Fix const generic panic

[HKalbasi]

fix #11688 (comment)

[lnicola]

bors r+

Merging this now to unblock the metrics.

[bors]

Build succeeded:

• Rust (macos-latest)
• Rust (ubuntu-latest)
• Rust (windows-latest)
• TypeScript (ubuntu-latest)
• TypeScript (windows-latest)

[HKalbasi]

Metrics is still broken, but for another reason I guess?

[lnicola]

They don't crash any more for me, but I still get [ERROR hir_ty::consteval] is_unknown was called on a non-concrete constant value! ^0.0.

[HKalbasi]

I still get [ERROR hir_ty::consteval] is_unknown was called on a non-concrete constant value! ^0.0.

I think it is fine for now. Current callers of is_unknown should be removed if I understand correctly, but it may miss some impls of arrays and keep it as-is is harmless. The tracing call can be removed.

And it seems metrics need some action: https://github.com/rust-analyzer/rust-analyzer/runs/5569046023?check_suite_focus=true

[lnicola]

Yeah, that's #11724 (comment).