Position Mapping

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:

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()