Qubit JSON

Lenient JSON decoder for Rust, designed for non-fully-trusted text inputs.

Overview

Qubit JSON provides a small and predictable decoding layer on top of serde_json. Its core type, LenientJsonDecoder, normalizes a limited set of common input issues before parsing and deserializing JSON values.

The crate is intended for cases where JSON text may come from sources such as:

Markdown-wrapped text
Markdown code blocks
copied snippets
CLI output streams
other text channels that may wrap otherwise valid JSON

It is intentionally narrow. The crate does not try to be a general JSON repair engine, and it does not attempt to guess missing quotes, commas, or braces.

Design Goals

Lenient but predictable: only handle a small set of well-defined input problems
Object-oriented API: use a reusable LenientJsonDecoder instance instead of a loose bag of helper functions
Serde-first: delegate actual parsing and deserialization to serde_json
Clear errors: report stable error kinds with enough context for callers
Low overhead: avoid unnecessary allocation when normalization can borrow the original input

Features

`LenientJsonDecoder`

Reusable decoder object that holds immutable decoding options
decode<T>(): decodes any JSON top-level value into T
decode_value(): decodes into serde_json::Value
decode_object<T>(): requires a top-level JSON object
decode_array<T>(): requires a top-level JSON array

`JsonDecodeOptions`

trim_whitespace: trims leading and trailing whitespace
strip_utf8_bom: strips a leading UTF-8 BOM
strip_markdown_code_fence: strips one outer Markdown code fence
escape_control_chars_in_strings: escapes ASCII control characters inside JSON string literals

Explicit Error Model

EmptyInput: input becomes empty after normalization
InvalidJson: normalized text is not valid JSON syntax
UnexpectedTopLevel: top-level JSON kind does not match the requested method
Deserialize: JSON is valid but cannot be deserialized into the target type

Installation

Add this to your Cargo.toml:

[dependencies]
qubit-json = "0.1.0"

Quick Start

Decode a JSON Object from a Markdown Code Fence

use serde::Deserialize;
use qubit_json::LenientJsonDecoder;

#[derive(Debug, Deserialize)]
struct User {
    name: String,
    age: u8,
}

fn main() {
    let decoder = LenientJsonDecoder::default();
    let user: User = decoder
        .decode_object("```json\n{\"name\":\"Alice\",\"age\":30}\n```")
        .expect("decoder should extract and decode the fenced JSON object");

    assert_eq!(user.name, "Alice");
    assert_eq!(user.age, 30);
}

Decode JSON Containing Raw Control Characters in Strings

use qubit_json::LenientJsonDecoder;

fn main() {
    let decoder = LenientJsonDecoder::default();
    let value = decoder
        .decode_value("{\"text\":\"line 1\nline 2\"}")
        .expect("decoder should escape raw control characters inside strings");

    assert_eq!(value["text"], "line 1\nline 2");
}

Customize Decoder Options

use qubit_json::{LenientJsonDecoder, JsonDecodeOptions};

fn main() {
    let decoder = LenientJsonDecoder::new(JsonDecodeOptions {
        strip_markdown_code_fence: false,
        ..JsonDecodeOptions::default()
    });

    let value = decoder
        .decode_value("{\"ok\":true}")
        .expect("plain JSON should still decode with custom options");

    assert_eq!(value["ok"], true);
}

Normalization Rules

When enabled, the decoder applies the following pipeline before parsing:

validate that the input is not empty
trim surrounding whitespace
strip a leading UTF-8 BOM
strip one outer Markdown code fence
escape ASCII control characters inside JSON string literals

The decoder does not:

add missing quotes
add missing commas
add missing braces or brackets
rewrite arbitrary malformed JSON into guessed valid JSON

When to Use

Qubit JSON is a good fit when:

you need a reusable, configurable JSON decoder object
your inputs are mostly valid JSON but may be wrapped or slightly noisy
you want stable error categories around serde_json

It is not a good fit when:

you need aggressive repair for heavily malformed JSON
your inputs are not actually JSON
a plain serde_json::from_str() call is already sufficient

License

This project is licensed under the Apache 2.0 License. See LICENSE for details.

Alignment Notes

This README reflects the current object model:

LenientJsonDecoder owns an internal LenientJsonNormalizer.
Public decoding APIs are decode, decode_object, decode_array, decode_value.
Normalization and error handling are implemented in src/lenient_json_normalizer.rs and src/json_decode_error.rs, which are covered by tests in tests/.
Product requirements and implementation behavior are aligned with doc/json_prd.zh_CN.md and doc/json_design.zh_CN.md.

qubit-json 0.1.0