reflect: add iterator equivalents for NumField, NumIn, NumOut and NumMethod

api/next/66631.txt

@@ -0,0 +1,6 @@
+pkg reflect, type Type interface, Fields() iter.Seq[StructField] #66631
+pkg reflect, type Type interface, Methods() iter.Seq[Method] #66631
+pkg reflect, type Type interface, Ins() iter.Seq[Type] #66631
+pkg reflect, type Type interface, Outs() iter.Seq[Type] #66631
+pkg reflect, method (Value) Fields() iter.Seq2[StructField, Value] #66631
+pkg reflect, method (Value) Methods() iter.Seq2[Method, Value] #66631

doc/next/6-stdlib/99-minor/reflect/66631.md

@@ -0,0 +1,3 @@
+[reflect.Type] includes new methods that return iterators for a type's fields, methods, inputs and outputs.
+[reflect.Value.Fields] and [reflect.Value.Methods] have also been introduced, these return iterators which
+yield both a [reflect.Value] as well as the corresponding [reflect.StructField] or [reflect.Method].

src/reflect/abi_test.go

@@ -175,8 +175,8 @@ func TestReflectCallABI(t *testing.T) {
 				t.Fatalf("test case has different number of inputs and outputs: %d in, %d out", typ.NumIn(), typ.NumOut())
 			}
 			var args []reflect.Value
-			for i := 0; i < typ.NumIn(); i++ {
-				args = append(args, genValue(t, typ.In(i), r))
+			for arg := range typ.Ins() {
+				args = append(args, genValue(t, arg, r))
 			}
 			results := fn.Call(args)
 			for i := range results {

src/reflect/all_test.go

@@ -8505,8 +8505,7 @@ func TestInitFuncTypes(t *testing.T) {
 		go func() {
 			defer wg.Done()
 			ipT := TypeOf(net.IP{})
-			for i := 0; i < ipT.NumMethod(); i++ {
-				_ = ipT.Method(i)
+			for range ipT.Methods() {
 			}
 		}()
 	}

src/reflect/benchmark_test.go

@@ -146,10 +146,8 @@ func BenchmarkIsZero(b *testing.B) {
 	s.ArrayInt_1024_NoZero[512] = 1
 	source := ValueOf(s)
 
-	for i := 0; i < source.NumField(); i++ {
-		name := source.Type().Field(i).Name
-		value := source.Field(i)
-		b.Run(name, func(b *testing.B) {
+	for field, value := range source.Fields() {
+		b.Run(field.Name, func(b *testing.B) {
 			for i := 0; i < b.N; i++ {
 				sink = value.IsZero()
 			}
@@ -175,9 +173,8 @@ func BenchmarkSetZero(b *testing.B) {
 		Struct    Value
 	})).Elem()
 
-	for i := 0; i < source.NumField(); i++ {
-		name := source.Type().Field(i).Name
-		value := source.Field(i)
+	for field, value := range source.Fields() {
+		name := field.Name
 		zero := Zero(value.Type())
 		b.Run(name+"/Direct", func(b *testing.B) {
 			for i := 0; i < b.N; i++ {

src/reflect/example_test.go

@@ -96,8 +96,7 @@ func ExampleStructTag_Lookup() {
 
 	s := S{}
 	st := reflect.TypeOf(s)
-	for i := 0; i < st.NumField(); i++ {
-		field := st.Field(i)
+	for field := range st.Fields() {
 		if alias, ok := field.Tag.Lookup("alias"); ok {
 			if alias == "" {
 				fmt.Println("(blank)")

src/reflect/type.go

@@ -18,6 +18,7 @@ package reflect
 import (
 	"internal/abi"
 	"internal/goarch"
+	"iter"
 	"runtime"
 	"strconv"
 	"sync"
@@ -64,6 +65,10 @@ type Type interface {
 	// This may make the executable binary larger but will not affect execution time.
 	Method(int) Method
 
+	// Methods returns an iterator over each method in the type's method set. The sequence is
+	// equivalent to calling Method successively for each index i in the range [0, NumMethod()).
+	Methods() iter.Seq[Method]
+
 	// MethodByName returns the method with that name in the type's
 	// method set and a boolean indicating if the method was found.
 	//
@@ -172,6 +177,11 @@ type Type interface {
 	// It panics if i is not in the range [0, NumField()).
 	Field(i int) StructField
 
+	// Fields returns an iterator over each struct field for struct type t. The sequence is
+	// equivalent to calling Field successively for each index i in the range [0, NumField()).
+	// It panics if the type's Kind is not Struct.
+	Fields() iter.Seq[StructField]
+
 	// FieldByIndex returns the nested field corresponding
 	// to the index sequence. It is equivalent to calling Field
 	// successively for each index i.
@@ -208,6 +218,11 @@ type Type interface {
 	// It panics if i is not in the range [0, NumIn()).
 	In(i int) Type
 
