The standard library is not a validator: 72 hours of zero-dependency JSON in Rust

I spent last weekend building a JSON toolkit in Rust under one rule: no third-party dependencies . Not "few". None. The [dependencies] table in Cargo.toml is present and empty, and Cargo.lock holds exactly one package — the project itself. No serde , No serde_json , No clap , No itoa , No ryu . That constraint is the premise of the Zero Dependency hackathon , and it is a good premise, because it forces you to find out what the standard library actually promises. Here is the thing I did not expect to find: About 10% of the JSON documents that RFC 8259 says a parser must reject are accepted by Rust's own number parser. Not a subtle 10%. NaN , Infinity , .5 , 5. , +1 and 012 are all invalid JSON, and f64::from_str and i64::from_str take every one of them. If you write a JSON parser the obvious way — scan to the end of the number token, hand the slice to from_str — you ship a parser that is silently non-conformant, with no warning anywhere. I have the number because I counted it against a real corpus before writing the parser. The rest of this post is what that measurement did to the design, and what generalizes to languages that are not Rust. What I built jaq-lite is a hand-rolled RFC 8259 parser, a serializer, and a jq-style query CLI with rustc -style caret diagnostics — 4,670 lines under src/ and 4,432 lines of tests, standard library only. $ echo '{"users":[{"name":"ada","age":36},{"name":"linus","age":54}]}' | jaq-lite '.users[] | .name' "ada" "linus" It supports identity, field access, quoted fields, indexes, iteration, pipes, commas, parentheses, the optional operator ? , and eleven builtins ( length , keys , keys_unsorted , type , to_entries , from_entries , flatten , first , last , reverse , not ). Exit codes follow jq: 2 for a bad flag, 3 for a filter that does not compile, 5 for input that is not JSON, 0 otherwise. The measurement JSONTestSuite is the standard conformance corpus: 318 files in test_parsing/ , named by what a parser is supposed to do with them