Sema: do not warn about unused const vars if main file is a header

[erikjv]

Summary

If we pass a header to libclang, e.g. because it's open in an editor in
an IDE, warnings about unused const vars are not useful: other files
that include the header might use those constants. So when -x *-header
is passed as command-line option, suppress this warning.

[bkramer]

This makes sense. While variable definitions in a header are weird, the warning that they're unused isn't adding any value.

[bkramer]

lib/Sema/Sema.cpp

870

Doesn't look clang-formatted.

[erikjv]

Committed as r285386.

lib/Sema/Sema.cpp

for (UnusedFileScopedDeclsType::iterator
       } else {
         const VarDecl *DiagD = cast<VarDecl>(*I)->getDefinition();
         if (!DiagD)
           DiagD = cast<VarDecl>(*I);
         if (DiagD->isReferenced()) {
           Diag(DiagD->getLocation(), diag::warn_unneeded_internal_decl)
                 << /*variable*/1 << DiagD->getDeclName();
         } else if (DiagD->getType().isConstQualified()) {
+          const SourceManager &SM = SourceMgr;
+          if (SM.getMainFileID() != SM.getFileID(DiagD->getLocation())
+              || !PP.getLangOpts().IsHeaderFile)
             Diag(DiagD->getLocation(), diag::warn_unused_const_variable)
                 << DiagD->getDeclName();
         } else {
           Diag(DiagD->getLocation(), diag::warn_unused_variable)
               << DiagD->getDeclName();
         }
       }
     }
 
     emitAndClearUnusedLocalTypedefWarnings();

test/Sema/no-warn-unused-const-variables.c

+// RUN: %clang_cc1 -fsyntax-only -Wunused-const-variable -x c-header -ffreestanding -verify %s
+// RUN: %clang_cc1 -fsyntax-only -Wunused-const-variable -x c++-header -ffreestanding -verify %s
+// expected-no-diagnostics
+static const int unused[] = { 2, 3, 5, 7, 11, 13 };