In Decode, reuse slices when possible

decode.go

@@ -308,9 +308,10 @@ func (md *MetaData) unifySlice(data interface{}, rv reflect.Value) error {
 		return badtype("slice", data)
 	}
 	sliceLen := datav.Len()
-	if rv.IsNil() || rv.Len() != datav.Len() {
+	if rv.IsNil() || rv.Len() < datav.Len() {
 		rv.Set(reflect.MakeSlice(rv.Type(), sliceLen, sliceLen))
 	}
+	rv.SetLen(datav.Len())
 	return md.unifySliceArray(datav, rv)
 }
 

decode_test.go

@@ -498,12 +498,17 @@ type ingredient struct {
 	Name string
 }
 
-func TestDecodeToEmptySlice(t *testing.T) {
+func TestDecodeSlices(t *testing.T) {
 	s := struct{ Test []string }{Test: []string{}}
-
-	_, err := Decode(`Test = ["test"]`, &s)
-	if err != nil {
-		t.Fatal(err)
+	if _, err := Decode(`Test = ["test"]`, &s); err != nil {
+		t.Errorf("Error decoding into empty slice: %s", err)
+	}
+	s.Test = []string{"a", "b", "c"}
+	if _, err := Decode(`Test = ["test"]`, &s); err != nil {
+		t.Errorf("Error decoding into oversized slice: %s", err)
+	}
+	if want := []string{"test"}; !reflect.DeepEqual(s.Test, want) {
+		t.Errorf("Got %v; want %v", s.Test, want)
 	}
 }