C#: Implement nullable warning suppression expressions.

Author: calumgrant

csharp/extractor/Semmle.Extraction.CSharp/Entities/Expressions/Factory.cs

@@ -242,6 +242,9 @@ internal static Expression Create(ExpressionNodeInfo info)
                     case SyntaxKind.SwitchExpression:
                         return Switch.Create(info);
 
+                    case SyntaxKind.SuppressNullableWarningExpression:
+                        return PostfixUnary.Create(info.SetKind(ExprKind.SUPPRESS_NULLABLE_WARNING), ((PostfixUnaryExpressionSyntax)info.Node).Operand);
+
                     default:
                         info.Context.ModelError(info.Node, $"Unhandled expression '{info.Node}' of kind '{info.Node.Kind()}'");
                         return new Unknown(info);

csharp/extractor/Semmle.Extraction.CSharp/Kinds/ExprKind.cs

@@ -113,6 +113,7 @@ public enum ExprKind
         PROPERTY_PATTERN = 116,
         POSITIONAL_PATTERN = 117,
         SWITCH_CASE = 118,
-        ASSIGN_COALESCE = 119 
+        ASSIGN_COALESCE = 119,
+        SUPPRESS_NULLABLE_WARNING = 120
     }
 }

csharp/ql/src/semmle/code/csharp/dataflow/internal/DataFlowPrivate.qll

@@ -83,6 +83,10 @@ module LocalFlow {
         scope = e2 and
         isSuccessor = true
         or
+        e1 = e2.(SuppressNullableWarningExpr).getExpr() and
+        scope = e2 and
+        isSuccessor = true
+        or
         e2 = any(ConditionalExpr ce |
             e1 = ce.getThen() or
             e1 = ce.getElse()

csharp/ql/src/semmle/code/csharp/exprs/Expr.qll

@@ -990,3 +990,20 @@ class IndexExpr extends Expr, @index_expr {
 
   override string toString() { result = "^..." }
 }
+
+/**
+ * A nullable warning suppression expression, for example `x!` in
+ * ```
+ * string GetName()
+ * {
+ *     string? x = ...;
+ *     return x!;
+ * }
+ * ```
+ */
+class SuppressNullableWarningExpr extends Expr, @suppress_nullable_warning_expr {
+  /** Gets the expression, for example `x` in `x!`. */
+  Expr getExpr() { result.getParent() = this }
+
+  override string toString() { result = "...!" }
+}

csharp/ql/src/semmlecode.csharp.dbscheme

@@ -987,6 +987,7 @@ case @expr.kind of
 | 117 = @positional_pattern_expr
 | 118 = @switch_case_expr
 | 119 = @assign_coalesce_expr
+| 120 = @suppress_nullable_warning_expr
 ;
 
 @switch = @switch_stmt | @switch_expr;

csharp/ql/test/library-tests/csharp8/NullableRefTypes.cs

@@ -0,0 +1,11 @@
+using System;
+
+class NullableRefTypes
+{
+    void TestNullableRefTypes()
+    {
+        string? x = "source";
+        string y = x!;
+        y = x!!;
+    }
+}

csharp/ql/test/library-tests/csharp8/NullableRefTypes.expected

@@ -0,0 +1,26 @@
+suppressNullableWarnings
+| NullableRefTypes.cs:8:20:8:21 | ...! | NullableRefTypes.cs:8:20:8:20 | access to local variable x |
+| NullableRefTypes.cs:9:13:9:14 | ...! | NullableRefTypes.cs:9:13:9:13 | access to local variable x |
+| NullableRefTypes.cs:9:13:9:15 | ...! | NullableRefTypes.cs:9:13:9:14 | ...! |
+nullableDataFlow
+| NullableRefTypes.cs:7:17:7:28 | SSA def(x) | NullableRefTypes.cs:8:20:8:20 | access to local variable x |
+| NullableRefTypes.cs:7:21:7:28 | "source" | NullableRefTypes.cs:7:17:7:28 | SSA def(x) |
+| NullableRefTypes.cs:8:20:8:20 | access to local variable x | NullableRefTypes.cs:8:20:8:21 | ...! |
+| NullableRefTypes.cs:8:20:8:20 | access to local variable x | NullableRefTypes.cs:9:13:9:13 | access to local variable x |
+| NullableRefTypes.cs:9:13:9:13 | access to local variable x | NullableRefTypes.cs:9:13:9:14 | ...! |
+| NullableRefTypes.cs:9:13:9:14 | ...! | NullableRefTypes.cs:9:13:9:15 | ...! |
+nullableControlFlow
+| NullableRefTypes.cs:5:10:5:29 | enter TestNullableRefTypes | NullableRefTypes.cs:6:5:10:5 | {...} | successor |
+| NullableRefTypes.cs:6:5:10:5 | {...} | NullableRefTypes.cs:7:9:7:29 | ... ...; | successor |
+| NullableRefTypes.cs:7:9:7:29 | ... ...; | NullableRefTypes.cs:7:21:7:28 | "source" | successor |
+| NullableRefTypes.cs:7:17:7:28 | String x = ... | NullableRefTypes.cs:8:9:8:22 | ... ...; | successor |
+| NullableRefTypes.cs:7:21:7:28 | "source" | NullableRefTypes.cs:7:17:7:28 | String x = ... | successor |
+| NullableRefTypes.cs:8:9:8:22 | ... ...; | NullableRefTypes.cs:8:20:8:20 | access to local variable x | successor |
+| NullableRefTypes.cs:8:16:8:21 | String y = ... | NullableRefTypes.cs:9:9:9:16 | ...; | successor |
+| NullableRefTypes.cs:8:20:8:20 | access to local variable x | NullableRefTypes.cs:8:20:8:21 | ...! | successor |
+| NullableRefTypes.cs:8:20:8:21 | ...! | NullableRefTypes.cs:8:16:8:21 | String y = ... | successor |
+| NullableRefTypes.cs:9:9:9:15 | ... = ... | NullableRefTypes.cs:5:10:5:29 | exit TestNullableRefTypes | successor |
+| NullableRefTypes.cs:9:9:9:16 | ...; | NullableRefTypes.cs:9:13:9:13 | access to local variable x | successor |
+| NullableRefTypes.cs:9:13:9:13 | access to local variable x | NullableRefTypes.cs:9:13:9:14 | ...! | successor |
+| NullableRefTypes.cs:9:13:9:14 | ...! | NullableRefTypes.cs:9:13:9:15 | ...! | successor |
+| NullableRefTypes.cs:9:13:9:15 | ...! | NullableRefTypes.cs:9:9:9:15 | ... = ... | successor |

csharp/ql/test/library-tests/csharp8/NullableRefTypes.ql

@@ -0,0 +1,17 @@
+import csharp
+
+query predicate suppressNullableWarnings(SuppressNullableWarningExpr e, Expr op) {
+  op = e.getExpr()
+}
+
+query predicate nullableDataFlow(DataFlow::Node src, DataFlow::Node sink) {
+  src.getEnclosingCallable().hasName("TestNullableRefTypes") and
+  DataFlow::localFlowStep(src, sink)
+}
+
+query predicate nullableControlFlow(
+  ControlFlow::Node a, ControlFlow::Node b, ControlFlow::SuccessorType t
+) {
+  a.getEnclosingCallable().hasName("TestNullableRefTypes") and
+  b = a.getASuccessorByType(t)
+}