all: statically enforce json/v2 interface satisfaction (#15154)

The json/v2 prototype is still in flux and the API can/will change.

Statically enforce that types implementing the v2 methods
satisfy the correct interface so that changes to the signature
can be statically detected by the compiler.

Updates tailscale/corp#791

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

types/prefs/prefs.go

@@ -158,6 +158,11 @@ func (p *preference[T]) SetReadOnly(readonly bool) {
 	p.s.Metadata.ReadOnly = readonly
 }
 
+var (
+	_ jsonv2.MarshalerTo     = (*preference[struct{}])(nil)
+	_ jsonv2.UnmarshalerFrom = (*preference[struct{}])(nil)
+)
+
 // MarshalJSONTo implements [jsonv2.MarshalerTo].
 func (p preference[T]) MarshalJSONTo(out *jsontext.Encoder) error {
 	return jsonv2.MarshalEncode(out, &p.s)