C#: Use * ID for Locations in buildless.

csharp/extractor/Semmle.Extraction.CSharp/Entities/Assembly.cs

@@ -72,15 +72,14 @@ public static Assembly CreateOutputAssembly(Context cx)
 
         public override void WriteId(EscapingTextWriter trapFile)
         {
-            if (isOutputAssembly && Context.ExtractionContext.IsStandalone)
+            if (Context.ExtractionContext.IsStandalone)
             {
-                trapFile.Write("buildlessOutputAssembly");
-            }
-            else
-            {
-                trapFile.Write(assembly.ToString());
+                WriteStarId(trapFile);
+                return;
             }
 
+            trapFile.Write(assembly.ToString());
+
             if (assemblyPath is not null)
             {
                 trapFile.Write("#file:///");

csharp/extractor/Semmle.Extraction.CSharp/Entities/Locations/EmptyLocation.cs

@@ -0,0 +1,43 @@
+using System.IO;
+
+namespace Semmle.Extraction.CSharp.Entities
+{
+    /// <summary>
+    /// Used to create a single canonical empty location entity.
+    /// </summary>
+    public class EmptyLocation : GeneratedLocation
+    {
+        private EmptyLocation(Context cx) : base(cx) { }
+
+        public override void Populate(TextWriter trapFile)
+        {
+            trapFile.locations_default(this, GenFile, -1, -1, -1, -1);
+        }
+
+        public override void WriteId(EscapingTextWriter trapFile)
+        {
+            if (Context.ExtractionContext.IsStandalone)
+            {
+                WriteStarId(trapFile);
+                return;
+            }
+
+            trapFile.Write("loc,");
+            trapFile.WriteSubId(GenFile);
+            trapFile.Write(",-1,-1,-1,-1");
+        }
+
+        public static new EmptyLocation Create(Context cx) => EmptyLocationFactory.Instance.CreateEntity(cx, typeof(EmptyLocation), null);
+
+        private class EmptyLocationFactory : CachedEntityFactory<string?, EmptyLocation>
+        {
+            public static EmptyLocationFactory Instance { get; } = new EmptyLocationFactory();
+
+            /// <summary>
+            /// The QL library assumes the presence of a single empty location element.
+            /// </summary>
+            public override EmptyLocation Create(Context cx, string? init) => new EmptyLocation(cx);
+        }
+    }
+
+}

csharp/extractor/Semmle.Extraction.CSharp/Entities/Locations/GeneratedLocation.cs

@@ -4,23 +4,29 @@ namespace Semmle.Extraction.CSharp.Entities
 {
     public class GeneratedLocation : SourceLocation
     {
-        private readonly File generatedFile;
+        protected File GenFile { get; init; }
 
-        private GeneratedLocation(Context cx)
+        protected GeneratedLocation(Context cx)
             : base(cx, null)
         {
-            generatedFile = GeneratedFile.Create(cx);
+            GenFile = GeneratedFile.Create(cx);
         }
 
         public override void Populate(TextWriter trapFile)
         {
-            trapFile.locations_default(this, generatedFile, 0, 0, 0, 0);
+            trapFile.locations_default(this, GenFile, 0, 0, 0, 0);
         }
 
         public override void WriteId(EscapingTextWriter trapFile)
         {
+            if (Context.ExtractionContext.IsStandalone)
+            {
+                WriteStarId(trapFile);
+                return;
+            }
+
             trapFile.Write("loc,");
-            trapFile.WriteSubId(generatedFile);
+            trapFile.WriteSubId(GenFile);
             trapFile.Write(",0,0,0,0");
         }
 

csharp/extractor/Semmle.Extraction.CSharp/Entities/Locations/Location.cs

@@ -5,7 +5,24 @@ public abstract class Location : CachedEntity<Microsoft.CodeAnalysis.Location?>
     {
 #nullable restore warnings
         protected Location(Context cx, Microsoft.CodeAnalysis.Location? init)
-            : base(cx, init) { }
+            : base(cx, init)
+        { }
+
+        protected static void WriteStarId(EscapingTextWriter writer)
+        {
+            writer.Write('*');
+        }
+
+        public sealed override void WriteQuotedId(EscapingTextWriter writer)
+        {
+            if (Context.ExtractionContext.IsStandalone)
+            {
+                WriteStarId(writer);
+                return;
+            }
+
+            base.WriteQuotedId(writer);
+        }
 
         public override Microsoft.CodeAnalysis.Location? ReportingLocation => Symbol;
 

csharp/extractor/Semmle.Extraction.CSharp/Entities/Locations/NonGeneratedSourceLocation.cs

@@ -42,6 +42,12 @@ public File FileEntity
 
         public override void WriteId(EscapingTextWriter trapFile)
         {
+            if (Context.ExtractionContext.IsStandalone)
+            {
+                WriteStarId(trapFile);
+                return;
+            }
+
             trapFile.Write("loc,");
             trapFile.WriteSubId(FileEntity);
             trapFile.Write(',');

csharp/extractor/Semmle.Extraction.CSharp/Extractor/Analyser.cs

@@ -238,6 +238,9 @@ private void DoAnalyseCompilation()
 
                 compilationEntity = Entities.Compilation.Create(cx);
 
+                // Add a single empty location
+                Entities.EmptyLocation.Create(cx);
+
                 ExtractionContext.CompilationInfos.ForEach(ci => trapWriter.Writer.compilation_info(compilationEntity, ci.key, ci.value));
 
                 ReportProgressTaskDone(currentTaskId, assemblyPath, trapWriter.TrapFile, stopwatch.Elapsed, AnalysisAction.Extracted);

csharp/ql/lib/semmle/code/csharp/Location.qll

@@ -74,15 +74,29 @@ class Location extends @location {
 }
 
 /** An empty location. */
-class EmptyLocation extends Location {
-  EmptyLocation() { this.hasLocationInfo("", 0, 0, 0, 0) }
+class EmptyLocation extends Location, @location_default {
+  EmptyLocation() { locations_default(this, _, -1, -1, -1, -1) }
+
+  override predicate hasLocationInfo(
+    string filepath, int startline, int startcolumn, int endline, int endcolumn
+  ) {
+    filepath = "" and
+    startline = 0 and
+    startcolumn = 0 and
+    endline = 0 and
+    endcolumn = 0
+  }
+
+  override string toString() { result = "empty location" }
 }
 
 /**
  * A location in source code, comprising of a source file and a segment of text
  * within the file.
  */
 class SourceLocation extends Location, @location_default {
+  SourceLocation() { not this instanceof EmptyLocation }
+
   /** Gets the location that takes into account `#line` directives, if any. */
   SourceLocation getMappedLocation() {
     locations_mapped(this, result) and