Drop dead IFreezable/Freeze apparatus from the C# AST

The C# AST inherited NRefactory's freezable model (IFreezable, Freeze,
IsFrozen, a frozen flag bit, and ThrowIfFrozen guards on every mutator),
but the decompiler never uses it: nothing calls Freeze(), not even the
generated null-node singletons, so every IsFrozen guard only ever
evaluated false. The decompiler is single-threaded and never shares or
freezes nodes. Remove the whole apparatus as preparation for the
slot-based AST rewrite, which has no place for it. Roles are untouched
here, so the flags word keeps its role index; only the freed frozen bit
goes away.

ICSharpCode.Decompiler/CSharp/Syntax/AstNode.cs

@@ -37,7 +37,7 @@ using ICSharpCode.Decompiler.TypeSystem;
 
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
-	public abstract class AstNode : AbstractAnnotatable, IFreezable, INode, ICloneable
+	public abstract partial class AstNode : AbstractAnnotatable, INode, ICloneable
 	{
 		// the Root role must be available when creating the null nodes, so we can't put it in the Roles class
 		internal static readonly Role<AstNode?> RootRole = new Role<AstNode?>("Root", null);
@@ -135,40 +135,12 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		// Flags, from least significant to most significant bits:
 		// - Role.RoleIndexBits: role index
-		// - 1 bit: IsFrozen
 		protected uint flags = RootRole.Index;
 		// Derived classes may also use a few bits,
 		// for example Identifier uses 1 bit for IsVerbatim
 
 		const uint roleIndexMask = (1u << Role.RoleIndexBits) - 1;
-		const uint frozenBit = 1u << Role.RoleIndexBits;
-		protected const int AstNodeFlagsUsedBits = Role.RoleIndexBits + 1;
-
-		protected AstNode()
-		{
-			if (IsNull)
-				Freeze();
-		}
-
-		public bool IsFrozen {
-			get { return (flags & frozenBit) != 0; }
-		}
-
-		public void Freeze()
-		{
-			if (!IsFrozen)
-			{
-				for (AstNode? child = firstChild; child != null; child = child.nextSibling)
-					child.Freeze();
-				flags |= frozenBit;
-			}
-		}
-
-		protected void ThrowIfFrozen()
-		{
-			if (IsFrozen)
-				throw new InvalidOperationException("Cannot mutate frozen " + GetType().Name);
-		}
+		protected const int AstNodeFlagsUsedBits = Role.RoleIndexBits;
 
 		public abstract NodeType NodeType {
 			get;
@@ -211,7 +183,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					throw new ArgumentNullException(nameof(value));
 				if (!value.IsValid(this))
 					throw new ArgumentException("This node is not valid in the new role.");
-				ThrowIfFrozen();
 				SetRole(value);
 			}
 		}
@@ -382,13 +353,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				throw new ArgumentNullException(nameof(role));
 			if (child == null || child.IsNull)
 				return;
-			ThrowIfFrozen();
 			if (child == this)
 				throw new ArgumentException("Cannot add a node to itself as a child.", nameof(child));
 			if (child.parent != null)
 				throw new ArgumentException("Node is already used in another tree.", nameof(child));
-			if (child.IsFrozen)
-				throw new ArgumentException("Cannot add a frozen node.", nameof(child));
 			AddChildUnsafe(child, role);
 		}
 
@@ -396,13 +364,10 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		{
 			if (child == null || child.IsNull)
 				return;
-			ThrowIfFrozen();
 			if (child == this)
 				throw new ArgumentException("Cannot add a node to itself as a child.", nameof(child));
 			if (child.parent != null)
 				throw new ArgumentException("Node is already used in another tree.", nameof(child));
-			if (child.IsFrozen)
-				throw new ArgumentException("Cannot add a frozen node.", nameof(child));
 			AddChildUnsafe(child, child.Role);
 		}
 
