[-Wunsafe-buffer-usage] Bail on fixits for arrays whose size is auto-deduced

C++ allows the array size to be omitted in declaration if it can be deduced from the
initializer.
E. g.:
int array [] = { 1, 2, 3 };

std::array<T, N> on the other hand requires the size N to be provided.

Currently the fixit produced would use empty space for N which would lead to a
compilation failure. This patch makes the fixit-producing machine just
bail in such cases and adds a FIXME to be addressed later.

Author: jkorous-apple

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -2516,6 +2516,15 @@ static FixItList fixVarDeclWithArray(const VarDecl *D, const ASTContext &Ctx,
     if (!MaybeArraySizeTxt)
       return {};
     const llvm::StringRef ArraySizeTxt = MaybeArraySizeTxt->trim();
+    if (ArraySizeTxt.empty()) {
+      // FIXME: Support array size getting determined from the initializer.
+      // Examples:
+      //    int arr1[] = {0, 1, 2};
+      //    int arr2{3, 4, 5};
+      // We might be able to preserve the non-specified size with `auto` and `std::to_array`:
+      //    auto arr1 = std::to_array<int>({0, 1, 2});
+      return {};
+    }
 
     std::optional<StringRef> IdentText =
         getVarDeclIdentifierText(D, Ctx.getSourceManager(), Ctx.getLangOpts());

clang/test/SemaCXX/warn-unsafe-buffer-usage-fixits-local-var-array.cpp

@@ -22,6 +22,22 @@ void comments_in_declaration(unsigned idx) {
   buffer_w[idx] = 0;
 }
 
+void auto_size(unsigned idx) {
+  int buffer[] = {0, 1, 2};
+// CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]
+// FIXME: implement support
+
+  buffer[idx] = 0;
+}
+
+void universal_initialization(unsigned idx) {
+  int buffer[] {0, 1, 2};
+// CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]
+// FIXME: implement support
+
+  buffer[idx] = 0;
+}
+
 void multi_decl1(unsigned idx) {
   int a, buffer[10];
 // CHECK-NOT: fix-it:"{{.*}}":{[[@LINE-1]]