reflect: add Value.Clear; support anytype->interface{}, Slice->(*)Array in Value.Convert

Author: Ben Krieger

src/reflect/type.go

@@ -423,8 +423,8 @@ type rawType struct {
 	meta uint8 // metadata byte, contains kind and flags (see constants above)
 }
 
-// All types that have an element type: named, chan, slice, array, map (but not
-// pointer because it doesn't have ptrTo).
+// All types that have an element type: named, chan, slice, array, map, interface
+// (but not pointer because it doesn't have ptrTo).
 type elemType struct {
 	rawType
 	numMethod uint16
@@ -438,6 +438,12 @@ type ptrType struct {
 	elem      *rawType
 }
 
+type interfaceType struct {
+	rawType
+	ptrTo *rawType
+	// TODO: methods
+}
+
 type arrayType struct {
 	rawType
 	numMethod uint16
@@ -970,10 +976,18 @@ func (t *rawType) FieldAlign() int {
 // AssignableTo returns whether a value of type t can be assigned to a variable
 // of type u.
 func (t *rawType) AssignableTo(u Type) bool {
-	if t == u.(*rawType) {
+	if u := u.(*rawType); t == u || t.underlying() == u || t == u.underlying() {
 		return true
 	}
 
+	if u.Kind() == Array && t.Kind() == Slice {
+		return u.Elem().(*rawType) == t.Elem().(*rawType)
+	}
+
+	if (u.Kind() == Pointer && u.Elem().Kind() == Array) && t.Kind() == Slice {
+		return u.Elem().Elem().(*rawType) == t.Elem().(*rawType)
+	}
+
 	if u.Kind() == Interface && u.NumMethod() == 0 {
 		return true
 	}
@@ -1039,6 +1053,9 @@ func (t *rawType) NumMethod() int {
 		return int((*ptrType)(unsafe.Pointer(t)).numMethod)
 	case Struct:
 		return int((*structType)(unsafe.Pointer(t)).numMethod)
+	case Interface:
+		//FIXME: Use len(methods)
+		return (*interfaceType)(unsafe.Pointer(t)).ptrTo.NumMethod()
 	}
 
 	// Other types have no methods attached.  Note we don't panic here.

src/reflect/value.go

@@ -689,6 +689,27 @@ func (v Value) Cap() int {
 	}
 }
 
+//go:linkname mapclear runtime.hashmapClear
+func mapclear(p unsafe.Pointer)
+
+// Clear clears the contents of a map or zeros the contents of a slice
+//
+// It panics if v's Kind is not Map or Slice.
+func (v Value) Clear() {
+	switch v.typecode.Kind() {
+	case Map:
+		mapclear(v.pointer())
+	case Slice:
+		hdr := (*sliceHeader)(v.value)
+		elemSize := v.typecode.underlying().elem().Size()
+		for i := uintptr(0); i < hdr.len; i++ {
+			memzero(unsafe.Add(hdr.data, i*elemSize), elemSize)
+		}
+	default:
+		panic(&ValueError{Method: "Clear", Kind: v.Kind()})
+	}
+}
+
 // NumField returns the number of fields of this struct. It panics for other
 // value types.
 func (v Value) NumField() int {
@@ -888,6 +909,9 @@ func (v Value) Index(i int) Value {
 }
 
 func (v Value) NumMethod() int {
+	if v.typecode == nil {
+		panic(&ValueError{Method: "reflect.Value.NumMethod", Kind: Invalid})
+	}
 	return v.typecode.NumMethod()
 }
 
@@ -1062,7 +1086,7 @@ func (v Value) Set(x Value) {
 	v.checkAddressable()
 	v.checkRO()
 	if !x.typecode.AssignableTo(v.typecode) {
-		panic("reflect: cannot set")
+		panic("reflect.Value.Set: value of type " + x.typecode.String() + " cannot be assigned to type " + v.typecode.String())
 	}
 
 	if v.typecode.Kind() == Interface && x.typecode.Kind() != Interface {
@@ -1240,7 +1264,9 @@ func (v Value) OverflowUint(x uint64) bool {
 }
 
 func (v Value) CanConvert(t Type) bool {
-	panic("unimplemented: (reflect.Value).CanConvert()")
+	// TODO: Optimize this to not actually perform a conversion
+	_, ok := convertOp(v, t)
+	return ok
 }
 
 func (v Value) Convert(t Type) Value {
@@ -1262,6 +1288,15 @@ func convertOp(src Value, typ Type) (Value, bool) {
 		}, true
 	}
 
