C#: Filter using var _ = ... results from DeadStoreOfLocal.ql

Author: hvitved

csharp/ql/src/Dead Code/DeadStoreOfLocal.ql

@@ -72,7 +72,12 @@ predicate mayEscape(LocalVariable v) {
 
 class RelevantDefinition extends AssignableDefinition {
   RelevantDefinition() {
-    this instanceof AssignableDefinitions::AssignmentDefinition
+    this.(AssignableDefinitions::AssignmentDefinition).getAssignment() =
+      any(Assignment a |
+        if a = any(UsingStmt us).getAVariableDeclExpr()
+        then not a.getTargetVariable().hasName("_")
+        else any()
+      )
     or
     this instanceof AssignableDefinitions::MutationDefinition
     or

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.cs

@@ -460,4 +460,10 @@ public static int Count<T>(this IEnumerable<T> items)
             count++;
         return count;
     }
+
+    public static void Using()
+    {
+        using var x = new System.IO.FileStream("", System.IO.FileMode.Open); // BAD
+        using var _ = new System.IO.FileStream("", System.IO.FileMode.Open); // GOOD
+    }
 }

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.expected

@@ -14,6 +14,7 @@
 | DeadStoreOfLocal.cs:331:9:331:32 | ... = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:327:23:327:23 | b | b |
 | DeadStoreOfLocal.cs:372:13:372:20 | String s = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:372:13:372:13 | s | s |
 | DeadStoreOfLocal.cs:398:13:398:21 | ... = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:396:13:396:13 | s | s |
+| DeadStoreOfLocal.cs:466:19:466:75 | FileStream x = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:466:19:466:19 | x | x |
 | DeadStoreOfLocalBad.cs:7:13:7:48 | Boolean success = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocalBad.cs:7:13:7:19 | success | success |
 | DeadStoreOfLocalBad.cs:23:32:23:32 | FormatException e | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocalBad.cs:23:32:23:32 | e | e |
 | DeadStoreOfLocalBad.cs:32:22:32:22 | String s | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocalBad.cs:32:22:32:22 | s | s |