implement debugPrintf in msl & spv

[39ali]

Connections
related to #7662

Description
now you can printf stuff in wgsl using metal backend, should be able add vulkan as well in the future :

        enable wgpu_debug_printf;
        
        @compute @workgroup_size(8, 1, 1)
        fn main(@builtin(local_invocation_index) idx: u32) {
             debugPrintf("WGSL_METAL_LOG: Thread index is %u", idx);
        }

Testing
added an example

Squash or Rebase?

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[39ali]

@ErichDonGubler test is failing :
error: error: cannot execute tool 'metal' due to missing Metal Toolchain; use: xcodebuild -downloadComponent MetalToolchain

[39ali]

added vulkan support as well

[atlv24]

Yay, it'll be great to have this! Some feedback, mostly style but also docs and a performance fix.

I'd appreciate safety comments on all the newly added unsafe blocks.

[atlv24]

Looking a lot better

[atlv24]

This needs some naga validation fail tests showing that string literals are banned from non debugPrintf locations, and also that debugPrintf is not allowed unless the extension is present.

[evilpie]

User defined format strings are famously unsafe and this seems to implicitly assume that all backends going forward will have the same format string formats. Maybe it would be a cool improvement to support (basic) Rust style format strings instead? Then again, maybe too much work for a debug feature.

See also gfx-rs/naga#2563. That seems to support printf in OpenGL via GL_EXT_debug_printf.

[39ali]

@atlv24 check it out

[atlv24]

This is good to merge now, thanks!

[atlv24]

Non-blocking nits and notes