---
title: Position Mapping
description: Bidirectional mapping between Mist source positions and Rust output positions for error remapping and LSP features.
icon: Map
---

Mist maintains a bidirectional mapping between Mist source positions and Rust output positions. This is essential for:

1. **Error remapping** — Rust compiler errors point to the correct Mist source location
2. **LSP features** — Go-to-definition, hover, and completion work from Mist source

### Data Structure

The mapping is stored as pairs:

```rust
pub struct Mapping {
    pub mist_path: PathBuf,
    pub map: HashSet<(RustMap, MistMap)>,
}
```

- `RustMap(usize, usize)` — line/column in generated Rust
- `MistMap(usize, usize)` — line/column in original Mist

### Mapping Lifecycle

1. **Codegen** — Each `Spanned<T>` AST node records its Mist position and the current Rust position in the codegen output buffer via `GenSpanTranslation`
2. **Persistence** — The mapping is serialized to `<output>.map.json` alongside the transpiled `.rs` output
3. **Build-time remapping** — The builder reads `.map.json` to remap `cargo` diagnostics back to Mist positions
4. **LSP remapping** — Both Mist→Rust and Rust→Mist direction queries are supported via `find()` and `find_by_mist()`