Use clang's enum_extensibility attribute to check enum values

We could avoid constructing bad enum values using enum_extensibility: https://clang.llvm.org/docs/AttributeReference.html#enum-extensibility

changed milestone to %Tor: unspecified

added code-correctness component::core tor/tor milestone::Tor: unspecified points::1 priority::medium severity::normal status::new type::enhancement labels

Here's how we can check for that feature in clang: https://clang.llvm.org/docs/LanguageExtensions.html#feature-checking-macros

I think the same syntax works on gcc, but we should check.

That first link (https://clang.llvm.org/docs/AttributeReference.html#enum-extensibility) gives me an empty page. Is there a better place to find out what this is?

Replying to nickm:

That first link (https://clang.llvm.org/docs/AttributeReference.html#enum-extensibility) gives me an empty page. Is there a better place to find out what this is?

That's really weird, I see that too. But it worked a few days ago. And some search engines still have it indexed :-)

Here's the version from clang 9.0.0, the latest release: https://releases.llvm.org/9.0.0/tools/clang/docs/AttributeReference.html#enum-extensibility

changed time estimate to 8h

moved to tpo/core/tor#32580 (closed)