Naga specific WGSL extensions

[teoxoy]

• naga_ext_push_constants
  • allows usage of push_constant Address Space
  • might become an official extension Investigation : Push Constants gpuweb/gpuweb#75
• naga_ext_f64
  • allows usage of f64 type
  • might become an official extension Double precision floats (IEEE-754 binary64) gpuweb/gpuweb#2805
• naga_ext_early_depth_test
  • allows usage of @early_depth_test attribute
  • might become an official extension Early Depth/Stencil test directive or flag in pipeline state creation gpuweb/gpuweb#398
• naga_ext_multiview (or naga_ext_view_index?)
  • allows usage of @builtin(view_index) builtin
• naga_ext_primitive_index
  • allows usage of @builtin(primitive_index) builtin
  • most likely won't be part of WebGPU Support for gl_PrimitiveID / SV_PrimitiveID -like construct? gpuweb/gpuweb#1786
  • might have a heavy cost to performance Support PrimitiveID when possible #1669 (comment)
• naga_ext_binding_array (this will end up having 8 Capabilities see Update binding_array related Capabilities #4407)
  • allows usage of binding_array type
  • might become an official extension
    • Investigation: Bindless resources gpuweb/gpuweb#380
    • wgsl: Sketch design for arrays of buffer-bindings gpuweb/gpuweb#2105

All of these have their own Capabilities therefore the validator is already aware of those non-standard items but I think we should still have our own set of extensions so that it's obvious when a shader uses those items.

Introducing a strict_extensions: bool field for the Parser will limit the extensions to only those defined by the spec (for now only f16).

Relevant errors should be different based on the strict_extensions field. Example: using an f64 type with strict_extensions: true should yield an error that informs the user that there is no such type; on the other hand, if strict_extensions were false, the error should inform the user that the naga_ext_f64 extension is required.

[jimblandy]

All of these have their own Capabilities therefore the validator is already aware of those non-standard items but I think we should still have our own set of extensions so that it's obvious when a shader uses those items.

So the idea is, to use some extension, you have to add the bit to Capabilities, and then also put enable BLAH; at the top? And the benefit would be that nobody would be writing Naga-specific shaders without them being clearly marked as such?

[teoxoy]

Capabilities that are not part of the WebGPU/WGSL specs will be set automatically by wgpu depending on some flag (see #3274).
Users will only have to enable naga_ext_<name>; in their shaders to use a naga specific extension (given they set the "Strict WebGPU compliance mode" to false).