[PATCH] D107095: Implement #pragma clang header_unsafe

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman added inline comments.


================
Comment at: clang/test/Lexer/Inputs/unsafe-macro-2.h:23-26
+// not-expected-warning at +1{{macro 'UNSAFE_MACRO_2' has been marked as unsafe for use in headers}}
+#undef UNSAFE_MACRO_2
+// not-expected-warning at +1{{macro 'UNSAFE_MACRO_2' has been marked as unsafe for use in headers}}
+#define UNSAFE_MACRO_2 2
----------------
beanz wrote:
> aaron.ballman wrote:
> > beanz wrote:
> > > aaron.ballman wrote:
> > > > Why do we not expect warnings for these cases? I would have expected that undefining a macro is just as unsafe for ABI reasons as defining a macro is.
> > > I kinda waffled on this myself. My thought was to treat this similarly to how we handle the macro redefinition warning. If you `undef`, you're kind of claiming the macro as your own and all bets are off...
> > > 
> > > That said, my next clang extension closes that loop hole too:
> > > https://github.com/llvm-beanz/llvm-project/commit/f0a5216e18f5ee0883039095169bd380295b1de0
> > So `header_unsafe` is "diagnose if someone expands this macro from outside the main source file" and `final` is "diagnose if someone defines or undefines this macro anywhere", correct? Would it make sense to have a shorthand to combine these effects for a "fully reserved" macro identifier (`#pragma clang reserve_macro(IDENT[, msg])` as a strawman)?
> My thought process for implementing them separately was that final would be useful independent of header_unsafe. I could, for example, see applying final to macros like MIN and MAX, where they can be safely used anywhere, but you really don’t want multiple definitions floating around. 
FWIW, I agree that having separation is useful -- I think these serve orthogonal (but related) purposes: macros which can only be used by "user code", and macros which cannot be redefined or undefined. I was thinking that would be an additional pragma instead of a replacement for the two proposed.

I should probably tell you my use cases so we're both on the same page. One of the most frustrating problems with trying to write a highly portable library is the fact that I have to worry about users defining macros that may conflict with my identifiers (like function names, structure names, template names, etc), but I have no way to reserve those identifiers. I'm hopeful we can find a way that I can "protect" those identifiers in a library with an extension that basically says "you can't use these identifiers for macro purposes without breaking me". I think that's a combination of `header_unsafe` and `final` -- I don't want other libraries to start using macros with the names of my library's functions if my header has been included somewhere, and I don't want user code defining macros that may conflict with my library.


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  https://reviews.llvm.org/D107095/new/

https://reviews.llvm.org/D107095