---
title: Builder & Error Remapping
description: How Mist invokes cargo and remaps Rust compiler errors back to Mist source locations.
icon: Bug
---

The builder module (`builder.mist` in the `mist_api` crate) wraps `cargo` to:

1. Spawn `cargo` with `--message-format=json`
2. Parse JSON compiler messages
3. For each diagnostic span, look up the corresponding `.map.json` file
4. Remap Rust line/column to Mist line/column using the mapping
5. Display errors/warnings with Mist source locations and context lines

```rust
pub fn build(args: Vec<String>, root: PathBuf) -> bool {
    // Spawn cargo with JSON output
    // Parse CompilerMessage for each span
    // Look up rev_mapper::Mapping from .map.json
    // Remap to Mist positions
    // Print diagnostics
}
```

### Diagnostic Output

Errors and warnings are displayed with Mist source context:

```
src/main.mist:10:5
 Error: mismatched types
    let x: i32 = "hello";
                ^^^^^^^
```