C#: Use Gvn comparison instead of StructuralComparisonConfiguration in UseTryGetValue.

Author: michaelnebel

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

@@ -190,8 +190,11 @@ private import Cached
 
 predicate toGvn = toGvnCached/1;
 
+/**
+ * Holds if the control flow elements `x` and `y` are structurally equal.
+ */
 pragma[inline]
-private predicate sameGvn(ControlFlowElement x, ControlFlowElement y) {
+predicate sameGvn(ControlFlowElement x, ControlFlowElement y) {
   pragma[only_bind_into](toGvn(pragma[only_bind_out](x))) =
     pragma[only_bind_into](toGvn(pragma[only_bind_out](y)))
 }

csharp/ql/src/Performance/UseTryGetValue.ql

@@ -13,19 +13,14 @@ import csharp
 import semmle.code.csharp.commons.StructuralComparison
 import semmle.code.csharp.controlflow.Guards as G
 
-class SameElement extends StructuralComparisonConfiguration {
-  SameElement() { this = "Same element" }
-
-  override predicate candidate(ControlFlowElement e1, ControlFlowElement e2) {
-    exists(MethodCall mc, IndexerRead access |
-      mc.getTarget().hasName("ContainsKey") and
-      access.getQualifier().(G::GuardedExpr).isGuardedBy(mc, mc.getQualifier(), _) and
-      e1 = mc.getArgument(0) and
-      e2 = access.getIndex(0)
-    )
-  }
+pragma[noinline]
+private predicate candidate(MethodCall mc, IndexerRead access) {
+  mc.getTarget().hasName("ContainsKey") and
+  access.getQualifier().(G::GuardedExpr).isGuardedBy(mc, mc.getQualifier(), _)
 }
 
-from SameElement element, MethodCall call, IndexerAccess index
-where element.same(call.getArgument(0), index.getIndex(0))
+from MethodCall call, IndexerRead index
+where
+  candidate(call, index) and
+  sameGvn(call.getArgument(0), index.getIndex(0))
 select call, "Inefficient use of 'ContainsKey' and $@.", index, "indexer"