Improve built-in entities

- Allow specifying just [Path] for entity, making it built-in
- Built-in entities error if the entity doesn't already exist

src/gaemstone.SourceGen/Descriptors.cs

@@ -36,6 +36,12 @@ public static class Descriptors
 
 	// Diagnostics relating to the combined usage of attributes.
 
+	public static readonly DiagnosticDescriptor AttributesRequireTypeOrPath = new(
+		$"gSG{_idCounter++:00}", "Entity attributes require type or path",
+		"Entity attributes require a type (such as [Entity]) or [Path].",
+		DiagnosticCategory, DiagnosticSeverity.Error, true);
+
+
 	public static readonly DiagnosticDescriptor InvalidAttributeCombination = new(
 		$"gSG{_idCounter++:00}", "Invalid combination of entity attributes",
 		"The combination of entity attributes {0} is not valid.",
@@ -61,7 +67,10 @@ public static class Descriptors
 		"A [BuiltIn, Module] must also have a [Path] set.",
 		DiagnosticCategory, DiagnosticSeverity.Info, true);
 
-
+	public static readonly DiagnosticDescriptor BuiltInModuleMustNotHaveMethods = new(
+		$"gSG{_idCounter++:00}", "BuiltIn Module must not have System / Observer",
+		"A [BuiltIn, Module] must not have [System] or [Observer] methods.",
+		DiagnosticCategory, DiagnosticSeverity.Info, true);
 
 
 	// Diagnostics relating to keywords / modifiers used with the symbol.
@@ -182,6 +191,7 @@ public static class Descriptors
 		DiagnosticCategory, DiagnosticSeverity.Error, true);
 
 
+	// TODO: Make this more descriptive. (Special = Has<>, Not<> and [Tag]?)
 	public static readonly DiagnosticDescriptor SpecialMustNotBeByRef = new(
 		$"gSG{_idCounter++:00}", "Special parameter must not be ByRef",
 		"Special parameter must not be ByRef (in/out/ref).",

src/gaemstone.SourceGen/ModuleGenerator.cs

@@ -107,6 +107,7 @@ public class ModuleGenerator
 
 			""");
 
+		// TODO: Built-in modules should not have dependencies.
 		var dependencies = module.GetDependencies().ToList();
 		sb.Append("\tstatic IReadOnlyList<string> IModule.Dependencies { get; } = ");
 		if (dependencies.Count > 0) {
@@ -155,26 +156,30 @@ public class ModuleGenerator
 		foreach (var e in entities) {
 			var @var = $"_{e.Name}_Entity";
 			var path = (e.EntityPath ?? e.Name).ToStringLiteral();
-			sb.AppendLine($"\t\tvar {@var} = world.New({path}, module)");
-			if (e.EntitySymbol != null)
-				sb.AppendLine($"\t\t\t.Symbol({e.EntitySymbol.ToStringLiteral()})");
+
+			// When looking for a BuiltIn entity, error if it doesn't exist.
+			var lookupFunc = (e.IsBuiltIn ? "LookupPathOrThrow" : "New");
+			sb.AppendLine($"\t\tvar {@var} = world.{lookupFunc}({path}, module)");
 
 			// Since this is a custom gaemstone tag, we want to add it even for [BuiltIn] modules.
 			if (e.IsRelation) sb.AppendLine("\t\t\t.Add<gaemstone.Doc.Relation>()");
 
+			// Other than [Relation], we don't add anything to entities defined in [BuiltIn] modules.
 			if (module.IsBuiltIn)
 			{
-				sb.AppendLine($"\t\t\t.Build().CreateLookup<{e.FullName}>()");
+				sb.AppendLine($"\t\t\t.CreateLookup<{e.FullName}>()");
 			}
 			else
 			{
-				// TODO: if (e.IsPublic) sb.AppendLine($"\t\t\t.Symbol(\"{e.Name}\")");
+				if (e.EntitySymbol != null)
+					sb.AppendLine($"\t\t\t.Symbol({e.EntitySymbol.ToStringLiteral()})");
 
 				// Tags and relations in Flecs are marked as empty components.
 				if (e.IsTag || e.IsRelation) sb.AppendLine("\t\t\t.Add<gaemstone.Flecs.Core.Component>()");
 				if (e.IsTag && e.IsRelation) sb.AppendLine("\t\t\t.Add<gaemstone.Flecs.Core.Tag>()");
 
-				                   sb.Append( "\t\t\t.Build()");
+				                   sb.Append( "\t\t\t");
+				if (!e.IsBuiltIn)  sb.Append(".Build()");
 				if (e.IsComponent) sb.Append($".InitComponent<{e.FullName}>()");
 				else               sb.Append($".CreateLookup<{e.FullName}>()");
 				                   sb.AppendLine();

src/gaemstone.SourceGen/Structure/MethodEntityInfo.cs

@@ -45,6 +45,9 @@ public class MethodEntityInfo : BaseEntityInfo
 		if (IsSystem && IsObserver) yield return Diagnostic.Create(
 			Descriptors.InvalidAttributeCombination, Location, "[System, Observer]");
 
