[clang] [Clang] raise extension warning for unknown namespaced attributes (PR #120925)

[via cfe-commits]

cor3ntin wrote:

Ultimately we want

1.  I know all of these attributes, don't tell me about them
2.  I know all of these attribute namespaces, don't tell me about them - which this patch is doing
3.   Have a way to disable unknown attributes warnings for all unknown namespaces - which this patch is doing by introducing a separate flag

I sort of question the last bullet point (the original issue), as it's a bit of a nuclear option. But maybe that's fine.
Then there is the question of whether 1. and 2. should have separate interfaces.

I will also point out that `-Wunknown-attribute-namespaces=foo` is a very weird way to spell "I know about `foo`, don't warn on it" - and I am concerned about discoverability.


And again, I apologize I did not see this patch sooner @a-tarasyuk.
What do people think about landing it early in the Clang 21 cycle, in case we find a better long term interface in the next few months?


https://github.com/llvm/llvm-project/pull/120925