Docs rewrite

content/docs/internals/codegen.mdx

@@ -0,0 +1,53 @@
+---
+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