Add queries to inspect and measure dispatch differences.

ql/src/semmle/go/dataflow/internal/CompareDispatch.qhelp

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+TBD

ql/src/semmle/go/dataflow/internal/CompareDispatch.ql

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+/**
+ * @kind problem
+ * @problem.severity warning
+ * @precision very-high
+ * @id go/compare-dispatch
+ */
+
+import go
+import DataFlowDispatch
+
+FuncDef viableCallableOld(CallExpr c) {
+  exists(DataFlow::CallNode call | call.asExpr() = c | result = call.getACallee())
+}
+
+from CallExpr c, FuncDef fn, string msg
+where
+  fn = viableCallableOld(c) and not fn = viableCallable(c) and msg = "Missing"
+  or
+  not fn = viableCallableOld(c) and fn = viableCallable(c) and msg = "New"
+select c, msg + " $@.", fn, "callee"

ql/src/semmle/go/dataflow/internal/DispatchStats.ql

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+/**
+ * @kind table
+ * @id go/dispatch-stats
+ */
+
+import go
+import DataFlowDispatch
+
+FuncDef viableCallableOld(CallExpr c) {
+  exists(DataFlow::CallNode call | call.asExpr() = c | result = call.getACallee())
+}
+
+from CallExpr c, int old, int new
+where
+  old = count(viableCallableOld(c)) and
+  new = count(viableCallable(c)) and
+  old != new
+select c, old + " -> " + new