Merge pull request #6799 from hvitved/csharp/dead-store-using-discard

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

Author: hvitved

csharp/change-notes/2021-10-04-dead-store-of-local.md

@@ -0,0 +1,2 @@
+lgtm,codescanning
+* `using` declarations are no longer flagged by the query "Useless assignment to local variable".

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

@@ -72,7 +72,8 @@ predicate mayEscape(LocalVariable v) {
 
 class RelevantDefinition extends AssignableDefinition {
   RelevantDefinition() {
-    this instanceof AssignableDefinitions::AssignmentDefinition
+    this.(AssignableDefinitions::AssignmentDefinition).getAssignment() =
+      any(Assignment a | not a = any(UsingDeclStmt uds).getAVariableDeclExpr())
     or
     this instanceof AssignableDefinitions::MutationDefinition
     or

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

@@ -461,3 +461,16 @@ public static int Count<T>(this IEnumerable<T> items)
         return count;
     }
 }
+
+public static class Using
+{
+    public static void M()
+    {
+        using var x = new System.IO.FileStream("", System.IO.FileMode.Open); // GOOD
+        using var _ = new System.IO.FileStream("", System.IO.FileMode.Open); // GOOD
+
+        using (var y = new System.IO.FileStream("", System.IO.FileMode.Open)) // BAD
+        {
+        }
+    }
+}

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:472:20:472:76 | FileStream y = ... | This assignment to $@ is useless, since its value is never read. | DeadStoreOfLocal.cs:472:20:472:20 | y | y |
 | 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 |