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#947 in Parser implementations

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Used in 10 crates (3 directly)

MIT license


High-fidelity JSON lexer and parser

Build status Rust 1.56+

hifijson is a Rust crate that provides a high-fidelity JSON lexer and parser. In this context, high-fidelity means that unlike many other parsers, hifijson aims to preserve input data very faithfully, in particular numbers.

  • Zero dependencies: Not even alloc is obligatory!
  • no_std: Can be used on embedded systems without standard library.
  • Reading from slices and from byte iterators: This is important if you are writing an application that should read from files as well as from standard input, for example.
  • Performance
  • Portability
  • Mostly zero-copy deserialisation: Due to the presence of escaped characters in JSON strings, full zero-copy deserialisation of JSON data is not possible. However, hifijson attempts to minimise allocations in presence of strings.
  • Deserialisation via serde

Comparison to serde_json

serde_json is currently the most popular JSON parser written in Rust. However, there are some deficiencies of serde_json:

  • Numbers can be parsed with arbitrary precision (via the feature flag arbitrary_precision), but they cannot be deserialised (by implementing the Deserialize trait) to anything else than a serde_json::Value #896. Instead, one has to deserialize to serde_json::Value, then convert that to something else, which costs time.
  • When using arbitrary_precision, serde_json incorrectly parses or rejects certain input; for example, it incorrectly parses {"$serde_json::private::Number": "1.0"} as number 1.0 and incorrectly rejects {"$serde_json::private::Number": "foo"}. I consider both of these to be bugs, but although they are known, the serde_json maintainers are "fine sticking with this behaviour".
  • The behaviour of serde_json can be customised to some degree via feature flags. However, this is a relatively inflexible solution; for example, you can specify whether to preserve the order of keys in objects by using the preserve_order feature flag, but what happens when you have an object that contains the same key several times, for example {"a": 1, "a": 2}? Currently, serde_json parses this as {"a": 2}, silently discarding information. What if you would like to fail in this case? Well, you can just implement Deserialize yourself. Except ... that you cannot, if you are using arbitrary_precision. Ouch.

You should probably use serde_json if you want to serialise / deserialise your existing Rust datatypes. However, if you want to process arbitrary JSON coming from the external world, require some control over what kind of input you read, or just care about fast build times and minimal dependencies, then hifijson might be for you.

There is also serde-json-core for embedded usage of JSON; however, this crate neither supports arbitrary-precision numbers, reading from byte iterators, nor escape sequences in strings.


Writing a JSON parser is remarkably easy --- the hard part is actually lexing. This is why hifijson provides you first and foremost with a lexer, which you can then use to build a parser yourself. Yes, you. You can do it. hifijson tries to give you some basic abstractions to help you. For example, the default parser is implemented in less than 40 lines of code.

Default parser

Parsing JSON is a minefield, because the JSON standard is underspecified or downright contradictory in certain aspects. For this reason, a parser has to make certain decisions which inputs to accept and which to reject.

hifijson comes with a default parser that might be good enough for many use cases. This parser makes the following choices:

  • Validation of strings: The parser validates that strings are valid UTF-8.
  • Concatenation of JSON values: Many JSON processing tools accept multiple root JSON values in a JSON file. For example, [] 42 true {"a": "b"}. However, defining formally what these tools actually accept or reject is not simple. For example, serde_json accepts []"a", but it rejects 42"a". The default behaviour of this parser is to accept any concatenation of JSON-text (as defined in RFC 8259) that can be somehow reconstructed. This allows for weird-looking things like nulltruefalse, 1.0"a", but some values cannot be reconstructed, such as 1.042.0, because this may be either a concatenation of 1.0 and 42.0 or a concatenation of 1.04 and 2.0. In that sense, hifijson attempts to implement a policy that is as permissive and easily describable as possible.

Furthermore, the parser passes all tests of the JSON parsing test suite.


To run the fuzzer, install cargo-fuzz. Then, if you do not wish to use the nightly Rust compiler as default, run the fuzzer by cargo +nightly fuzz run all.