+	// Ins returns an iterator over each input parameter of function type t. The sequence
+	// is equivalent to calling In successively for each index i in the range [0, NumIn()).
+	// It panics if the type's Kind is not Func.
+	Ins() iter.Seq[Type]
+
 	// Key returns a map type's key type.
 	// It panics if the type's Kind is not Map.
 	Key() Type
@@ -233,6 +248,11 @@ type Type interface {
 	// It panics if i is not in the range [0, NumOut()).
 	Out(i int) Type
 
+	// Outs returns an iterator over each output parameter of function type t. The sequence
+	// is equivalent to calling Out successively for each index i in the range [0, NumOut()).
+	// It panics if the type's Kind is not Func.
+	Outs() iter.Seq[Type]
+
 	// OverflowComplex reports whether the complex128 x cannot be represented by type t.
 	// It panics if t's Kind is not Complex64 or Complex128.
 	OverflowComplex(x complex128) bool
@@ -934,6 +954,55 @@ func canRangeFunc2(t *abi.Type) bool {
 	return yield.InCount == 2 && yield.OutCount == 1 && yield.Out(0).Kind() == abi.Bool
 }
 
+func (t *rtype) Fields() iter.Seq[StructField] {
+	if t.Kind() != Struct {
+		panic("reflect: Fields of non-struct type " + t.String())
+	}
+	return func(yield func(StructField) bool) {
+		for i := range t.NumField() {
+			if !yield(t.Field(i)) {
+				return
+			}
+		}
+	}
+}
+
+func (t *rtype) Methods() iter.Seq[Method] {
+	return func(yield func(Method) bool) {
+		for i := range t.NumMethod() {
+			if !yield(t.Method(i)) {
+				return
+			}
+		}
+	}
+}
+
+func (t *rtype) Ins() iter.Seq[Type] {
+	if t.Kind() != Func {
+		panic("reflect: Ins of non-func type " + t.String())
+	}
+	return func(yield func(Type) bool) {
+		for i := range t.NumIn() {
+			if !yield(t.In(i)) {
+				return
+			}
+		}
+	}
+}
+
+func (t *rtype) Outs() iter.Seq[Type] {
+	if t.Kind() != Func {
+		panic("reflect: Outs of non-func type " + t.String())
+	}
+	return func(yield func(Type) bool) {
+		for i := range t.NumOut() {
+			if !yield(t.Out(i)) {
+				return
+			}
+		}
+	}
+}
+
 // add returns p+x.
 //
 // The whySafe string is ignored, so that the function still inlines

src/reflect/value.go

@@ -10,6 +10,7 @@ import (
 	"internal/goarch"
 	"internal/itoa"
 	"internal/unsafeheader"
+	"iter"
 	"math"
 	"runtime"
 	"unsafe"
@@ -2620,6 +2621,48 @@ func (v Value) UnsafePointer() unsafe.Pointer {
 	panic(&ValueError{"reflect.Value.UnsafePointer", v.kind()})
 }
 
+// Fields returns an iterator over each [StructField] of v along with its [Value].
+//
+// The sequence is equivalent to calling [Value.Field] successively
+// for each index i in the range [0, NumField()).
+//
+// It panics if v's Kind is not Struct.
+func (v Value) Fields() iter.Seq2[StructField, Value] {
+	if t.Kind() != Struct {
+		panic("reflect: Fields of non-struct type " + t.String())
+	}
+	return func(yield func(StructField, Value) bool) {
+		rtype := v.Type()
+		for i := range v.NumField() {
+			if !yield(rtype.Field(i), v.Field(i)) {
+				return
+			}
+		}
+	}
+}
+
+// Methods returns an iterator over each [Method] of v along with the corresponding
+// method [Value]; this is a function with v bound as the receiver. As such, the
+// receiver shouldn't be included in the arguments to [Value.Call].
+//
+// The sequence is equivalent to calling [Value.Method] successively
+// for each index i in the range [0, NumMethod()).
+//
+// Methods panics if v is a nil interface value.
+//
+// Calling this method will force the linker to retain all exported methods in all packages.
+// This may make the executable binary larger but will not affect execution time.
+func (v Value) Methods() iter.Seq2[Method, Value] {
+	return func(yield func(Method, Value) bool) {
+		rtype := v.Type()
+		for i := range v.NumMethod() {
+			if !yield(rtype.Method(i), v.Method(i)) {
+				return
+			}
+		}
+	}
+}
+
 // StringHeader is the runtime representation of a string.
 // It cannot be used safely or portably and its representation may
 // change in a later release.
@@ -3221,8 +3264,8 @@ func (v Value) Comparable() bool {
 		return v.IsNil() || v.Elem().Comparable()
 
 	case Struct:
-		for i := 0; i < v.NumField(); i++ {
-			if !v.Field(i).Comparable() {
+		for _, value := range v.Fields() {
+			if !value.Comparable() {
 				return false
 			}
 		}