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Add ParamTag[T] and ResultTag[T] type-targeted annotations #1289

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Add ParamTag[T] and ResultTag[T] type-targeted annotations #1289

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5a9faf8
Refactor tag builders to use annotated state
destel May 8, 2026
c333b44
Add type-based parameter tag annotation
destel May 8, 2026
43e1b76
Add type-based result tag annotation
destel May 9, 2026
7446e3d
Handle terminal error result tags in a backward compatible way
destel May 9, 2026
324fb6e
Use fxreflect.FuncName in tag annotation errors
destel May 9, 2026
86a27a3
Validate nth >= 1 and reword tag annotation errors
destel May 9, 2026
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222 changes: 205 additions & 17 deletions annotated.go
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Original file line number Diff line number Diff line change
Expand Up @@ -248,6 +248,10 @@ func (pt paramTagsAnnotation) apply(ann *annotated) error {

// build builds and returns a constructor after applying a ParamTags annotation
func (pt paramTagsAnnotation) build(ann *annotated) (any, error) {
if ann.paramTagsBuilt {
return ann.Target, nil
}
ann.paramTagsBuilt = true
paramTypes, remap := pt.parameters(ann)
resultTypes, _ := ann.currentResultTypes()

Expand All @@ -267,6 +271,7 @@ func (pt paramTagsAnnotation) parameters(ann *annotated) (
types []reflect.Type,
remap func([]reflect.Value) []reflect.Value,
) {
tags := ann.ParamTags
ft := reflect.TypeOf(ann.Target)
types = make([]reflect.Type, ft.NumIn())
for i := 0; i < ft.NumIn(); i++ {
Expand All @@ -275,7 +280,7 @@ func (pt paramTagsAnnotation) parameters(ann *annotated) (

// No parameter annotations. Return the original types
// and an identity function.
if len(pt.tags) == 0 {
if len(tags) == 0 {
return types, func(args []reflect.Value) []reflect.Value {
return args
}
Expand All @@ -295,8 +300,8 @@ func (pt paramTagsAnnotation) parameters(ann *annotated) (
Type: origField.Type,
Tag: origField.Tag,
}
if i-1 < len(pt.tags) {
field.Tag = reflect.StructTag(pt.tags[i-1])
if i-1 < len(tags) {
field.Tag = reflect.StructTag(tags[i-1])
}

inFields = append(inFields, field)
Expand All @@ -318,8 +323,8 @@ func (pt paramTagsAnnotation) parameters(ann *annotated) (
Name: fmt.Sprintf("Field%d", i),
Type: t,
}
if i < len(pt.tags) {
field.Tag = reflect.StructTag(pt.tags[i])
if i < len(tags) {
field.Tag = reflect.StructTag(tags[i])
}

inFields = append(inFields, field)
Expand Down Expand Up @@ -353,6 +358,76 @@ func ParamTags(tags ...string) Annotation {
return paramTagsAnnotation{tags}
}

type genericParamTagAnnotation struct {
typ reflect.Type
tag string
nth int
}

var _ Annotation = genericParamTagAnnotation{}

func (pt genericParamTagAnnotation) apply(ann *annotated) error {
if err := verifyAnnotateTag(pt.tag); err != nil {
return err
}
if pt.nth < 1 {
return fmt.Errorf("fx.ParamTag[%v]: nth must be >= 1, got %d", pt.typ, pt.nth)
}

targetTyp := reflect.TypeOf(ann.Target)
if targetTyp == nil || targetTyp.Kind() != reflect.Func {
return fmt.Errorf("fx.ParamTag[%v]: must annotate a function, got %T", pt.typ, ann.Target)
}

idx, total := nthOfType(targetTyp, pt.typ, pt.nth, "in")
if total == 0 {
return fmt.Errorf("fx.ParamTag[%v]: %s has no %v parameters",
pt.typ, fxreflect.FuncName(ann.Target), pt.typ)
}
if idx < 0 {
return fmt.Errorf("fx.ParamTag[%v]: %s has only %d %v parameters, requested #%d",
pt.typ, fxreflect.FuncName(ann.Target), total, pt.typ, pt.nth)
}