+	if rtype := typ.(*rawType); rtype.Kind() == Interface && rtype.NumMethod() == 0 {
+		iface := composeInterface(unsafe.Pointer(src.typecode), src.value)
+		return Value{
+			typecode: rtype,
+			value:    unsafe.Pointer(&iface),
+			flags:    valueFlagExported,
+		}, true
+	}
+
 	switch src.Kind() {
 	case Int, Int8, Int16, Int32, Int64:
 		switch rtype := typ.(*rawType); rtype.Kind() {
@@ -1302,25 +1337,47 @@ func convertOp(src Value, typ Type) (Value, bool) {
 		*/
 
 	case Slice:
-		if typ.Kind() == String && !src.typecode.elem().isNamed() {
-			rtype := typ.(*rawType)
-
-			switch src.Type().Elem().Kind() {
-			case Uint8:
-				return cvtBytesString(src, rtype), true
-			case Int32:
-				return cvtRunesString(src, rtype), true
+		switch rtype := typ.(*rawType); rtype.Kind() {
+		case Array:
+			if src.typecode.elem() == rtype.elem() && rtype.Len() <= src.Len() {
+				return Value{
+					typecode: rtype,
+					value:    (*sliceHeader)(src.value).data,
+					flags:    src.flags,
+				}, true
+			}
+		case Pointer:
+			if rtype.Elem().Kind() != Array {
+				break
+			}
+			if src.typecode.elem() == rtype.elem().elem() && rtype.elem().Len() <= src.Len() {
+				return Value{
+					typecode: rtype,
+					value:    (*sliceHeader)(src.value).data,
+					flags:    src.flags & (valueFlagExported | valueFlagRO),
+				}, true
+			}
+		case String:
+			if !src.typecode.elem().isNamed() {
+				switch src.Type().Elem().Kind() {
+				case Uint8:
+					return cvtBytesString(src, rtype), true
+				case Int32:
+					return cvtRunesString(src, rtype), true
+				}
 			}
 		}
 
 	case String:
-		rtype := typ.(*rawType)
-		if typ.Kind() == Slice && !rtype.elem().isNamed() {
-			switch typ.Elem().Kind() {
-			case Uint8:
-				return cvtStringBytes(src, rtype), true
-			case Int32:
-				return cvtStringRunes(src, rtype), true
+		switch rtype := typ.(*rawType); rtype.Kind() {
+		case Slice:
+			if !rtype.elem().isNamed() {
+				switch typ.Elem().Kind() {
+				case Uint8:
+					return cvtStringBytes(src, rtype), true
+				case Int32:
+					return cvtStringRunes(src, rtype), true
+				}
 			}
 		}
 	}

src/reflect/value_test.go

@@ -1,6 +1,7 @@
 package reflect_test
 
 import (
+	"bytes"
 	"encoding/base64"
 	. "reflect"
 	"sort"
@@ -599,6 +600,29 @@ func TestAssignableTo(t *testing.T) {
 	if got, want := refa.Interface().(int), 4; got != want {
 		t.Errorf("AssignableTo / Set failed, got %v, want %v", got, want)
 	}
+
+	b := []byte{0x01, 0x02}
+	refb := ValueOf(&b).Elem()
+	refb.Set(ValueOf([]byte{0x02, 0x03}))
+	if got, want := refb.Interface().([]byte), []byte{0x02, 0x03}; !bytes.Equal(got, want) {
+		t.Errorf("AssignableTo / Set failed, got %v, want %v", got, want)
+	}
+
+	type bstr []byte
+
+	c := bstr{0x01, 0x02}
+	refc := ValueOf(&c).Elem()
+	refc.Set(ValueOf([]byte{0x02, 0x03}))
+	if got, want := refb.Interface().([]byte), []byte{0x02, 0x03}; !bytes.Equal(got, want) {
+		t.Errorf("AssignableTo / Set failed, got %v, want %v", got, want)
+	}
+
+	d := []byte{0x01, 0x02}
+	refd := ValueOf(&d).Elem()
+	refd.Set(ValueOf(bstr{0x02, 0x03}))
+	if got, want := refb.Interface().([]byte), []byte{0x02, 0x03}; !bytes.Equal(got, want) {
+		t.Errorf("AssignableTo / Set failed, got %v, want %v", got, want)
+	}
 }
 
 func TestConvert(t *testing.T) {
@@ -624,6 +648,123 @@ func TestConvert(t *testing.T) {
 	}
 }
 
