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

The new methods are Type.Fields, Type.Methods, Type.Ins, Type.Outs,
Value.Fields and Value.Methods.

These methods have been introduced into the reflect package (as well
as tests) replacing any three-clause for loops where possible.

Fixes #66631

improve comments

Author: Splizard

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 +954,46 @@ 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] {
+	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] {
+	return func(yield func(Type) bool) {
+		for i := range t.NumIn() {
+			if !yield(t.In(i)) {
+				return
+			}
+		}
+	}
+}
+
+func (t *rtype) Outs() iter.Seq[Type] {
+	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 +2621,45 @@ 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] {
+	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 +3261,8 @@ 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
 			}
 		}