C#: Add a verbatim copy of the structural comparison for internal use only.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/commons/StructuralComparison.qll

@@ -235,3 +235,58 @@ abstract class StructuralComparisonConfiguration extends string {
     toGvn(x) = toGvn(y)
   }
 }
+
+/**
+ * INTERNAL: Do not use.
+ *
+ * A verbatim copy of the class `StructuralComparisonConfiguration` for internal
+ * use.
+ *
+ * A copy is needed in order to use structural comparison within the standard
+ * library without running into caching issues.
+ */
+module Internal {
+  // Import all uses of the internal library to make sure caching works
+  private import semmle.code.csharp.controlflow.Guards as G
+
+  /**
+   * A configuration for performing structural comparisons of program elements
+   * (expressions and statements).
+   *
+   * The predicate `candidate()` must be overridden, in order to identify the
+   * elements for which to perform structural comparison.
+   *
+   * Each use of the library is identified by a unique string value.
+   */
+  abstract class InternalStructuralComparisonConfiguration extends string {
+    bindingset[this]
+    InternalStructuralComparisonConfiguration() { any() }
+
+    /**
+     * Holds if elements `x` and `y` are candidates for testing structural
+     * equality.
+     *
+     * Subclasses are expected to override this predicate to identify the
+     * top-level elements which they want to compare. Care should be
+     * taken to avoid identifying too many pairs of elements, as in general
+     * there are very many structurally equal subtrees in a program, and
+     * in order to keep the computation feasible we must focus attention.
+     *
+     * Note that this relation is not expected to be symmetric -- it's
+     * fine to include a pair `(x, y)` but not `(y, x)`.
+     * In fact, not including the symmetrically implied fact will save
+     * half the computation time on the structural comparison.
+     */
+    abstract predicate candidate(ControlFlowElement x, ControlFlowElement y);
+
+    /**
+     * Holds if elements `x` and `y` structurally equal. `x` and `y` must be
+     * flagged as candidates for structural equality, that is,
+     * `candidate(x, y)` must hold.
+     */
+    predicate same(ControlFlowElement x, ControlFlowElement y) {
+      candidate(x, y) and
+      toGvn(x) = toGvn(y)
+    }
+  }
+}