Don't diagnose unused but set when the Cleanup attribute is used.

[mbenfield]

Summary

This applies to -Wunused-but-set-variable and
-Wunused-but-set-parameter.

This addresses bug 51865.

[aaron.ballman]

I think this is reasonable, but it's missing test coverage. Can you add some tests to clang/test/Sema to demonstrate that we purposefully do not issue a diagnostic when the cleanup attribute is used?

[mbenfield]

Added a test case.

[aaron.ballman]

LGTM, thank you!

clang/lib/Sema/SemaDecl.cpp

else if (isa<LabelDecl>(D))
     DiagID = diag::warn_unused_label;
   else
     DiagID = diag::warn_unused_variable;
 
   Diag(D->getLocation(), DiagID) << D << Hint;
 }
 
 void Sema::DiagnoseUnusedButSetDecl(const VarDecl *VD) {
-  // If it's not referenced, it can't be set.
-  if (!VD->isReferenced() || !VD->getDeclName() || VD->hasAttr<UnusedAttr>())
+  // If it's not referenced, it can't be set. If it has the Cleanup attribute,
+  // it's not really unused.
+  if (!VD->isReferenced() || !VD->getDeclName() || VD->hasAttr<UnusedAttr>() ||
+      VD->hasAttr<CleanupAttr>())
     return;
 
   const auto *Ty = VD->getType().getTypePtr()->getBaseElementTypeUnsafe();
 
   if (Ty->isReferenceType() || Ty->isDependentType())
     return;
 
   if (const TagType *TT = Ty->getAs<TagType>()) {

clang/test/Sema/warn-unused-but-set-variables.c

 }
 
 void f2 (void) {
   // Don't warn, even if it's only used in a non-ODR context.
   int x;
   x = 0;
   (void) sizeof(x);
 }
+
+void for_cleanup(int *x) {
+  *x = 0;
+}
+
+void f3(void) {
+  // Don't warn if the __cleanup__ attribute is used.
+  __attribute__((__cleanup__(for_cleanup))) int x;
+  x = 5;
+}