cm: Implement json Marshal/Unmarshal for List type

[lxfontes]

Relates to #239

This will also respect json nulls as per https://pkg.go.dev/encoding/json#Unmarshaler

[ydnar]

This will miss List[T] where T is a named uint8 type.

Try something like this and convert the underlying slice: https://go.dev/play/p/MSlhH5LesNN

Update: updated link.

package main

import (
	"encoding/json"
	"fmt"
	"unsafe"
)

func main() {
	j, _ := json.Marshal([]uint8{0, 1, 2, 3})
	fmt.Println(string(j))

	j, _ = json.Marshal(sliceOf([]uint8{0, 1, 2, 3}))
	fmt.Println(string(j))
}

type slice[T any] []entry[T]

func sliceOf[S ~[]E, E any](s S) slice[E] {
	return *(*slice[E])(unsafe.Pointer(&s))
}

type entry[T any] [1]T

func (v entry[T]) MarshalJSON() ([]byte, error) {
	return json.Marshal(v[0])
}

[lxfontes]

makes sense! was avoiding the unsafe.Pointer route but totally forgot about type aliases.

[ydnar]

Is all this machinery necessary?

If List[T] implements json.Marshaler and json.Unmarshaler, then the test cases can have these lists inline.

[lxfontes]

listTestItem is to test structs.

The machinery is necessary to generalize the Slice() method & carry the type information to reflect.DeepEqual. It's either this or unrolling into many smaller test functions per type.

While List implements Marshaler/Unmarshaler, accessing the underlying slice is thru typed Slice() []T ( not a common interface ). So the options are:

• make a generic wrapper ( what I did )
• cast it ( which requires knowing which type to cast to )

Unmarshal is where it is needed, and I used it in Marshal as well for consistency/less typing.

[ydnar]

The caller can mutate the value of nullLiteral. Maybe mix this optimization for now?

[ydnar]

Merged as part of #279.