@@ -437,11 +402,8 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 			if (child == null || child.IsNull)
 				return;
-			ThrowIfFrozen();
 			if (child.parent != null)
 				throw new ArgumentException("Node is already used in another tree.", nameof(child));
-			if (child.IsFrozen)
-				throw new ArgumentException("Cannot add a frozen node.", nameof(child));
 			if (nextSibling.parent != this)
 				throw new ArgumentException("NextSibling is not a child of this node.", nameof(nextSibling));
 			// No need to test for "Cannot add children to null nodes",
@@ -481,7 +443,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		{
 			if (parent != null)
 			{
-				ThrowIfFrozen();
 				if (prevSibling != null)
 				{
 					Debug.Assert(prevSibling.nextSibling == this);
@@ -524,7 +485,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			{
 				throw new InvalidOperationException(this.IsNull ? "Cannot replace the null nodes" : "Cannot replace the root node");
 			}
-			ThrowIfFrozen();
 			// Because this method doesn't statically check the new node's type with the role,
 			// we perform a runtime test:
 			if (!this.Role.IsValid(newNode))
@@ -545,9 +505,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 					throw new ArgumentException("Node is already used in another tree.", nameof(newNode));
 				}
 			}
-			if (newNode.IsFrozen)
-				throw new ArgumentException("Cannot add a frozen node.", nameof(newNode));
-
 			newNode.parent = parent;
 			newNode.SetRole(this.Role);
 			newNode.prevSibling = prevSibling;
@@ -623,7 +580,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			copy.lastChild = null;
 			copy.prevSibling = null;
 			copy.nextSibling = null;
-			copy.flags &= ~frozenBit; // unfreeze the copy
 
 			// Then perform a deep copy:
 			for (AstNode? cur = firstChild; cur != null; cur = cur.nextSibling)

ICSharpCode.Decompiler/CSharp/Syntax/CSharpModifierToken.cs

@@ -38,7 +38,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public Modifiers Modifier {
 			get { return modifier; }
 			set {
-				ThrowIfFrozen();
 				this.modifier = value;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Syntax/DocumentationReference.cs

@@ -43,7 +43,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public SymbolKind SymbolKind {
 			get { return symbolKind; }
 			set {
-				ThrowIfFrozen();
 				symbolKind = value;
 			}
 		}
@@ -55,7 +54,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public OperatorType OperatorType {
 			get { return operatorType; }
 			set {
-				ThrowIfFrozen();
 				operatorType = value;
 			}
 		}
@@ -66,7 +64,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public bool HasParameterList {
 			get { return hasParameterList; }
 			set {
-				ThrowIfFrozen();
 				hasParameterList = value;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/AnonymousMethodExpression.cs

@@ -41,7 +41,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public bool IsAsync {
 			get { return isAsync; }
-			set { ThrowIfFrozen(); isAsync = value; }
+			set { isAsync = value; }
 		}
 
 		// used to tell the difference between delegate {} and delegate () {}
@@ -49,7 +49,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public bool HasParameterList {
 			get { return hasParameterList || Parameters.Any(); }
-			set { ThrowIfFrozen(); hasParameterList = value; }
+			set { hasParameterList = value; }
 		}
 
 		public CSharpTokenNode DelegateToken {

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/LambdaExpression.cs

@@ -43,7 +43,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public bool IsAsync {
 			get { return isAsync; }
-			set { ThrowIfFrozen(); isAsync = value; }
+			set { isAsync = value; }
 		}
 
 		public CSharpTokenNode LParToken {

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/NullReferenceExpression.cs

@@ -40,7 +40,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		internal void SetStartLocation(TextLocation value)
 		{
-			ThrowIfFrozen();
 			this.location = value;
 		}
 

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/PrimitiveExpression.cs

@@ -57,7 +57,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		internal void SetLocation(TextLocation startLocation, TextLocation endLocation)
 		{
-			ThrowIfFrozen();
 			this.startLocation = startLocation;
 			this.endLocation = endLocation;
 		}
@@ -68,7 +67,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public object Value {
 			get { return this.value; }
 			set {
-				ThrowIfFrozen();
 				this.value = value;
 			}
 		}
@@ -76,7 +74,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public LiteralFormat Format {
 			get { return format; }
 			set {
-				ThrowIfFrozen();
 				format = value;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/Comment.cs

