05 — Hand-Rolled JSON

Context sources: ./04-cutover-remove-tokio-axum.md, ../../prototypes/wo-db/src/value.hpp.

Goal

Replace serde_json::Value / serde_json::Map with a hand-rolled Value type covering exactly the shapes the runtime reads and writes: request bodies, response bodies, and the Engine's in-memory Row. Remove serde + serde_json from crates/rt/Cargo.toml. After this phase the dep list is anyhow + libc.

Design decisions (locked)

Minimal surface. The runtime's actual JSON needs are small:
- Parse request body bytes → Value::Object (single top-level object on every sample endpoint).
- Emit Value::Object / Value::Array → bytes for the response.
- Pretty-printing is not required. Operators reach for | python3 -m json.tool if they want it.
No #[derive(Serialize/Deserialize)]. Value is the union type; every Row, Product, Article is already a Value::Object at the boundary. The only thing that "serializes" is Value. The ecosystem of derive-based types doesn't exist in rt today — engine::Row is HashMap<String, Value> via serde_json today, becomes HashMap<String, Value> via the new module tomorrow.
RFC 8259 compliant, but strict. No unquoted keys, no trailing commas, no comments. Standard JSON. The runtime isn't serving JSON5.
Parser is recursive descent, zero-copy where possible. String values borrow from the input buffer unless they contain escapes; objects own their keys. Preserves the "no heavy abstraction" pattern of phase 02 / 03.
Module at crates/rt/src/json/. Same "extract when a second consumer appears" rule. Eventual home is the empty crates/value/ sibling — not this phase.

Scope

New files inside `crates/rt/src/json/`

File	Responsibility	Approx LOC
`mod.rs`	Re-exports `Value`, `Object`, `Array`, `parse`, `emit`	~10
`value.rs`	`pub enum Value { Null, Bool(bool), Int(i64), Float(f64), Str(String), Array(Vec<Value>), Object(BTreeMap<String, Value>) }` + `impl Value` helpers (`as_str`, `as_i64`, `get`, indexing)	~200
`parse.rs`	`parse(&[u8]) -> Result<Value, ParseError>` — recursive descent: `parse_value` → `parse_object` / `parse_array` / `parse_string` / `parse_number` / `parse_keyword`. Single-pass, no backtracking.	~300
`emit.rs`	`emit(value: &Value, buf: &mut Vec<u8>)` — iterative-ish writer, escapes strings per RFC 8259 §7	~150

Total: ~660 LOC. No v1 precedent — no sample parser to port. Reference the target shape against prototypes/wo-db/src/value.hpp for the Value variants (same six kinds as the C++ prototype, minus Float which that prototype folds into Int but we need for HTTP request bodies like {"qty": 2.5}).

`Cargo.toml` delta

 [dependencies]
 anyhow     = "1"
-serde      = { version = "1", features = ["derive"] }
-serde_json = "1"
 libc       = "0.2"

Consumers to update

Search: rg 'serde_json|serde::' crates/rt/src | wc -l — expected ~20 call sites. Each is a mechanical swap:

Current	After
`serde_json::json!({"key": value})`	`json::Value::Object(…)` or a small `json!` macro we ship
`serde_json::Value`	`json::Value`
`serde_json::Map<String, Value>`	`BTreeMap<String, json::Value>` (the runtime already uses BTreeMap for stable order)
`serde_json::from_slice::<Value>(&bytes)?`	`json::parse(&bytes)?`
`Json(json!(rows)).into_response()`	`Response::ok().json_body(&rows)` (new helper on phase-03 `Response`)
`#[derive(Serialize, Deserialize)]` on any `rt` struct	deleted — no consumer after this phase

The biggest consumer is crates/rt/src/engine.rs — Row is a serde_json::Map<String, Value> today. It becomes BTreeMap<String, json::Value>. The eval_default() function's json!(n) / json!(b) calls become Value::Int(n) / Value::Bool(b). Minor, all local.

A compact `json!` macro (for ergonomics)

Without serde_json::json!, the most-used construction pattern (json!({"runtime": "wo", "stage": 2})) gets verbose. Ship a minimal macro:

#[macro_export]
macro_rules! json {
    (null)             => ($crate::json::Value::Null);
    (true)             => ($crate::json::Value::Bool(true));
    (false)            => ($crate::json::Value::Bool(false));
    ([$($e:tt),* $(,)?]) => (
        $crate::json::Value::Array(vec![$($crate::json!($e)),*])
    );
    ({$($k:tt : $v:tt),* $(,)?}) => ({
        let mut m = std::collections::BTreeMap::new();
        $( m.insert(stringify!($k).trim_matches('"').to_string(), $crate::json!($v)); )*
        $crate::json::Value::Object(m)
    });
    ($e:expr) => ($crate::json::Value::from($e));
}

Covers 95% of current serde_json::json!(...) uses in the codebase. For the other 5%, build Value by hand.

Exit criteria

cargo build — compiles with three deps (anyhow, libc + Cargo.toml itself).
New tests in crates/rt/src/json/:
- parse_object_simple — {"a":1,"b":"x"} round-trips.
- parse_nested_and_array — {"xs":[1,2,3],"meta":{"k":"v"}} round-trips.
- parse_escapes — "\\n\\t\\\"\\u0041" → "\n\t\"A".
- emit_stable_key_order — emitting a BTreeMap-backed object produces keys in sorted order (matters for .rest expected-body stability).
- parse_errors — unterminated string, trailing comma, missing comma, unclosed object all return ParseError with line/col.
All 14 existing rt tests pass after the swap (the engine::Engine and server::* tests most affected).
reference/rest/blog.rest — 20 assertions all return the same HTTP status AND the same response body shape (may differ in key ordering if BTreeMap ordering differs from serde_json's insertion order — document the shift).
Dep audit. cargo tree -p rt --depth 1 shows zero serde* lines.

Non-scope

No streaming parse. Request bodies are small (< 1 MB on every sample endpoint). A buffered full-body parse is fine.
No serde_json-compat feature flag. Clean break; this is the only consumer that matters, and we control it.
No JSON Pointer, no JSON Schema, no JSON Patch. If needed later, layer on top.
No crate extraction to crates/value/. Same rule as phase 02/03: wait for a second consumer.

Verification

cargo build                                    # three deps
cargo test --lib json                          # new parser/emitter tests
cargo test --lib                               # 14 existing tests still green
# full .rest smoke — same script as phase 04 exit criterion 3
cd reference/crates && cargo build && cargo test

After this phase

Two deps left: anyhow and libc. Phase 06 removes anyhow. After that, libc is the only external crate — the stated end goal.

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