming up soon!