[Wunsafe-buffer-usage] Turn off unsafe-buffer warning for methods annotated with clang::unsafe_buffer_usage attribute (#125671)

Unsafe operation in methods that are already annotated with
clang::unsafe_buffer_usage attribute, should not trigger a warning. This
is because, the developer has already identified the method as unsafe
and warning at every unsafe operation is redundant.

rdar://138644831

---------

Co-authored-by: MalavikaSamak <malavika2@apple.com>

clang/docs/ReleaseNotes.rst

@@ -124,6 +124,8 @@ Removed Compiler Flags
 
 Attribute Changes in Clang
 --------------------------
+Adding [[clang::unsafe_buffer_usage]] attribute to a method definition now turns off all -Wunsafe-buffer-usage
+related warnings within the method body.
 
 - The ``no_sanitize`` attribute now accepts both ``gnu`` and ``clang`` names.
 - Clang now diagnoses use of declaration attributes on void parameters. (#GH108819)

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -2566,6 +2566,9 @@ void clang::sema::AnalysisBasedWarnings::IssueWarnings(
 
   // The Callback function that performs analyses:
   auto CallAnalyzers = [&](const Decl *Node) -> void {
+    if (Node->hasAttr<UnsafeBufferUsageAttr>())
+      return;
+
     // Perform unsafe buffer usage analysis:
     if (!Diags.isIgnored(diag::warn_unsafe_buffer_operation,
                          Node->getBeginLoc()) ||

clang/test/SemaCXX/warn-unsafe-buffer-usage-function-attr.cpp

@@ -119,16 +119,15 @@ struct HoldsUnsafeMembers {
 
     [[clang::unsafe_buffer_usage]]
     HoldsUnsafeMembers(int i)
-        : FromCtor(i),  // expected-warning{{function introduces unsafe buffer manipulation}}
-          FromCtor2{i}  // expected-warning{{function introduces unsafe buffer manipulation}}
-    {}
+        : FromCtor(i),
+          FromCtor2{i} {}
 
     HoldsUnsafeMembers(float f)
         : HoldsUnsafeMembers(0) {}  // expected-warning{{function introduces unsafe buffer manipulation}}
 
     UnsafeMembers FromCtor;
     UnsafeMembers FromCtor2;
-    UnsafeMembers FromField{3};  // expected-warning 2{{function introduces unsafe buffer manipulation}}
+    UnsafeMembers FromField{3};  // expected-warning {{function introduces unsafe buffer manipulation}}
 };
 
 struct SubclassUnsafeMembers : public UnsafeMembers {
@@ -138,8 +137,7 @@ struct SubclassUnsafeMembers : public UnsafeMembers {
 
     [[clang::unsafe_buffer_usage]]
     SubclassUnsafeMembers(int i)
-        : UnsafeMembers(i)  // expected-warning{{function introduces unsafe buffer manipulation}}
-    {}
+        : UnsafeMembers(i){}
 };
 
 // https://github.com/llvm/llvm-project/issues/80482
@@ -245,3 +243,68 @@ struct AggregateViaDefaultInit {
 void testAggregateViaDefaultInit() {
     AggregateViaDefaultInit A;
 };
+
+struct A {
+  int arr[2];
+
+  [[clang::unsafe_buffer_usage]]
+  int *ptr;
+};
+
+namespace std{
+  template <typename T> class span {
+
+   T *elements;
+
+   public:
+
+   constexpr span(T *, unsigned){}
+
+   template<class Begin, class End>
+   constexpr span(Begin first, End last){}
+
+   constexpr T* data() const noexcept {
+     return elements;
+   }
+ };
+}
+
+[[clang::unsafe_buffer_usage]]
+void check_no_warnings(unsigned idx) {
+  int *arr = new int[20];
+
+  int k = arr[idx]; // no-warning
+
+  std::span<int> sp = {arr, 20}; // no-warning
+  A *ptr = reinterpret_cast<A*> (sp.data()); // no-warning
+  A a;
+  a.ptr = arr; // no-warning
+}
+
+[[clang::unsafe_buffer_usage]]
+void check_no_warning_variadic(unsigned idx, int arr[20], ...) {
+  int k = arr[idx]; // no-warning
+
+  std::span<int> sp = {arr, 20}; // no-warning
+  A *ptr = reinterpret_cast<A*> (sp.data()); // no-warning
+  A a;
+  a.ptr = arr; // no-warning
+}
+
+template<typename T>
+[[clang::unsafe_buffer_usage]]
+void check_no_warnings_template(unsigned idx, T* arr) {
+  int k = arr[idx]; // no-warning
+
+  std::span<int> sp = {arr, 20}; // no-warning
+  A *ptr = reinterpret_cast<A*> (sp.data()); // no-warning
+  A a;
+  a.ptr = arr; // no-warning
+}
+
+void invoke_methods() {
+  int array[20];
+  check_no_warnings(30); //expected-warning{{function introduces unsafe buffer manipulation}}
+  check_no_warning_variadic(15, array); //expected-warning{{function introduces unsafe buffer manipulation}}
+  check_no_warnings_template(10, array); //expected-warning{{function introduces unsafe buffer manipulation}}
+}