Merge pull request #2568 from calumgrant/cs/assignment-to-_

C#: Remove FP in useless assignment to _

Author: hvitved

change-notes/1.24/analysis-csharp.md

@@ -13,6 +13,7 @@ The following changes in version 1.24 affect C# analysis in all applications.
 
 | **Query**                    | **Expected impact**    | **Change**                        |
 |------------------------------|------------------------|-----------------------------------|
+| Useless assignment to local variable (`cs/useless-assignment-to-local`) | Fewer false positive results | Results have been removed when the variable is named `_` in a `foreach` statement. | 
 
 ## Removal of old queries
 

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

@@ -83,8 +83,10 @@ class RelevantDefinition extends AssignableDefinition {
     //or
     //this.(AssignableDefinitions::OutRefDefinition).getTargetAccess().isOutArgument()
     this.(AssignableDefinitions::LocalVariableDefinition).getDeclaration() = any(LocalVariableDeclExpr lvde |
-        lvde = any(SpecificCatchClause scc).getVariableDeclExpr() or
-        lvde = any(ForeachStmt fs).getVariableDeclExpr()
+        lvde = any(SpecificCatchClause scc).getVariableDeclExpr()
+        or
+        lvde = any(ForeachStmt fs).getVariableDeclExpr() and
+        not lvde.getName() = "_"
       )
     or
     this instanceof AssignableDefinitions::PatternDefinition

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

@@ -259,7 +259,7 @@ void M7()
         fn(() =>
         {
             var x = y;  // BAD: Dead store in lambda
-        return 0;
+            return 0;
         });
     }
 
@@ -425,3 +425,14 @@ int M(bool b)
         return i;
     }
 }
+
+public static class AnonymousVariable
+{
+    public static int Count<T>(this IEnumerable<T> items)
+    {
+        int count = 0;
+        foreach (var _ in items) // GOOD
+            count++;
+        return count;
+    }
+}