Add opendream_noconstfold, add it to some world vars

DMCompiler/Compiler/DM/DMParser.cs

@@ -2801,6 +2801,7 @@ private DMValueType SingleAsType(out DreamPath? path, bool allowPath = false) {
                 case "path": return DMValueType.Path;
                 case "opendream_unimplemented": return DMValueType.Unimplemented;
                 case "opendream_compiletimereadonly": return DMValueType.CompiletimeReadonly;
+                case "opendream_noconstfold": return DMValueType.NoConstFold;
                 default:
                     Emit(WarningCode.BadToken, typeToken.Location, $"Invalid value type '{typeToken.Text}'");
                     return 0;

DMCompiler/DM/Builders/DMExpressionBuilder.cs

@@ -952,7 +952,7 @@ private DMExpression BuildDereference(DMASTDereference deref, DreamPath? inferre
                         }
 
                         if (property == null && fromObject.GetGlobalVariableId(field) is { } globalId) {
-                            property = ObjectTree.Globals [globalId];
+                            property = ObjectTree.Globals[globalId];
 
                             expr = new GlobalField(expr.Location, property.Type, globalId, property.ValType);
 

DMCompiler/DM/DMValueType.cs

@@ -28,6 +28,7 @@ public enum DMValueType {
     //Byond here be dragons
     Unimplemented = 0x4000, // Marks that a method or property is not implemented. Throws a compiler warning if accessed.
     CompiletimeReadonly = 0x8000, // Marks that a property can only ever be read from, never written to. This is a const-ier version of const, for certain standard values like list.type
+    NoConstFold = 0x10000 // Marks that a const var cannot be const-folded during compile
 }
 
 /// <summary>

DMCompiler/DM/DMVariable.cs

@@ -14,6 +14,9 @@ internal sealed class DMVariable {
     public DMExpression? Value;
     public DMComplexValueType ValType;
 
+    public bool CanConstFold => (IsConst || ValType.MatchesType(DMValueType.CompiletimeReadonly)) &&
+                                !ValType.MatchesType(DMValueType.NoConstFold);
+
     public DMVariable(DreamPath? type, string name, bool isGlobal, bool isConst, bool isTmp, DMComplexValueType? valType = null) {
         Type = type;
         Name = name;

DMCompiler/DM/Expressions/Dereference.cs

@@ -303,14 +303,8 @@
 
         if (operation is FieldOperation fieldOperation && prevPath is not null && compiler.DMObjectTree.TryGetDMObject(prevPath.Value, out var obj)) {
             var variable = obj.GetVariable(fieldOperation.Identifier);
-            if (variable != null) {
-                if (variable.IsConst)
-                    return variable.Value.TryAsConstant(compiler, out constant);
-                if (variable.ValType.IsCompileTimeReadOnly) {
-                    variable.Value.TryAsConstant(compiler, out constant!);
-                    return true; // MUST be true.
-                }
-            }
+            if (variable is { CanConstFold: true })
+                return variable.Value.TryAsConstant(compiler, out constant);
         }
 
         constant = null;

DMCompiler/DM/Expressions/LValue.cs

@@ -144,7 +144,7 @@ public override DMReference EmitReference(ExpressionContext ctx, string endLabel
     public override string GetNameof(ExpressionContext ctx) => variable.Name;
 
     public override bool TryAsConstant(DMCompiler compiler, [NotNullWhen(true)] out Constant? constant) {
-        if (variable is { IsConst: true, Value: not null }) {
+        if (variable is { CanConstFold: true, Value: not null }) {
             return variable.Value.TryAsConstant(compiler, out constant);
         }
 
@@ -181,7 +181,7 @@ public override string GetNameof(ExpressionContext ctx) {
 
     public override bool TryAsConstant(DMCompiler compiler, [NotNullWhen(true)] out Constant? constant) {
         DMVariable global = compiler.DMObjectTree.Globals[Id];
-        if (global.IsConst) {
+        if (global.CanConstFold) {
             return global.Value.TryAsConstant(compiler, out constant);
         }
 

DMCompiler/DMStandard/Types/World.dm

@@ -42,7 +42,7 @@
 
 	var/sleep_offline = 0
 
-	var/system_type
+	var/const/system_type as opendream_noconstfold
 
 	var/map_cpu = 0 as opendream_unimplemented
 	var/hub as opendream_unimplemented
@@ -56,7 +56,7 @@
 
 	// An OpenDream read-only var that tells you what port Topic() is listening on
 	// Remove OPENDREAM_TOPIC_PORT_EXISTS if this is ever removed
-	var/const/opendream_topic_port
+	var/const/opendream_topic_port as opendream_noconstfold
 
 	proc/New()
 	proc/Del()