+func TestConvertSliceToArrayOrArrayPointer(t *testing.T) {
+	s := make([]byte, 2, 4)
+	// a0 := [0]byte(s)
+	// a1 := [1]byte(s[1:])     // a1[0] == s[1]
+	// a2 := [2]byte(s)         // a2[0] == s[0]
+	// a4 := [4]byte(s)         // panics: len([4]byte) > len(s)
+
+	v := ValueOf(s).Convert(TypeFor[[0]byte]())
+	if v.Kind() != Array || v.Type().Len() != 0 {
+		t.Error("Convert([]byte -> [0]byte)")
+	}
+	v = ValueOf(s[1:]).Convert(TypeFor[[1]byte]())
+	if v.Kind() != Array || v.Type().Len() != 1 {
+		t.Error("Convert([]byte -> [1]byte)")
+	}
+	v = ValueOf(s).Convert(TypeFor[[2]byte]())
+	if v.Kind() != Array || v.Type().Len() != 2 {
+		t.Error("Convert([]byte -> [2]byte)")
+	}
+	if ValueOf(s).CanConvert(TypeFor[[4]byte]()) {
+		t.Error("Converting a slice with len smaller than array to array should fail")
+	}
+
+	// s0 := (*[0]byte)(s)      // s0 != nil
+	// s1 := (*[1]byte)(s[1:])  // &s1[0] == &s[1]
+	// s2 := (*[2]byte)(s)      // &s2[0] == &s[0]
+	// s4 := (*[4]byte)(s)      // panics: len([4]byte) > len(s)
+	v = ValueOf(s).Convert(TypeFor[*[0]byte]())
+	if v.Kind() != Pointer || v.Elem().Kind() != Array || v.Elem().Type().Len() != 0 {
+		t.Error("Convert([]byte -> *[0]byte)")
+	}
+	v = ValueOf(s[1:]).Convert(TypeFor[*[1]byte]())
+	if v.Kind() != Pointer || v.Elem().Kind() != Array || v.Elem().Type().Len() != 1 {
+		t.Error("Convert([]byte -> *[1]byte)")
+	}
+	v = ValueOf(s).Convert(TypeFor[*[2]byte]())
+	if v.Kind() != Pointer || v.Elem().Kind() != Array || v.Elem().Type().Len() != 2 {
+		t.Error("Convert([]byte -> *[2]byte)")
+	}
+	if ValueOf(s).CanConvert(TypeFor[*[4]byte]()) {
+		t.Error("Converting a slice with len smaller than array to array pointer should fail")
+	}
+}
+
+func TestConvertToEmptyInterface(t *testing.T) {
+	anyType := TypeFor[interface{}]()
+
+	v := ValueOf(false).Convert(anyType)
+	if v.Kind() != Interface || v.NumMethod() > 0 {
+		t.Error("Convert(bool -> interface{})")
+	}
+	_ = v.Interface().(interface{}).(bool)
+
+	v = ValueOf(int64(3)).Convert(anyType)
+	if v.Kind() != Interface || v.NumMethod() > 0 {
+		t.Error("Convert(int64 -> interface{})")
+	}
+	_ = v.Interface().(interface{}).(int64)
+
+	v = ValueOf(struct{}{}).Convert(anyType)
+	if v.Kind() != Interface || v.NumMethod() > 0 {
+		t.Error("Convert(struct -> interface{})")
+	}
+	_ = v.Interface().(interface{}).(struct{})
+
+	v = ValueOf([]struct{}{}).Convert(anyType)
+	if v.Kind() != Interface || v.NumMethod() > 0 {
+		t.Error("Convert(slice -> interface{})")
+	}
+	_ = v.Interface().(interface{}).([]struct{})
+
+	v = ValueOf(map[string]string{"A": "B"}).Convert(anyType)
+	if v.Kind() != Interface || v.NumMethod() > 0 {
+		t.Error("Convert(map -> interface{})")
+	}
+	_ = v.Interface().(interface{}).(map[string]string)
+}
+
+func TestClearSlice(t *testing.T) {
+	type stringSlice []string
+	for _, test := range []struct {
+		slice  any
+		expect any
+	}{
+		{
+			slice:  []bool{true, false, true},
+			expect: []bool{false, false, false},
+		},
+		{
+			slice:  []byte{0x00, 0x01, 0x02, 0x03},
+			expect: []byte{0x00, 0x00, 0x00, 0x00},
+		},
+		{
+			slice:  [][]int{[]int{2, 1}, []int{3}, []int{}},
+			expect: [][]int{nil, nil, nil},
+		},
+		{
+			slice:  []stringSlice{stringSlice{"hello", "world"}, stringSlice{}, stringSlice{"goodbye"}},
+			expect: []stringSlice{nil, nil, nil},
+		},
+	} {
+		v := ValueOf(test.slice)
+		expectLen, expectCap := v.Len(), v.Cap()
+
+		v.Clear()
+		if len := v.Len(); len != expectLen {
+			t.Errorf("Clear(slice) altered len, got %d, expected %d", len, expectLen)
+		}
+		if cap := v.Cap(); cap != expectCap {
+			t.Errorf("Clear(slice) altered cap, got %d, expected %d", cap, expectCap)
+		}
+		if !DeepEqual(test.slice, test.expect) {
+			t.Errorf("Clear(slice) got %v, expected %v", test.slice, test.expect)
+		}
+	}
+}
+
 func TestIssue4040(t *testing.T) {
 	var value interface{} = uint16(0)