+		if (Parent is ModuleEntityInfo { IsBuiltIn: true }) yield return Diagnostic.Create(
+			Descriptors.BuiltInModuleMustNotHaveMethods, Location);
+
 		if (Symbol.IsAbstract) yield return Diagnostic.Create(
 			Descriptors.MethodMustNotBeAbstract, Location);
 		if (Symbol.IsAsync) yield return Diagnostic.Create(

src/gaemstone.SourceGen/Structure/ModuleEntityInfo.cs

@@ -10,7 +10,6 @@ namespace gaemstone.SourceGen.Structure;
 public class ModuleEntityInfo : TypeEntityInfo
 {
 	public bool IsPartial { get; }
-	public bool IsBuiltIn { get; }
 	public bool HasInitializer { get; }
 
 	public ModuleEntityInfo(ISymbol symbol)
@@ -19,10 +18,10 @@ public class ModuleEntityInfo : TypeEntityInfo
 		var classDecl = (TypeDeclarationSyntax)Symbol.DeclaringSyntaxReferences.First().GetSyntax();
 		IsPartial = classDecl.Modifiers.Any(t => t.IsKind(SyntaxKind.PartialKeyword));
 
-		IsBuiltIn = Has("BuiltIn");
-
 		HasInitializer = Symbol.AllInterfaces.Any(i =>
 			i.GetFullName(FullNameStyle.NoGeneric) == "gaemstone.ECS.IModuleInitializer");
+
+		IsBuiltIn = Has("BuiltIn");
 	}
 
 	protected override IEnumerable<Diagnostic> ValidateSelf()

src/gaemstone.SourceGen/Structure/TypeEntityInfo.cs

@@ -17,6 +17,11 @@ public class TypeEntityInfo : BaseEntityInfo
 	public bool IsSingleton { get; }
 	public bool IsRelation { get; }
 
+	// Built-in entities error if they don't exist.
+	// When a module is built-in, most attributes become descriptive,
+	// and don't change or affect the entity they're attached to.
+	public bool IsBuiltIn { get; protected set; }
+
 	public bool IsValueType     => Symbol.IsValueType;
 	public bool IsReferenceType => Symbol.IsReferenceType;
 	// TODO: Make sure that component has (instance) fields, non-component entity does not have fields.
@@ -30,6 +35,10 @@ public class TypeEntityInfo : BaseEntityInfo
 		IsComponent = Has("Component");
 		IsSingleton = Has("Singleton");
 		IsRelation  = Has("Relation");
+
+		IsBuiltIn = !IsModule && !IsEntity && !IsTag
+				 && !IsComponent && !IsSingleton && !IsRelation
+				 && (EntityPath != null);
 	}
 
 	protected override IEnumerable<Diagnostic> ValidateSelf()
@@ -49,7 +58,14 @@ public class TypeEntityInfo : BaseEntityInfo
 		if (IsComponent) attributeList.Add("Component");
 		if (IsSingleton) attributeList.Add("Singleton");
 
-		switch (attributeList) {
+		switch (attributeList)
+		{
+			// No attributes are only valid if [Path] is set.
+			case [  ]:
+				if (!IsBuiltIn) yield return Diagnostic.Create(
+					Descriptors.AttributesRequireTypeOrPath, Location,
+					$"[{string.Join(", ", attributeList)}]");
+				break;
 			// A single attribute is valid.
 			case [ _ ]:
 			// The following are valid attribute combinations.
@@ -70,6 +86,10 @@ public class TypeEntityInfo : BaseEntityInfo
 				break;
 		}
 
+		// In in a built-in module, this entity is built-in, too.
+		if (Parent is ModuleEntityInfo { IsBuiltIn: true })
+			IsBuiltIn = true;
+
 		// Singletons are special kinds of components.
 		if (IsSingleton) IsComponent = true;
 	}

src/gaemstone/Universe+Modules.cs

@@ -91,11 +91,18 @@ public class ModuleManager<TContext>
 
 		public ModuleInfo(Universe<TContext> universe)
 		{
-			var world   = universe.World;
-			var builder = world.New(T.Path);
-			var deps    = new List<ModuleDependency>();
+			var world = universe.World;
+
+			if (T.IsBuiltIn)
+			{
+				Entity = world.LookupPathOrThrow(T.Path);
+				Dependencies = Array.Empty<ModuleDependency>();
+			}
+			else
+			{
+				var builder = world.New(T.Path);
+				var deps    = new List<ModuleDependency>();
 
-			if (!T.IsBuiltIn) {
 				builder.Add<Module>();
 				foreach (var dependsPath in T.Dependencies) {
 					var dependency = world.LookupPathOrNull(dependsPath) ??
@@ -108,15 +115,14 @@ public class ModuleManager<TContext>
 					if (!isDepInit) builder.Add<Disabled>();
 					builder.Add<DependsOn>(dependency);
 				}
-			}
 
-			Entity = builder.Build().CreateLookup<T>();
-			Dependencies = deps.AsReadOnly();
+				Entity = builder.Build().CreateLookup<T>();
+				Dependencies = deps.AsReadOnly();
 
-			if (!T.IsBuiltIn)
 				// Ensure all parent entities have the Module tag set.
 				for (var p = Entity.Parent; p is Entity<TContext> parent; p = parent.Parent)
 					parent.Add<Module>();
+			}
 		}
 
 		public void Enable()