do not emit -Wunused-macros warnings in -frewrite-includes mode (PR15614)

[llunak]

Summary

This is an updated patch from https://bugs.llvm.org/show_bug.cgi?id=15614. See there for testcase etc.

[llunak]

Ping?

[dblaikie]

A test case would be good (in the clang/test directory - probably near/in the other tests for -frewrite-includes)

And does the same bug occur for other preprocessor-related warnings? Maybe it's not practical to disable them all this way & there should be a different solution? (or maybe we shouldn't fix these and users can pass -w to disable warnings when using -frewrite-includes?)

[llunak]

Added a test.

[llunak]

In D65371#1659929, @dblaikie wrote:

A test case would be good (in the clang/test directory - probably near/in the other tests for -frewrite-includes)

Done.

And does the same bug occur for other preprocessor-related warnings? Maybe it's not practical to disable them all this way & there should be a different solution? (or maybe we shouldn't fix these and users can pass -w to disable warnings when using -frewrite-includes?)

I've never seen any other warning from -frewrite-includes besides -Wunused-macros. Given that I use and more or less maintain Icecream, which uses -frewrite-includes for distributed builds, I'd say any other warnings there either don't exist or are very rare (which rather makes sense, given that -frewrite-includes only does limited macro expansion when analysing the input and that's about it). Given that, I'd prefer not to disable warnings globally - they are unlikely to show up, if they do, they can hopefully be handled individually, but on the other hand maybe Clang one day gets warnings about #include or similar that would be a pity to miss when using -rewrite-includes.

[dblaikie]

Fair enough - thanks!

clang/lib/Lex/PPDirectives.cpp

void Preprocessor::HandleDefineDirective(
 
   DefMacroDirective *MD =
       appendDefMacroDirective(MacroNameTok.getIdentifierInfo(), MI);
 
   assert(!MI->isUsed());
   // If we need warning for not using the macro, add its location in the
   // warn-because-unused-macro set. If it gets used it will be removed from set.
   if (getSourceManager().isInMainFile(MI->getDefinitionLoc()) &&
-      !Diags->isIgnored(diag::pp_macro_not_used, MI->getDefinitionLoc())) {
+      !Diags->isIgnored(diag::pp_macro_not_used, MI->getDefinitionLoc()) &&
+      !MacroExpansionInDirectivesOverride) {
     MI->setIsWarnIfUnused(true);
     WarnUnusedMacroLocs.insert(MI->getDefinitionLoc());
   }
 
   // If the callbacks want to know, tell them about the macro definition.
   if (Callbacks)
     Callbacks->MacroDefined(MacroNameTok, MD);
 }

clang/test/Frontend/rewrite-includes-warnings.c

-// RUN: %clang_cc1 -verify -Wall -Wextra -E -frewrite-includes %s
+// RUN: %clang_cc1 -verify -Wall -Wextra -Wunused-macros -E -frewrite-includes %s
 // expected-no-diagnostics
 
 #pragma GCC visibility push (default)
+
+#define USED_MACRO 1
+int test() { return USED_MACRO; }