[clang] Respect the [[clang::unsafe_buffer_usage]] attribute for constructors (PR #91777)

[Artem Dergachev via cfe-commits]

https://github.com/haoNoQ commented:

> https://github.com/llvm/llvm-project/blob/2ff43ce87e66d9324370e35ea6743ef57400c76e/clang/lib/Analysis/UnsafeBufferUsage.cpp#L1373-L1374
> 
> These assert that exactly one gadget matched. I think it's kinda worthwhile keeping the warnings independent too, so I don't see this as a bad thing. If we were to mark the span ctor as `[[clang::unsafe_buffer_usage]]` for instance, it the span-specific warning gives a better output still, and generating two warnings for the same piece of code is noisy.

Hmm this isn't really what these assertions are about. They don't protect against duplicate gadgets, they protect against duplicate `.bind()` tags across gadgets. Because gadgets are connected by the `anyOf()` matcher (as opposed to `eachOf()`), a single match result will always have exactly one gadget, the first one in the list, even if they match the same statement.

So I think you're right that there's a problem: if you have two gadgets match the same statement, only one will be reported. So your solution is probably necessary. We should also maybe do something about that, so that warning gadgets weren't conflicting this way. But I don't think *this* assertion guards against that.

If this still doesn't sound right to you, can you include a test where the assertion fails for you? No matter how ridiculous the test is. The machine is complex enough to appreciate ridiculous tests. I think we want to get to the bottom of this.

https://github.com/llvm/llvm-project/pull/91777