[-Wunsafe-buffer-usage] Do not emit fixits for C++ interfaces with C linkage

[NoQ]

Nice, LGTM!

I guess it could also be nice to suppress note_safe_buffer_usage_suggestions_disabled in such functions but that's an extremely minor issue.

[ziqingluo-90]

clang/lib/Analysis/UnsafeBufferUsage.cpp

2146

nitpick: since we already checked the Kind here, can we use cast instead of dyn_cast?

2148

Shall we break the loop?

[t-rasmud]

Address comments.

[NoQ]

clang/lib/Analysis/UnsafeBufferUsage.cpp

2145–2146

IIRC redecls() picks up on the type, so this can be made even more idiomatic this way (didn't actually try).

[t-rasmud]

Address feedback.

[ziqingluo-90]

Looks great to me! I never thought of this case before.

clang/lib/Analysis/UnsafeBufferUsage.cpp

getNaiveStrategy(const llvm::SmallVectorImpl<const VarDecl *> &UnsafeVars) {
   }
   return S;
 }
 
 void clang::checkUnsafeBufferUsage(const Decl *D,
                                    UnsafeBufferUsageHandler &Handler,
                                    bool EmitSuggestions) {
   assert(D && D->getBody());
+  
+  // Do not emit fixit suggestions for functions declared in an
+  // extern "C" block.
+  if (const auto *FD = dyn_cast<FunctionDecl>(D)) {
+      for (FunctionDecl *FReDecl : FD->redecls()) {
+      if (FReDecl->isExternC()) {
+        EmitSuggestions = false;
+        break;
+      }
+    }
+  }
+  
   WarningGadgetSets UnsafeOps;
   FixableGadgetSets FixablesForAllVars;
 
   auto [FixableGadgets, WarningGadgets, Tracker] =
     findGadgets(D, Handler, EmitSuggestions);
 
   if (!EmitSuggestions) {
     // Our job is very easy without suggestions. Just warn about

clang/test/SemaCXX/warn-unsafe-buffer-usage-c-linkage.cpp

+// RUN: %clang_cc1 -std=c++20 -Wunsafe-buffer-usage -fsafe-buffer-usage-suggestions -verify %s
+
+extern "C" {
+void foo(int *ptr) {
+  ptr[5] = 10;  // expected-warning{{unsafe buffer access}}
+}
+
+void bar(int *ptr);
+
+struct c_struct {
+  char *name;
+};
+}
+
+void bar(int *ptr) {
+  ptr[5] = 10;  // expected-warning{{unsafe buffer access}}
+}
+
+void call_foo(int *p) {
+  foo(p);
+  struct c_struct str;
+  str.name[7] = 9;  // expected-warning{{unsafe buffer access}}
+  bar(p);
+}