reflect: allow conversion from slice to array

Updates #46505

Change-Id: Ib8f52d6ae199338f278731267c966da85dd0acdd
Reviewed-on: https://go-review.googlesource.com/c/go/+/430475
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/reflect/all_test.go

@@ -4323,6 +4323,25 @@ var convertTests = []struct {
 	{V(MyString("runes♝")), V(MyRunes("runes♝"))},
 	{V(MyRunes("runes♕")), V(MyString("runes♕"))},
 
+	// slice to array
+	{V([]byte(nil)), V([0]byte{})},
+	{V([]byte{}), V([0]byte{})},
+	{V([]byte{1}), V([1]byte{1})},
+	{V([]byte{1, 2}), V([2]byte{1, 2})},
+	{V([]byte{1, 2, 3}), V([3]byte{1, 2, 3})},
+	{V(MyBytes([]byte(nil))), V([0]byte{})},
+	{V(MyBytes{}), V([0]byte{})},
+	{V(MyBytes{1}), V([1]byte{1})},
+	{V(MyBytes{1, 2}), V([2]byte{1, 2})},
+	{V(MyBytes{1, 2, 3}), V([3]byte{1, 2, 3})},
+	{V([]byte(nil)), V(MyBytesArray0{})},
+	{V([]byte{}), V(MyBytesArray0([0]byte{}))},
+	{V([]byte{1, 2, 3, 4}), V(MyBytesArray([4]byte{1, 2, 3, 4}))},
+	{V(MyBytes{}), V(MyBytesArray0([0]byte{}))},
+	{V(MyBytes{5, 6, 7, 8}), V(MyBytesArray([4]byte{5, 6, 7, 8}))},
+	{V([]MyByte{}), V([0]MyByte{})},
+	{V([]MyByte{1, 2}), V([2]MyByte{1, 2})},
+
 	// slice to array pointer
 	{V([]byte(nil)), V((*[0]byte)(nil))},
 	{V([]byte{}), V(new([0]byte))},
@@ -4399,6 +4418,8 @@ var convertTests = []struct {
 	// cannot convert mismatched array sizes
 	{V([2]byte{}), V([2]byte{})},
 	{V([3]byte{}), V([3]byte{})},
+	{V(MyBytesArray0{}), V([0]byte{})},
+	{V([0]byte{}), V(MyBytesArray0{})},
 
 	// cannot convert other instances
 	{V((**byte)(nil)), V((**byte)(nil))},
@@ -4574,6 +4595,34 @@ func TestConvertPanic(t *testing.T) {
 	shouldPanic("reflect: cannot convert slice with length 4 to pointer to array with length 8", func() {
 		_ = v.Convert(pt)
 	})
+
+	if v.CanConvert(pt.Elem()) {
+		t.Errorf("slice with length 4 should not be convertible to [8]byte")
+	}
+	shouldPanic("reflect: cannot convert slice with length 4 to array with length 8", func() {
+		_ = v.Convert(pt.Elem())
+	})
+}
+
+func TestConvertSlice2Array(t *testing.T) {
+	s := make([]int, 4)
+	p := [4]int{}
+	pt := TypeOf(p)
+	ov := ValueOf(s)
+	v := ov.Convert(pt)
+	// Converting a slice to non-empty array needs to return
+	// a non-addressable copy of the original memory.
+	if v.CanAddr() {
+		t.Fatalf("convert slice to non-empty array returns a addressable copy array")
+	}
+	for i := range s {
+		ov.Index(i).Set(ValueOf(i + 1))
+	}
+	for i := range s {
+		if v.Index(i).Int() != 0 {
+			t.Fatalf("slice (%v) mutation visible in converted result (%v)", ov, v)
+		}
+	}
 }
 
 var gFloat32 float32

src/reflect/value.go

@@ -3245,10 +3245,14 @@ func (v Value) CanConvert(t Type) bool {
 	if !vt.ConvertibleTo(t) {
 		return false
 	}
-	// Currently the only conversion that is OK in terms of type
-	// but that can panic depending on the value is converting
-	// from slice to pointer-to-array.
-	if vt.Kind() == Slice && t.Kind() == Pointer && t.Elem().Kind() == Array {
+	// Converting from slice to array or to pointer-to-array can panic
+	// depending on the value.
+	switch {
+	case vt.Kind() == Slice && t.Kind() == Array:
+		if t.Len() > v.Len() {
+			return false
+		}
+	case vt.Kind() == Slice && t.Kind() == Pointer && t.Elem().Kind() == Array:
 		n := t.Elem().Len()
 		if n > v.Len() {
 			return false
@@ -3401,6 +3405,11 @@ func convertOp(dst, src *rtype) func(Value, Type) Value {
 		if dst.Kind() == Pointer && dst.Elem().Kind() == Array && src.Elem() == dst.Elem().Elem() {
 			return cvtSliceArrayPtr
 		}
+		// "x is a slice, T is a array type,
+		// and the slice and array types have identical element types."
+		if dst.Kind() == Array && src.Elem() == dst.Elem() {
+			return cvtSliceArray
+		}
 
 	case Chan:
 		if dst.Kind() == Chan && specialChannelAssignability(dst, src) {
@@ -3604,6 +3613,22 @@ func cvtSliceArrayPtr(v Value, t Type) Value {
 	return Value{t.common(), h.Data, v.flag&^(flagIndir|flagAddr|flagKindMask) | flag(Pointer)}
 }
 
+// convertOp: []T -> [N]T
+func cvtSliceArray(v Value, t Type) Value {
+	n := t.Len()
+	if n > v.Len() {
+		panic("reflect: cannot convert slice with length " + itoa.Itoa(v.Len()) + " to array with length " + itoa.Itoa(n))
+	}
+	h := (*unsafeheader.Slice)(v.ptr)
+	typ := t.common()
+	ptr := h.Data
+	c := unsafe_New(typ)
+	typedmemmove(typ, c, ptr)
+	ptr = c
+
+	return Value{typ, ptr, v.flag&^(flagAddr|flagKindMask) | flag(Array)}
+}
+
 // convertOp: direct copy
 func cvtDirect(v Value, typ Type) Value {
 	f := v.flag