@@ -62,7 +62,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public CommentType CommentType {
 			get { return commentType; }
-			set { ThrowIfFrozen(); commentType = value; }
+			set { commentType = value; }
 		}
 
 		/// <summary>
@@ -78,14 +78,14 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public bool StartsLine {
 			get { return startsLine; }
-			set { ThrowIfFrozen(); startsLine = value; }
+			set { startsLine = value; }
 		}
 
 		string content;
 
 		public string Content {
 			get { return content; }
-			set { ThrowIfFrozen(); content = value; }
+			set { content = value; }
 		}
 
 		TextLocation startLocation;
@@ -104,13 +104,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		internal void SetStartLocation(TextLocation value)
 		{
-			ThrowIfFrozen();
 			this.startLocation = value;
 		}
 
 		internal void SetEndLocation(TextLocation value)
 		{
-			ThrowIfFrozen();
 			this.endLocation = value;
 		}
 

ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/TypeDeclaration.cs

@@ -83,7 +83,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public ClassType ClassType {
 			get { return classType; }
 			set {
-				ThrowIfFrozen();
 				classType = value;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Syntax/GeneralScope/TypeParameterDeclaration.cs

@@ -45,7 +45,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		public VarianceModifier Variance {
 			get { return variance; }
-			set { ThrowIfFrozen(); variance = value; }
+			set { variance = value; }
 		}
 
 		public CSharpTokenNode VarianceToken {

ICSharpCode.Decompiler/CSharp/Syntax/Identifier.cs

@@ -72,7 +72,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			set {
 				if (value == null)
 					throw new ArgumentNullException(nameof(value));
-				ThrowIfFrozen();
 				this.name = value;
 			}
 		}
@@ -86,7 +85,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		internal void SetStartLocation(TextLocation value)
 		{
-			ThrowIfFrozen();
 			this.startLocation = value;
 		}
 
@@ -97,7 +95,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				return (flags & verbatimBit) != 0;
 			}
 			set {
-				ThrowIfFrozen();
 				if (value)
 					flags |= verbatimBit;
 				else

ICSharpCode.Decompiler/CSharp/Syntax/MemberType.cs

@@ -37,7 +37,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public bool IsDoubleColon {
 			get { return isDoubleColon; }
 			set {
-				ThrowIfFrozen();
 				isDoubleColon = value;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Syntax/PrimitiveType.cs

@@ -41,7 +41,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 			set {
 				if (value == null)
 					throw new ArgumentNullException();
-				ThrowIfFrozen();
 				keyword = value;
 			}
 		}
@@ -73,7 +72,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 
 		internal void SetStartLocation(TextLocation value)
 		{
-			ThrowIfFrozen();
 			this.location = value;
 		}
 

ICSharpCode.Decompiler/CSharp/Syntax/SyntaxTree.cs

@@ -49,7 +49,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public string FileName {
 			get { return fileName; }
 			set {
-				ThrowIfFrozen();
 				fileName = value;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/OperatorDeclaration.cs

@@ -173,7 +173,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public OperatorType OperatorType {
 			get { return operatorType; }
 			set {
-				ThrowIfFrozen();
 				operatorType = value;
 			}
 		}

ICSharpCode.Decompiler/CSharp/Syntax/TypeMembers/ParameterDeclaration.cs

@@ -110,7 +110,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public bool HasThisModifier {
 			get { return hasThisModifier; }
 			set {
-				ThrowIfFrozen();
 				hasThisModifier = value;
 			}
 		}
@@ -118,7 +117,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public bool IsParams {
 			get { return isParams; }
 			set {
-				ThrowIfFrozen();
 				isParams = value;
 			}
 		}
@@ -126,7 +124,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public bool IsScopedRef {
 			get { return isScopedRef; }
 			set {
-				ThrowIfFrozen();
 				isScopedRef = value;
 			}
 		}
@@ -136,7 +133,6 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public ReferenceKind ParameterModifier {
 			get { return parameterModifier; }
 			set {
-				ThrowIfFrozen();
 				parameterModifier = value;
 			}
 		}