[clang] [-Wunsafe-buffer-usage] Add FixableGadget for AddAssign in UnspecifiedUntypedContext (PR #71862)

[Ziqing Luo via cfe-commits]

================
@@ -1028,6 +1028,50 @@ class UPCPreIncrementGadget : public FixableGadget {
   }
 };
 
+// Representing a pointer type expression of the form `Ptr += n` in an
+// Unspecified Untyped Context (UUC):
+class UUCAddAssignGadget : public FixableGadget {
+private:
+  static constexpr const char *const UUCAddAssignTag =
+      "PointerAddAssignUnderUUC";
+  static constexpr const char *const IntOffsetTag = "IntOffset";
+  static constexpr const char *const OffsetTag = "Offset";
+
+  const BinaryOperator *Node; // the `Ptr += n` node
+  const IntegerLiteral *IntOffset = nullptr;
+  const DeclRefExpr *Offset = nullptr;
+
+public:
+  UUCAddAssignGadget(const MatchFinder::MatchResult &Result)
+      : FixableGadget(Kind::UUCAddAssign),
+        Node(Result.Nodes.getNodeAs<BinaryOperator>(UUCAddAssignTag)),
+        IntOffset(Result.Nodes.getNodeAs<IntegerLiteral>(IntOffsetTag)),
+        Offset(Result.Nodes.getNodeAs<DeclRefExpr>(OffsetTag)) {
+    assert(Node != nullptr && "Expecting a non-null matching result");
+  }
+
+  static bool classof(const Gadget *G) {
+    return G->getKind() == Kind::UUCAddAssign;
+  }
+
+  static Matcher matcher() {
+    return stmt(isInUnspecifiedUntypedContext(expr(ignoringImpCasts(
+        binaryOperator(
+            hasOperatorName("+="), hasLHS(declRefExpr(toSupportedVariable())),
+            hasRHS(expr(anyOf(ignoringImpCasts(declRefExpr().bind(OffsetTag)),
+                              integerLiteral().bind(IntOffsetTag)))))
----------------
ziqingluo-90 wrote:

For RHS,  if it is not a constant,  all we need to know about it is whether it is an unsigned integer.    It doesn't have to be a DRE, e.g., `p += (x + 1)` can be transformed to `p = p.subspan(x + 1)` as long as `(x + 1)` has unsigned integer type. 

https://github.com/llvm/llvm-project/pull/71862