Disallow using In/Out structs with From/As annotations

[tchung1118]

In #1053, we eased up the requirement for function signature for applying annotations to allow using In/Out structs with ResultTags/ParamTags. This had an unintended change in behavior where users are now able to use In/Out structs with From/As annotations. For example, the below code used to error out:

type Foo interface { ... }
type Bar struct { ... } // Bar implements Foo
struct Res {
  fx.Out

  ABar Bar
}
fx.Annotate(
  func() Res { ... },
  fx.As(new(Foo)),
)

Currently the code above applies the As annotation to ABar field and transforms the target function's signature accordingly. However, this was an unintended change in behavior, and we should have a design discussion about whether we actually want this before cutting a new release. Since it will require a breaking change to disable this after a new release is cut, I think the safe approach is to disable this for now and enable it after having decided that this is indeed what we want.

[ankit209]

Would love to pick up this issue. Suggested change in annotated.go.

@tchung1118 Can you please add me as a contributor to uber/fx repo ?

diff --git a/annotated.go b/annotated.go
index 3c9ecf7..80c5243 100644
--- a/annotated.go
+++ b/annotated.go
@@ -1591,8 +1591,8 @@ func (ann *annotated) cleanUpAsResults() {
 }

 // checks and returns a non-nil error if the target function:
-// - returns an fx.Out struct as a result.
-// - takes in an fx.In struct as a parameter.
+// - returns an fx.Out struct as a result and has either of ResultTags, As or From annotation
+// - takes in an fx.In struct as a parameter and has either of ParamTags, As or From annotation
 // - has an error result not as the last result.
 func (ann *annotated) typeCheckOrigFn() error {
        ft := reflect.TypeOf(ann.Target)
@@ -1608,8 +1608,11 @@ func (ann *annotated) typeCheckOrigFn() error {
                if ot.Kind() != reflect.Struct {
                        continue
                }
-               if len(ann.ResultTags) > 0 && dig.IsOut(reflect.New(ft.Out(i)).Elem().Interface()) {
-                       return errors.New("fx.Out structs cannot be annotated with fx.ResultTags")
+               if !dig.IsOut(reflect.New(ft.Out(i)).Elem().Interface()) {
+                       continue
+               }
+               if len(ann.ResultTags) > 0 || len(ann.As) > 0 || len(ann.From) > 0 {
+                       return errors.New("fx.Out structs cannot be annotated with either of fx.ResultTags, fx.As or fx.From")
                }
        }

@@ -1618,8 +1621,11 @@ func (ann *annotated) typeCheckOrigFn() error {
                if it.Kind() != reflect.Struct {
                        continue
                }
-               if len(ann.ParamTags) > 0 && dig.IsIn(reflect.New(ft.In(i)).Elem().Interface()) {
-                       return errors.New("fx.In structs cannot be annotated with fx.ParamTags")
+               if !dig.IsIn(reflect.New(ft.In(i)).Elem().Interface()) {
+                       continue
+               }
+               if len(ann.ParamTags) > 0 || len(ann.As) > 0 || len(ann.From) > 0 {
+                       return errors.New("fx.In structs cannot be annotated with either of fx.ParamTags, fx.As or fx.From")
                }
        }
        return nil