---
title: Code Generation
description: How the Mist AST is translated into Rust source code, including class vtables and position mapping.
icon: Wrench
---

The code generator converts the Mist AST into Rust source code directly — no intermediate representation.

### Key files

- `src/lib.rs` — `RustCodegen` struct with output buffer, indentation tracking, `Mapping` for position translation
- `src/top_level.rs` — Generates Rust for top-level items (structs, enums, traits, functions, impls, imports, type aliases, const/static)
- `src/statement.rs` — Generates Rust for statements and blocks
- `src/expr.rs` — Generates Rust for expressions (literals, paths, binary ops, closures, arrays, prefix/postfix)
- `src/class_decl.rs` — Class-specific code generation (struct + vtable + impl blocks + Deref)

### Codegen Design

The `RustCodegen` maintains:
- A string `output` buffer
- An `indent` level (4 spaces per level)
- A `Mapping` that records `(RustMap, MistMap)` pairs for source position remapping
- A current `position` tracker (`RustMap(line, column)`)

Generation follows the `GenRust` trait:

```rust
pub trait GenRust {
    fn gen_rust(&self, ctx: &mut Context, cg: &mut RustCodegen);
}
```

And `GetRust` for simple string-returning types:

```rust
pub trait GetRust {
    fn get_rust(&self) -> String;
}
```

The `Context` carries optional expression path information for class `super` / `Super` resolution.

### Class Codegen

Classes are the most complex codegen path. `ClassProcessedData` analyzes a class declaration and emits:

1. **Struct declaration** — `struct ClassName<G> { pub _super: Parent, pub field1: T1, ... }` (or `_vptr` for root classes)
2. **Vtable constants** — Index constants `__FN_METHOD` and a `__V_TABLE` static array of function pointers. For inherited classes, parent vtable entries are copied and overridden entries replaced.
3. **Constructor** — `pub fn new(...) -> Self` that creates an uninitialized instance via `MaybeUninit`, sets the vtable pointer, writes field defaults, calls `self.constructor(...)`, and returns `this`
4. **Method trampolines** — Public methods with `self` get wrapper functions `__m_method` stored in the vtable, plus virtual dispatch methods
5. **Override support** — Methods marked `override` are validated at compile time via generated test code
6. **Deref impls** — `impl Deref<Target = Parent> for Child` and `DerefMut` for inherited classes
7. **Impl declarations** — Inner `impl` blocks are rewritten to use the self type