Introduce common base class for ElementReadNode and FieldReadNode

Author: smowton

ql/src/semmle/go/dataflow/SSA.qll

@@ -306,7 +306,7 @@ private newtype TSsaWithFields =
   TStep(SsaWithFields base, Field f) { exists(accessPathAux(base, f)) }
 
 /**
- * Gets a representation of `insn` as an ssa-with-fields-and-elements value if there is one.
+ * Gets a representation of `nd` as an ssa-with-fields value if there is one.
  */
 private TSsaWithFields accessPath(IR::Instruction insn) {
   exists(SsaVariable v | insn = v.getAUse() | result = TRoot(v))

ql/src/semmle/go/dataflow/internal/DataFlowUtil.qll

@@ -590,14 +590,22 @@ class ReadNode extends InstructionNode {
   }
 }
 
+/**
+ * A data-flow node that reads the value of a field from a struct, or an element from an array, slice, map or string.
+ */
+class ReadFromAggregateNode extends ReadNode {
+  /** Gets the data-flow node representing the base from which the field or element is read. */
+  abstract Node getBase();
+}
+
 /**
  * A data-flow node that reads an element of an array, map, slice or string.
  */
-class ElementReadNode extends ReadNode {
+class ElementReadNode extends ReadFromAggregateNode {
   override IR::ElementReadInstruction insn;
 
   /** Gets the data-flow node representing the base from which the element is read. */
-  Node getBase() { result = instructionNode(insn.getBase()) }
+  override Node getBase() { result = instructionNode(insn.getBase()) }
 
   /** Gets the data-flow node representing the index of the element being read. */
   Node getIndex() { result = instructionNode(insn.getIndex()) }
@@ -744,11 +752,11 @@ class AddressOperationNode extends UnaryOperationNode, ExprNode {
 /**
  * A data-flow node that reads the value of a field.
  */
-class FieldReadNode extends ReadNode {
+class FieldReadNode extends ReadFromAggregateNode {
   override IR::FieldReadInstruction insn;
 
   /** Gets the base node from which the field is read. */
-  Node getBase() { result = instructionNode(insn.getBase()) }
+  override Node getBase() { result = instructionNode(insn.getBase()) }
 
   /** Gets the field this node reads. */
   Field getField() { result = insn.getField() }