[clang] Pass LangOpts from CompilerInstance to DependencyScanningWorker (PR #93753)

[Nishith Kumar M Shah via cfe-commits]

nishithshah2211 wrote:

Thanks for the comments @jansvoboda11 . I am new to all these different moving parts and want to understand better. I have a few questions.

> If you concurrently scan the same file under two language standards with the same scanning service, it becomes non-deterministic which one gets cached in the filesystem cache.

That is true. But until your comment, I did not even know that it is possible (and supported) to be able to invoke the same scanning service on the same file under two language options (say c++14 and c++11). How would someone do that? Asking so I can test this out locally and try to come up with a better solution. (Also, why would someone do that?)

> You need to make the language standard a part of the cache key.

This was kind of one of my concerns that I had called out here: https://discourse.llvm.org/t/looking-for-help-with-accessing-langopts-from-the-actual-compiler-invocation/79228/3. Specifically:

> would it look a bit off to someone if they were to look at the header for DependencyScanningWorkerFilesystem and see that the ensureDirectiveTokensArePopulated API took a LangOpts specifically

Given that `LangOpts` is kind of becoming a feature within `DependencyScanningWorkerFilesystem`'s APIs, I am kind of inclined towards having `LangOpts` as part of the cache key for disambiguation - but again, I am very very new to this.

> An alternative solution (that I prefer) is to set up the scanner in a way that always accepts ' in integer constants.

Would this be considered "hacky"? Because if we go this way, the Scanner would technically be operating in a different language mode for integers, potentially overriding the language mode arg that was passed in during invocation. I am not opposed to it - just trying to understand the implications better. We do turn on specific `LangOpts` (like `Objc`) for the lexer during the Scanning phase as can be seen here - https://github.com/llvm/llvm-project/blob/83646590afe222cfdd792514854549077e17b005/clang/lib/Lex/DependencyDirectivesScanner.cpp#L71-L79.

I guess the general question is - is it acceptable to have the Scanner operating in a language standard different than the passed in language mode and different than the compiler language standard?

https://github.com/llvm/llvm-project/pull/93753