beanz added inline comments.
================
Comment at: clang/test/Lexer/Inputs/unsafe-macro-2.h:23-26
+// not-expected-warning@+1{{macro 'UNSAFE_MACRO_2' has been marked as unsafe
for use in headers}}
+#undef UNSAFE_MACRO_2
+// not-expected-warning@+1{{macro 'UNSAFE_MACRO_2' has been marked as unsafe
for use in headers}}
+#define UNSAFE_MACRO_2 2
----------------
aaron.ballman wrote:
> 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.
Ah! That makes perfect sense. I think I misunderstood your comment. Totally
agree, having a shorthand that groups both together would be super useful. I
suspect we'll have some uses where we will want both `final` and
`header_unsafe` too!
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D107095/new/
https://reviews.llvm.org/D107095
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