Added AsIn to mark struct as In.

dig_test.go

@@ -264,39 +264,63 @@ func TestEndToEndSuccess(t *testing.T) {
 	})
 
 	t.Run("param recurse", func(t *testing.T) {
-		type anotherParam struct {
+		type anotherParamEmbedded struct {
 			dig.In
 
 			Buffer *bytes.Buffer
 		}
 
-		type someParam struct {
-			dig.In
+		type anotherParam struct {
+			Reader *bytes.Reader
+		}
 
-			Buffer  *bytes.Buffer
-			Another anotherParam
+		type someParam struct {
+			Buffer          *bytes.Buffer
+			Reader          *bytes.Reader
+			AnotherEmbedded anotherParamEmbedded
+			Another         anotherParam
 		}
 
 		var (
-			buff   *bytes.Buffer
-			called bool
+			buff         *bytes.Buffer
+			reader       *bytes.Reader
+			calledBuffer bool
+			calledReader bool
 		)
 
 		c := digtest.New(t)
 		c.RequireProvide(func() *bytes.Buffer {
-			require.False(t, called, "constructor must be called exactly once")
-			called = true
+			require.False(t, calledBuffer, "constructor must be calledBuffer exactly once")
+			calledBuffer = true
 			buff = new(bytes.Buffer)
 			return buff
 		})
 
+		c.RequireProvide(func() *bytes.Reader {
+			require.False(t, calledReader, "constructor must be calledBuffer exactly once")
+			calledReader = true
+			reader = new(bytes.Reader)
+			return reader
+		})
+
+		c.RequireProvide(dig.AsIn(someParam{}))
+		c.RequireProvide(dig.AsIn(reflect.TypeOf(anotherParam{})))
+
 		c.RequireInvoke(func(p someParam) {
-			require.True(t, called, "constructor must be called first")
+			require.True(t, calledReader, "constructor must be calledBuffer first")
+			require.True(t, calledReader, "constructor must be calledReader first")
+
+			require.NotNil(t, p.Buffer, "someParam.Reader must not be nil")
+			require.NotNil(t, p.Reader, "someParam.Reader must not be nil")
+
+			require.NotNil(t, p.Another.Reader, "anotherParam.Reader must not be nil")
+			require.True(t, p.Reader == p.Another.Reader, "readers fields must match")
+
+			require.True(t, p.Reader == reader, "buffer must match constructor's return value")
 
-			require.NotNil(t, p.Buffer, "someParam.Buffer must not be nil")
-			require.NotNil(t, p.Another.Buffer, "anotherParam.Buffer must not be nil")
+			require.NotNil(t, p.AnotherEmbedded.Buffer, "anotherParamEmbedded.Reader must not be nil")
+			require.True(t, p.Buffer == p.AnotherEmbedded.Buffer, "buffers fields must match")
 
-			require.True(t, p.Buffer == p.Another.Buffer, "buffers fields must match")
 			require.True(t, p.Buffer == buff, "buffer must match constructor's return value")
 		})
 	})
@@ -638,6 +662,13 @@ func TestEndToEndSuccess(t *testing.T) {
 			A1 A `name:"first"`  // should come from ret1 through ret2
 			A2 A `name:"second"` // should come from ret2
 		}
+
+		type paramAsIn struct {
+			A1 A `name:"first"`  // should come from ret1 through ret2
+			A2 A `name:"second"` // should come from ret2
+		}
+		c.RequireProvide(dig.AsIn(paramAsIn{}))
+
 		c.RequireProvide(func() Ret2 {
 			return Ret2{
 				Ret1: Ret1{
@@ -651,6 +682,11 @@ func TestEndToEndSuccess(t *testing.T) {
 			assert.Equal(t, 1, p.A1.idx)
 			assert.Equal(t, 2, p.A2.idx)
 		})
+
+		c.RequireInvoke(func(p paramAsIn) {
+			assert.Equal(t, 1, p.A1.idx)
+			assert.Equal(t, 2, p.A2.idx)
+		})
 	})
 
 	t.Run("named instances do not cause cycles", func(t *testing.T) {
@@ -709,7 +745,7 @@ func TestEndToEndSuccess(t *testing.T) {
 
 		require.Error(t, c.Invoke(func(*bytes.Buffer) {
 			t.Fatalf("must not be called")
-		}), "must not have a *bytes.Buffer in the container")
+		}), "must not have a *bytes.Reader in the container")
 	})
 
 	t.Run("As with Name", func(t *testing.T) {

inout.go

@@ -158,6 +158,89 @@ func embedsType(i interface{}, e reflect.Type) bool {
 	return false
 }
 
+// AsIn marks struct as In by creating reflect.StructOf.
+func AsIn(i any) any {
+	t, ok := inType(i)
+	if !ok {
+		return nil
+	}
+
+	embeddingType := reflect.TypeOf(embeddingIn(t))
+	fnType := reflect.FuncOf([]reflect.Type{embeddingType}, []reflect.Type{t}, false)
+
+	fn := reflect.MakeFunc(fnType, func(args []reflect.Value) []reflect.Value {
+		in := args[0]
+		out := reflect.New(t).Elem()
+
+		outIndex := 0
+		for inIndex := 0; inIndex < in.NumField(); inIndex++ {
+			if in.Field(inIndex).Type() == _inType {
+				continue
+			}
+
+			out.Field(outIndex).Set(in.Field(inIndex))
+			outIndex++
+		}
+
+		return []reflect.Value{out}
+	})
+
+	return fn.Interface()
+}
+
+func embeddingIn(t reflect.Type) any {
+	return embedding(t, "In", _inType)
+}
+
+func embedding(i any, name string, _type reflect.Type) any {
+	t, ok := inType(i)
+	if !ok {
+		return nil
+	}
+
+	if t.Kind() == reflect.Ptr {
+		t = t.Elem()
+	}
+
+	if t.Kind() != reflect.Struct {
+		return nil
+	}
+
+	// Build fields: start with embedded In
+	fields := make([]reflect.StructField, 0, t.NumField()+1)
+	fields = append(fields, reflect.StructField{
+		Name:      name,
+		Type:      _type,
+		Anonymous: true,
+	})
+
+	// Add all original fields
+	for i := 0; i < t.NumField(); i++ {
+		f := t.Field(i)
+		fields = append(fields, reflect.StructField{
+			Name: f.Name,
+			Type: f.Type,
+			Tag:  f.Tag,
+		})
+	}
+
+	newType := reflect.StructOf(fields)
+	return reflect.New(newType).Elem().Interface()
+}
+
+func inType(i any) (reflect.Type, bool) {
+	if i == nil {
+		return nil, false
+	}
+
+	t, ok := i.(reflect.Type)
+	if !ok {
+		t = reflect.TypeOf(i)
+	}
+
+	return t, true
+}
+
 // Checks if a field of an In struct is optional.
 func isFieldOptional(f reflect.StructField) (bool, error) {
 	tag := f.Tag.Get(_optionalTag)