Make feature experimental

Author: adityapatwardhan

src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

@@ -114,7 +114,10 @@ static ExperimentalFeature()
                     description: "New parameter set for ForEach-Object to run script blocks in parallel"),
                 new ExperimentalFeature(
                     name: "PSTernaryOperator",
-                    description: "Support the ternary operator in PowerShell language")
+                    description: "Support the ternary operator in PowerShell language"),
+                new ExperimentalFeature(
+                    name: "PSNullCoalescingOperators",
+                    description: "Support the null coalescing operator and null coalescing assignment operator in PowerShell language")
             };
             EngineExperimentalFeatures = new ReadOnlyCollection<ExperimentalFeature>(engineFeatures);
 

src/System.Management.Automation/engine/parser/Compiler.cs

@@ -806,17 +806,20 @@ internal Expression ReduceAssignment(ISupportsAssignment left, TokenKind tokenKi
 
         private Expression GetCoalesceExpression(Expression rhs, IAssignableValue leftAssignableValue, TokenKind tokenKind)
         {
-            var exprs = new List<Expression>();
-            var temps = new List<ParameterExpression>();
-            var leftExpr = leftAssignableValue.GetValue(this, exprs, temps);
-
-            if (tokenKind == TokenKind.QuestionQuestionEquals)
+            if (ExperimentalFeature.IsEnabled("PSNullCoalescingOperators"))
             {
-                exprs.Add(leftAssignableValue.SetValue(this, Expression.MakeBinary(ExpressionType.Coalesce,leftExpr,rhs)));
-                return Expression.Block(temps, exprs);
-            }
+                var exprs = new List<Expression>();
+                var temps = new List<ParameterExpression>();
+                var leftExpr = leftAssignableValue.GetValue(this, exprs, temps);
 
-            return null;
+                if (tokenKind == TokenKind.QuestionQuestionEquals)
+                {
+                    exprs.Add(leftAssignableValue.SetValue(this, Expression.MakeBinary(ExpressionType.Coalesce, leftExpr, rhs)));
+                    return Expression.Block(temps, exprs);
+                }
+            }
+            
+            return null;            
         }
 
         internal Expression GetLocal(int tupleIndex)
@@ -5247,7 +5250,7 @@ public object VisitBinaryExpression(BinaryExpressionAst binaryExpressionAst)
                         CachedReflectionInfo.ParserOps_SplitOperator,
                         _executionContextParameter, Expression.Constant(binaryExpressionAst.ErrorPosition), lhs.Cast(typeof(object)), rhs.Cast(typeof(object)),
                         ExpressionCache.Constant(false));
-                case TokenKind.QuestionQuestion:
+                case TokenKind.QuestionQuestion when ExperimentalFeature.IsEnabled("PSNullCoalescingOperators") :
                     if (lhs is ConstantExpression lhsConstExpr && lhsConstExpr.Value != null)
                     {
                         return lhs;

src/System.Management.Automation/engine/parser/token.cs

@@ -416,7 +416,7 @@ public enum TokenKind
         /// <summary>The ternary operator '?'.</summary>
         QuestionMark = 100,
 
-        /// <summary>The null conditional assignment operator '?='.</summary>
+        /// <summary>The null conditional assignment operator '??='.</summary>
         QuestionQuestionEquals = 101,
 
         /// <summary>The null coalesce operator '??'.</summary>
@@ -675,7 +675,7 @@ public enum TokenFlags
         SpecialOperator = 0x00001000,
 
         /// <summary>
-        /// The token is one of the assignment operators: '=', '+=', '-=', '*=', '/=', '%=' or '?='
+        /// The token is one of the assignment operators: '=', '+=', '-=', '*=', '/=', '%=' or '??='
         /// </summary>
         AssignmentOperator = 0x00002000,
 

src/System.Management.Automation/engine/parser/tokenizer.cs

@@ -4996,24 +4996,23 @@ internal Token NextToken()
                 case '?' when InExpressionMode():
                     c1 = PeekChar();
 
-                    if (c1 == '?')
+                    if (ExperimentalFeature.IsEnabled("PSNullCoalescingOperators"))
                     {
-                        SkipChar();
-
-                        c1 = PeekChar();
-
-                        if (c1 == '=')
+                        if (c1 == '?')
                         {
                             SkipChar();
-                            return CheckOperatorInCommandMode(c, c1, TokenKind.QuestionQuestionEquals);
-                        }
-
-                        return CheckOperatorInCommandMode(c, c1, TokenKind.QuestionQuestion);
-                    }
-
 
+                            c1 = PeekChar();
 
+                            if (c1 == '=')
+                            {
+                                SkipChar();
+                                return CheckOperatorInCommandMode(c, c1, TokenKind.QuestionQuestionEquals);
+                            }
 
+                            return CheckOperatorInCommandMode(c, c1, TokenKind.QuestionQuestion);
+                        }
+                    }
 
                     return this.NewToken(TokenKind.QuestionMark);