+++
title = "Rust"
description = "Rust code generator"
weight = 3
+++

The `mlf-codegen-rust` generator emits Rust structs from MLF lexicons, with `serde` derives for round-trip JSON support.

## Usage

```toml
[[output]]
type = "rust"
directory = "./crates/lex-types/src/generated"
```

```bash
mlf generate code -g rust
```

This is the generator the MLF project itself uses for its `mlf-generated-lexicon` crate — see [the lol.mlf.package lexicon](https://github.com/stavola-xyz/mlf/blob/main/lexicons/lol/mlf/package.mlf) and the corresponding [generated struct](https://github.com/stavola-xyz/mlf/blob/main/mlf-generated-lexicon/src/generated/lol/mlf/package.rs) for a live example.

## Type mapping

| MLF type | Rust |
|---|---|
| `null` | `serde_json::Value` |
| `boolean` | `bool` |
| `integer` | `i64` |
| `string` | `String` |
| `bytes` | `Vec<u8>` |
| `blob` | `Vec<u8>` |
| `Datetime` | `String` (ISO 8601) |
| `Did`, `AtUri`, `Cid`, `Handle`, `Nsid`, `Tid`, `RecordKey`, `Uri`, `Language`, `AtIdentifier` | `String` |
| `T[]` (array) | `Vec<T>` |
| `T \| U` (union) | `#[serde(untagged)] enum` |
| `{ …fields… }` (object) | named `struct` |
| Custom `def type Foo` | `pub struct Foo { … }` |
| `record foo` | `pub struct Foo { … }` (outer collection is the file's NSID) |

## Field syntax

- `field: T` → `pub field: Option<T>` with `#[serde(skip_serializing_if = "Option::is_none")]`
- `field!: T` → `pub field: T` (required)

Field names are snake_cased (Rust convention); a `#[serde(rename = "…")]` attribute preserves the original camelCase for on-wire fidelity.

## Derives

Generated structs carry `#[derive(Debug, Clone, Serialize, Deserialize)]`. Nothing else — no `PartialEq`, no `Eq`, no `Hash`. If you need those, wrap or extend the generated types in a sibling module.

## Doc comments

`///` doc comments on defs and fields are preserved as `///` Rust doc comments. They show up in rustdoc + LSP hover + IDE completion.

## What isn't covered yet

- **Validators.** No generated `Validate` trait impls. Use `mlf-validation`'s `RecordValidator` if you need runtime validation against the lexicon.
- **Enum types.** `token` declarations emit as a string newtype with known-value constants; no `#[non_exhaustive]` enum yet.