ensureLen(&ann.ParamTags, targetTyp.NumIn())

if ann.ParamTags[idx] != "" {
return fmt.Errorf("fx.ParamTag[%v]: parameter %d of %s already has tag %q",
pt.typ, idx, fxreflect.FuncName(ann.Target), ann.ParamTags[idx])
}

ann.ParamTags[idx] = pt.tag
return nil
}

func (pt genericParamTagAnnotation) build(ann *annotated) (any, error) {
return paramTagsAnnotation{}.build(ann) // delegate
}

// ParamTag is an Annotation that annotates a function parameter of type T.
//
// By default, it annotates the first such parameter. If there are multiple parameters of type T,
// nth selects the 1-based occurrence to annotate.
//
// For example, this tags the second [*sql.DB] parameter:
//
// fx.Annotate(constructor, fx.ParamTag[*sql.DB](`name:"ro"`, 2))
//
// Multiple ParamTag annotations may be applied to the same function as long as
// they target different parameters.
func ParamTag[T any](tag string, nth ...int) Annotation {
finalNth := 1
if len(nth) > 0 {
finalNth = nth[0]
}

return genericParamTagAnnotation{
typ: reflect.TypeFor[T](),
tag: tag,
nth: finalNth,
}
}

type resultTagsAnnotation struct {
tags []string
}
Expand Down Expand Up @@ -391,6 +466,10 @@ func (rt resultTagsAnnotation) apply(ann *annotated) error {

// build builds and returns a constructor after applying a ResultTags annotation
func (rt resultTagsAnnotation) build(ann *annotated) (any, error) {
if ann.resultTagsBuilt {
return ann.Target, nil
}
ann.resultTagsBuilt = true
paramTypes := ann.currentParamTypes()
resultTypes, remapResults := rt.results(ann)
origFn := reflect.ValueOf(ann.Target)
Expand All @@ -409,6 +488,7 @@ func (rt resultTagsAnnotation) results(ann *annotated) (
types []reflect.Type,
remap func([]reflect.Value) []reflect.Value,
) {
tags := ann.ResultTags
types, hasError := ann.currentResultTypes()

if hasError {
Expand All @@ -417,7 +497,7 @@ func (rt resultTagsAnnotation) results(ann *annotated) (

// No result annotations. Return the original types
// and an identity function.
if len(rt.tags) == 0 {
if len(tags) == 0 {
return types, func(results []reflect.Value) []reflect.Value {
return results
}
Expand All @@ -441,8 +521,8 @@ func (rt resultTagsAnnotation) results(ann *annotated) (
Name: fmt.Sprintf("Field%d", i),
Type: t,
}
if i < len(rt.tags) {
field.Tag = reflect.StructTag(rt.tags[i])
if i < len(tags) {
field.Tag = reflect.StructTag(tags[i])
}
newOut.Offsets = append(newOut.Offsets, len(newOut.Fields))
newOut.Fields = append(newOut.Fields, field)
Expand All @@ -452,7 +532,7 @@ func (rt resultTagsAnnotation) results(ann *annotated) (
// apply the tags to the existing type
taggedFields := make([]reflect.StructField, t.NumField())
taggedFields[0] = _outAnnotationField
for j, tag := range rt.tags {
for j, tag := range tags {
if j+1 < t.NumField() {
field := t.Field(j + 1)
taggedFields[j+1] = reflect.StructField{
Expand Down Expand Up @@ -543,6 +623,76 @@ func ResultTags(tags ...string) Annotation {
return resultTagsAnnotation{tags}
}

type genericResultTagAnnotation struct {
typ reflect.Type
tag string
nth int
}

var _ Annotation = genericResultTagAnnotation{}

func (rt genericResultTagAnnotation) apply(ann *annotated) error {
if err := verifyAnnotateTag(rt.tag); err != nil {
return err
}
if rt.nth < 1 {
return fmt.Errorf("fx.ResultTag[%v]: nth must be >= 1, got %d", rt.typ, rt.nth)
}

