feat(wgsl): recognize enable primitive-index;

[ErichDonGubler]

Connections

• Contributes to, but does not resolve, Support enable primitive-index; in WGSL #8236.

Testing

• Yes.

Squash or Rebase?

squashplz

Checklist

• If this contains user-facing changes, add a CHANGELOG.md entry.

[ErichDonGubler]

Not sure if we needs tests or a CHANGELOG yet. Thoughts, @gfx-rs/wgpu?

[andyleiserson]

I created some infrastructure for testing extension availability, but it's for the case where the extension is supported.

I do think it's worth adding a test, mostly to document the state of affairs. Nothing fancy, just a wgsl_errors test for the error that gets raised when enable primitive-index appears in WGSL.

[ErichDonGubler]

Welp, it was definitely worth making those tests, because I'd incorrectly written primitive-index as the WGSL enable-extension identifier, but that's not even an identifier. +1 for testing!

I had to add UnimplementedEnableExtension to the public API (and I CHANGELOG'd it), but we already do this with UnimplementedLanguageExtension, so I think it's fine.