Fix a crash with invalid FILE_DIR defines

DMCompiler/Compiler/CompilerError.cs

@@ -33,6 +33,7 @@ public enum WarningCode {
     FileAlreadyIncluded = 1000,
     MissingIncludedFile = 1001,
     InvalidWarningCode = 1002,
+    InvalidFileDirDefine = 1003,
     MisplacedDirective = 1100,
     UndefineMissingDirective = 1101,
     DefinedMissingParen = 1150,

DMCompiler/Compiler/DMPreprocessor/DMPreprocessor.cs

@@ -304,7 +304,7 @@ private void HandleDefineDirective(Token defineToken) {
 
             DMPreprocessorLexer currentLexer = _lexerStack.Peek();
             string dir = Path.Combine(currentLexer.IncludeDirectory, dirTokenValue);
-            compiler.AddResourceDirectory(dir);
+            compiler.AddResourceDirectory(dir, dirToken.Location);
 
             // In BYOND it goes on to set the FILE_DIR macro's value to the added directory
             // I don't see any reason to do that

DMCompiler/DMCompiler.cs

@@ -21,12 +21,13 @@ namespace DMCompiler;
 public class DMCompiler {
     public int ErrorCount;
     public int WarningCount;
-    public HashSet<WarningCode> UniqueEmissions = new();
+    public readonly HashSet<WarningCode> UniqueEmissions = new();
     public DMCompilerSettings Settings;
     public IReadOnlyList<string> ResourceDirectories => _resourceDirectories;
 
     private readonly DMCompilerConfiguration Config = new();
     private readonly List<string> _resourceDirectories = new();
+    private string _codeDirectory;
     private DateTime _compileStartTime;
 
     internal readonly DMCodeTree DMCodeTree;
@@ -97,8 +98,13 @@ public bool Compile(DMCompilerSettings settings) {
         return successfulCompile;
     }
 
-    public void AddResourceDirectory(string dir) {
+    public void AddResourceDirectory(string dir, Location loc) {
         dir = dir.Replace('\\', Path.DirectorySeparatorChar);
+        if (!Directory.Exists(dir)) {
+            Emit(WarningCode.InvalidFileDirDefine, loc,
+                $"Folder \"{Path.GetRelativePath(_codeDirectory, dir)}\" does not exist");
+            return;
+        }
 
         _resourceDirectories.Add(dir);
     }
@@ -129,7 +135,10 @@ public void AddResourceDirectory(string dir) {
             }
 
             // Adds the root of the DM project to FILE_DIR
-            compiler.AddResourceDirectory(Path.GetDirectoryName(files[0]) ?? "/");
+            _codeDirectory = Path.GetDirectoryName(files[0]) ?? "";
+            if (string.IsNullOrWhiteSpace(_codeDirectory))
+                _codeDirectory = Path.GetFullPath(".");
+            compiler.AddResourceDirectory(_codeDirectory, Location.Internal);
 
             string compilerDirectory = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location) ?? string.Empty;
             string dmStandardDirectory = Path.Join(compilerDirectory, "DMStandard");

DMCompiler/DMStandard/DefaultPragmaConfig.dm

@@ -5,6 +5,7 @@
 #pragma FileAlreadyIncluded warning
 #pragma MissingIncludedFile error
 #pragma InvalidWarningCode warning
+#pragma InvalidFileDirDefine warning
 #pragma MisplacedDirective error
 #pragma UndefineMissingDirective warning
 #pragma DefinedMissingParen error