reflect: allow Copy to a byte array or byte slice from a string

This somewhat mirrors the special case behavior of the copy built-in.

Fixes #22215

Change-Id: Ic353003ad3de659d3a6b4e9d97295b42510f3bf7
Reviewed-on: https://go-review.googlesource.com/70431Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/reflect/all_test.go

@@ -668,6 +668,47 @@ func TestCopy(t *testing.T) {
 	}
 }
 
+func TestCopyString(t *testing.T) {
+	t.Run("Slice", func(t *testing.T) {
+		s := bytes.Repeat([]byte{'_'}, 8)
+		val := ValueOf(s)
+
+		n := Copy(val, ValueOf(""))
+		if expecting := []byte("________"); n != 0 || !bytes.Equal(s, expecting) {
+			t.Errorf("got n = %d, s = %s, expecting n = 0, s = %s", n, s, expecting)
+		}
+
+		n = Copy(val, ValueOf("hello"))
+		if expecting := []byte("hello___"); n != 5 || !bytes.Equal(s, expecting) {
+			t.Errorf("got n = %d, s = %s, expecting n = 5, s = %s", n, s, expecting)
+		}
+
+		n = Copy(val, ValueOf("helloworld"))
+		if expecting := []byte("hellowor"); n != 8 || !bytes.Equal(s, expecting) {
+			t.Errorf("got n = %d, s = %s, expecting n = 8, s = %s", n, s, expecting)
+		}
+	})
+	t.Run("Array", func(t *testing.T) {
+		s := [...]byte{'_', '_', '_', '_', '_', '_', '_', '_'}
+		val := ValueOf(&s).Elem()
+
+		n := Copy(val, ValueOf(""))
+		if expecting := []byte("________"); n != 0 || !bytes.Equal(s[:], expecting) {
+			t.Errorf("got n = %d, s = %s, expecting n = 0, s = %s", n, s[:], expecting)
+		}
+
+		n = Copy(val, ValueOf("hello"))
+		if expecting := []byte("hello___"); n != 5 || !bytes.Equal(s[:], expecting) {
+			t.Errorf("got n = %d, s = %s, expecting n = 5, s = %s", n, s[:], expecting)
+		}
+
+		n = Copy(val, ValueOf("helloworld"))
+		if expecting := []byte("hellowor"); n != 8 || !bytes.Equal(s[:], expecting) {
+			t.Errorf("got n = %d, s = %s, expecting n = 8, s = %s", n, s[:], expecting)
+		}
+	})
+}
+
 func TestCopyArray(t *testing.T) {
 	a := [8]int{1, 2, 3, 4, 10, 9, 8, 7}
 	b := [11]int{11, 22, 33, 44, 1010, 99, 88, 77, 66, 55, 44}

src/reflect/value.go

@@ -1864,6 +1864,8 @@ func AppendSlice(s, t Value) Value {
 // It returns the number of elements copied.
 // Dst and src each must have kind Slice or Array, and
 // dst and src must have the same element type.
+//
+// As a special case, src can have kind String if the element type of dst is kind Uint8.
 func Copy(dst, src Value) int {
 	dk := dst.kind()
 	if dk != Array && dk != Slice {
@@ -1875,14 +1877,20 @@ func Copy(dst, src Value) int {
 	dst.mustBeExported()
 
 	sk := src.kind()
+	var stringCopy bool
 	if sk != Array && sk != Slice {
-		panic(&ValueError{"reflect.Copy", sk})
+		stringCopy = sk == String && dst.typ.Elem().Kind() == Uint8
+		if !stringCopy {
+			panic(&ValueError{"reflect.Copy", sk})
+		}
 	}
 	src.mustBeExported()
 
 	de := dst.typ.Elem()
-	se := src.typ.Elem()
-	typesMustMatch("reflect.Copy", de, se)
+	if !stringCopy {
+		se := src.typ.Elem()
+		typesMustMatch("reflect.Copy", de, se)
+	}
 
 	var ds, ss sliceHeader
 	if dk == Array {
@@ -1896,8 +1904,13 @@ func Copy(dst, src Value) int {
 		ss.Data = src.ptr
 		ss.Len = src.Len()
 		ss.Cap = ss.Len
-	} else {
+	} else if sk == Slice {
 		ss = *(*sliceHeader)(src.ptr)
+	} else {
+		sh := *(*stringHeader)(src.ptr)
+		ss.Data = sh.Data
+		ss.Len = sh.Len
+		ss.Cap = sh.Len
 	}
 
 	return typedslicecopy(de.common(), ds, ss)