targetTyp := reflect.TypeOf(ann.Target)
if targetTyp == nil || targetTyp.Kind() != reflect.Func {
return fmt.Errorf("fx.ResultTag[%v]: must annotate a function, got %T", rt.typ, ann.Target)
}

idx, total := nthOfType(targetTyp, rt.typ, rt.nth, "out")
if total == 0 {
return fmt.Errorf("fx.ResultTag[%v]: %s has no %v results",
rt.typ, fxreflect.FuncName(ann.Target), rt.typ)
}
if idx < 0 {
return fmt.Errorf("fx.ResultTag[%v]: %s has only %d %v results, requested #%d",
rt.typ, fxreflect.FuncName(ann.Target), total, rt.typ, rt.nth)
}

ensureLen(&ann.ResultTags, targetTyp.NumOut())

if ann.ResultTags[idx] != "" {
return fmt.Errorf("fx.ResultTag[%v]: return value %d of %s already has tag %q",
rt.typ, idx, fxreflect.FuncName(ann.Target), ann.ResultTags[idx])
}

ann.ResultTags[idx] = rt.tag
return nil
}

func (rt genericResultTagAnnotation) build(ann *annotated) (any, error) {
return resultTagsAnnotation{}.build(ann) // delegate
}

// ResultTag is an Annotation that annotates a function return value of type T.
//
// By default, it annotates the first such return value. If there are multiple return
// values of type T, nth selects the 1-based occurrence to annotate.
//
// For example, this tags the second [*sql.DB] return value:
//
// fx.Annotate(constructor, fx.ResultTag[*sql.DB](`name:"ro"`, 2))
//
// Multiple ResultTag annotations may be applied to the same function as long as
// they target different return values.
func ResultTag[T any](tag string, nth ...int) Annotation {
finalNth := 1
if len(nth) > 0 {
finalNth = nth[0]
}

return genericResultTagAnnotation{
typ: reflect.TypeFor[T](),
tag: tag,
nth: finalNth,
}
}

type outStructInfo struct {
Fields []reflect.StructField // fields of the struct
Offsets []int // Offsets[i] is the index of result i in Fields
Expand Down Expand Up @@ -1210,6 +1360,42 @@ func isIn(t reflect.Type) bool {
dig.IsIn(reflect.New(t).Elem().Interface()))
}

// find the index of the n-th occurrence (nth >= 1) of typ in the
// function funcTyp parameters or results. Also returns the total number of occurrences.
func nthOfType(funcTyp, typ reflect.Type, nth int, op string) (idx, seen int) {
count := funcTyp.NumIn()
get := funcTyp.In
if op == "out" {
count = funcTyp.NumOut()
get = funcTyp.Out
}

idx = -1
for i := 0; i < count; i++ {
if get(i) != typ {
continue
}

seen++
if seen == nth {
idx = i
}
}

return
}

// grows the slice and fills it with zero values
func ensureLen[T any](s *[]T, targetLen int) {
if len(*s) >= targetLen {
return
}

res := make([]T, targetLen)
copy(res, *s)
*s = res
}

var _ Annotation = (*asAnnotation)(nil)

// As is an Annotation that annotates the result of a function (i.e. a
Expand Down Expand Up @@ -1613,14 +1799,16 @@ func (fr *fromAnnotation) parameters(ann *annotated) (
}

type annotated struct {
Target any
Annotations []Annotation
ParamTags []string
ResultTags []string
As [][]asType
From []reflect.Type
FuncPtr uintptr
Hooks []*lifecycleHookAnnotation
Target any
Annotations []Annotation
ParamTags []string
ResultTags []string
As [][]asType
From []reflect.Type
FuncPtr uintptr
Hooks []*lifecycleHookAnnotation
paramTagsBuilt bool
resultTagsBuilt bool
// container is used to build private scopes for lifecycle hook functions
// added via fx.OnStart and fx.OnStop annotations.
container *dig.Container
Expand Down
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