Function remap_project_errors 

pub fn remap_project_errors(
    source: &str,
    cache_dir: &Path,
    file_map: Option<&SourceMap>,
) -> Result<Vec<RemappedError>, CompileError>

Remap every rustc error over a project’s GENERATED Rust back to the LOGOS source line it came from — the build path uses this so a generated-code failure points at the LOGOS the user wrote, not main.rs. Compiles with the source-mapped codegen (whose line numbers the map indexes), runs cargo check --message-format=json, and remaps each diagnostic’s primary span. Returns Ok(vec![]) when nothing maps (caller falls back to raw cargo output).