fix: Allow non-static CustomHandler method if scan method is non-static

ServiceScan.SourceGenerator.Tests/CustomHandlerTests.cs

@@ -397,6 +397,53 @@ public static partial class ModelBuilderExtensions
         Assert.Equal(expected, results.GeneratedTrees[1].ToString());
     }
 
+    [Fact]
+    public void UseInstanceCustomHandlerMethod()
+    {
+        var source = $$"""
+            using ServiceScan.SourceGenerator;
+
+            namespace GeneratorTests;
+
+            public partial class ServicesExtensions
+            {
+                [GenerateServiceRegistrations(AssignableTo = typeof(IService), CustomHandler = nameof(HandleType))]
+                public partial void ProcessServices();
+
+                private void HandleType<T>() => System.Console.WriteLine(typeof(T).Name);
+            }
+            """;
+
+        var services =
+            """
+            namespace GeneratorTests;
+
+            public interface IService { }
+            public class MyService1 : IService { }
+            public class MyService2 : IService { }
+            """;
+
+        var compilation = CreateCompilation(source, services);
+
+        var results = CSharpGeneratorDriver
+            .Create(_generator)
+            .RunGenerators(compilation)
+            .GetRunResult();
+
+        var expected = $$"""
+            namespace GeneratorTests;
+
+            public partial class ServicesExtensions
+            {
+                public partial void ProcessServices()
+                {
+                    HandleType<global::GeneratorTests.MyService1>();
+                    HandleType<global::GeneratorTests.MyService2>();
+                }
+            }
+            """;
+        Assert.Equal(expected, results.GeneratedTrees[1].ToString());
+    }
 
     private static Compilation CreateCompilation(params string[] source)
     {

ServiceScan.SourceGenerator/DependencyInjectionGenerator.ParseMethodModel.cs

@@ -54,7 +54,7 @@ public partial class DependencyInjectionGenerator
             if (attribute.CustomHandler != null)
             {
                 var customHandlerMethod = method.ContainingType.GetMembers().OfType<IMethodSymbol>()
-                    .FirstOrDefault(m => m.IsStatic && m.Name == attribute.CustomHandler);
+                    .FirstOrDefault(m => m.Name == attribute.CustomHandler);
 
                 if (customHandlerMethod is null)
                     return Diagnostic.Create(CustomHandlerMethodNotFound, attribute.Location);

ServiceScan.SourceGenerator/DiagnosticDescriptors.cs

@@ -4,70 +4,70 @@
 
 public static class DiagnosticDescriptors
 {
     public static readonly DiagnosticDescriptor NotPartialDefinition = new("DI0001",
         "Method is not partial",
         "Method with GenerateServiceRegistrations attribute must have partial modifier",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor WrongReturnType = new("DI0002",
         "Wrong return type",
         "Method with GenerateServiceRegistrations attribute must return void or IServiceCollection",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor WrongMethodParameters = new("DI0003",
         "Wrong method parameters",
         "Method with GenerateServiceRegistrations attribute must have a single IServiceCollection parameter",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor MissingSearchCriteria = new("DI0004",
         "Missing search criteria",
         "GenerateServiceRegistrations must have at least one search criteria",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor NoMatchingTypesFound = new("DI0005",
         "No matching types found",
         "There are no types matching attribute's search criteria",
         "Usage",
         DiagnosticSeverity.Warning,
         true);
 
     public static readonly DiagnosticDescriptor KeySelectorMethodHasIncorrectSignature = new("DI0007",
         "Provided KeySelector method has incorrect signature",
         "KeySelector should have non-void return type, and either be generic with no parameters, or non-generic with a single Type parameter",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor CantMixRegularAndCustomHandlerRegistrations = new("DI0008",
         "It's not allowed to mix GenerateServiceRegistrations attributes with and without CustomHandler on the same method",
         "It's not allowed to mix GenerateServiceRegistrations attributes with and without CustomHandler on the same method",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor WrongReturnTypeForCustomHandler = new("DI0009",
         "Wrong return type",
         "Method with CustomHandler must return void or the type of its first parameter",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor CustomHandlerMethodNotFound = new("DI0012",
         "Provided CustomHandler method is not found",
-        "CustomHandler parameter should point to a static method in the class",
+        "CustomHandler parameter should point to a method in the class",
         "Usage",
         DiagnosticSeverity.Error,
         true);
 
     public static readonly DiagnosticDescriptor CustomHandlerMethodHasIncorrectSignature = new("DI0011",
         "Provided CustomHandler method has incorrect signature",
         "CustomHandler method must be generic, and must have the same parameters as the method with the attribute",
         "Usage",