unified: Add a hook for writebacks

Writebacks are a language-specific thing so let's add a hook for it

Author: asgerf

unified/ql/lib/codeql/unified/internal/dataflow/DataFlowCall.qll

@@ -20,7 +20,8 @@ private newtype TDataFlowCall =
       isInAssignmentContext(expr, _)
       or
       // Write back a value that was mutated in-place, such as `x.p` in `x.p.f = y`
-      isMutatedByCallOrAssign(expr, _)
+      isMutatedByCallOrAssign(expr, _) and
+      mayWriteBackViaSetterCall(expr)
     ) and
     (
       N::getStaticBindingTargetFromRef(expr) = any(SetterDeclaration g).getNameNode()

unified/ql/lib/codeql/unified/internal/dataflow/DataFlowPlugin.qll

@@ -26,6 +26,15 @@ class DataFlowPlugin extends Unit {
    * This affects the steps generated by the language-agnostic code as well as steps from the plugin itself.
    */
   predicate bypassContentSet(ContentSet contents) { none() }
+
+  /**
+   * Holds if mutating the value of `expr` in-place, such as `expr.f = y` or `expr.mutate()`,
+   * may cause the mutated value to be written back via a setter call.
+   *
+   * `expr` is an expression in getter position, such as a member access.
+   */
+  bindingset[expr]
+  predicate mayWriteBackViaSetterCall(Expr expr) { none() }
 }
 
 /**
@@ -40,3 +49,12 @@ predicate bypassContentSet(ContentSet contents) { any(DataFlowPlugin p).bypassCo
 predicate exprStep(Expr expr1, Expr expr2, boolean mustFlow) {
   any(DataFlowPlugin p).exprStep(expr1, expr2, mustFlow)
 }
+
+/**
+ * Holds if mutating the value of `expr` in-place, such as `expr.f = y` or `expr.mutate()`,
+ * may cause the mutated value to be written back via a setter call.
+ */
+bindingset[expr]
+predicate mayWriteBackViaSetterCall(Expr expr) {
+  any(DataFlowPlugin p).mayWriteBackViaSetterCall(expr)
+}

unified/ql/lib/codeql/unified/internal/dataflow/DataFlowPluginSwift.qll

@@ -11,6 +11,10 @@ private class SwiftDataFlowPlugin extends DataFlowPlugin {
     contents.asSingleton().asNamedMember() = "some.0"
   }
 
+  // Swift writes back mutated values of computed properties via their setter
+  bindingset[expr]
+  override predicate mayWriteBackViaSetterCall(Expr expr) { any() }
+
   override predicate exprStep(Expr expr1, Expr expr2, boolean mustFlow) {
     mustFlow = true and
     exists(UnaryExpr expr |