Feature suggestion: add new option disallow_duplicate_fields

[peterzeller]

This option makes unmarshalling return an error if a field appears in the json more than once.

This option can be useful as a precaution against JSON Interoperability Vulnerabilities, see for example "Inconsistent Duplicate Key Precedence" in https://bishopfox.com/blog/json-interoperability-vulnerabilities

Also, a similar option is planned for the Golang standard library: golang/go#48298

For reference, here is the generated code with the option:

// Code generated by easyjson for marshaling/unmarshaling. DO NOT EDIT.

package tests

import (
	json "encoding/json"
	fmt "fmt"
	easyjson "github.com/mailru/easyjson"
	jlexer "github.com/mailru/easyjson/jlexer"
	jwriter "github.com/mailru/easyjson/jwriter"
)

// suppress unused package warning
var (
	_ *json.RawMessage
	_ *jlexer.Lexer
	_ *jwriter.Writer
	_ easyjson.Marshaler
)

func easyjson4f43fa15DecodeGithubComMailruEasyjsonTests(in *jlexer.Lexer, out *Dupl) {
	isTopLevel := in.IsStart()
	if in.IsNull() {
		if isTopLevel {
			in.Consumed()
		}
		in.Skip()
		return
	}
	var ASet bool
	var BSet bool
	in.Delim('{')
	for !in.IsDelim('}') {
		key := in.UnsafeFieldName(false)
		in.WantColon()
		if in.IsNull() {
			in.Skip()
			in.WantComma()
			continue
		}
		switch key {
		case "a":
			if ASet {
				in.AddError(fmt.Errorf("duplicate field A"))
			}
			out.A = int(in.Int())
			ASet = true
		case "b":
			if BSet {
				in.AddError(fmt.Errorf("duplicate field B"))
			}
			out.B = int(in.Int())
			BSet = true
		default:
			in.SkipRecursive()
		}
		in.WantComma()
	}
	in.Delim('}')
	if isTopLevel {
		in.Consumed()
	}
}
func easyjson4f43fa15EncodeGithubComMailruEasyjsonTests(out *jwriter.Writer, in Dupl) {
	out.RawByte('{')
	first := true
	_ = first
	{
		const prefix string = ",\"a\":"
		out.RawString(prefix[1:])
		out.Int(int(in.A))
	}
	{
		const prefix string = ",\"b\":"
		out.RawString(prefix)
		out.Int(int(in.B))
	}
	out.RawByte('}')
}

// MarshalJSON supports json.Marshaler interface
func (v Dupl) MarshalJSON() ([]byte, error) {
	w := jwriter.Writer{}
	easyjson4f43fa15EncodeGithubComMailruEasyjsonTests(&w, v)
	return w.Buffer.BuildBytes(), w.Error
}

// MarshalEasyJSON supports easyjson.Marshaler interface
func (v Dupl) MarshalEasyJSON(w *jwriter.Writer) {
	easyjson4f43fa15EncodeGithubComMailruEasyjsonTests(w, v)
}

// UnmarshalJSON supports json.Unmarshaler interface
func (v *Dupl) UnmarshalJSON(data []byte) error {
	r := jlexer.Lexer{Data: data}
	easyjson4f43fa15DecodeGithubComMailruEasyjsonTests(&r, v)
	return r.Error()
}

// UnmarshalEasyJSON supports easyjson.Unmarshaler interface
func (v *Dupl) UnmarshalEasyJSON(l *jlexer.Lexer) {
	easyjson4f43fa15DecodeGithubComMailruEasyjsonTests(l, v)
}