FuelLabs/sway · error · std::io::Error

Source file was modified, and the mapping is now out of rang

Error message

Source file was modified, and the mapping is now out of range

What it means

`forc addr2line` reconstructs a source snippet from a source map by reading the .sw file and locating the recorded LineCol range. start_pos remains None when the read loop reaches EOF without ever reaching range_start — i.e. the file now has fewer lines/bytes than when the source map was recorded. This io::Error (UnexpectedEof) reports that the source file was modified after the build, invalidating the line/column mapping.

Source

Thrown at forc/src/cli/commands/addr2line.rs:137

        if start_pos.is_none() {
            if position + n > range_start {
                let cbl: usize = context_buffer.iter().map(|c| c.len()).sum();
                start_pos = Some((line_num, position, range_start - (position + n - cbl)));
            } else if context_buffer.len() > context_lines {
                let _ = context_buffer.pop_front();
            }
        } else if context_buffer.len() > context_lines * 2 {
            break;
        }

        position += n;
    }

    let source = context_buffer.make_contiguous().join("");
    let length = range_end - range_start;

    let (source_start_line, _source_start_byte, offset) = start_pos.ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::UnexpectedEof,
            "Source file was modified, and the mapping is now out of range",
        )
    })?;

    if offset + length > source.len() {
        return Err(io::Error::new(
            io::ErrorKind::UnexpectedEof,
            "Source file was modified, and the mapping is now out of range",
        ));
    }

    Ok(ReadRange {
        source,
        _source_start_byte,
        source_start_line,
        offset,
        length,

View on GitHub (pinned to 47e5e902fa)

Solutions

1. Rebuild so the source map matches the sources: `forc build`, then re-run the addr2line command.
2. Check out the exact commit the artifact was built from before running addr2line.
3. Restore the file via `git status`/`git stash` so its content matches the build, then retry.

Example fix

// before
$ vi src/main.sw        # shrink the file
$ forc addr2line -s out/debug/my_pkg-source_map.json 0x3a  -> UnexpectedEof
// after
$ forc build
$ forc addr2line -s out/debug/my_pkg-source_map.json 0x3a  -> snippet prints

# rebuild whenever any source is newer than the source map
smap=out/debug/my_pkg-source_map.json
if [ "$(find src -name '*.sw' -newer "$smap" | head -n1)" ]; then forc build; fi
forc addr2line -s "$smap" 0x3a

match res {
    Err(e) if e.kind() == std::io::ErrorKind::UnexpectedEof => {
        eprintln!("source changed since build; re-run forc build");
    }
    other => other,
}

When it happens

Trigger: Editing or shortening the .sw source between `forc build` and `forc addr2line`; using a source map built from a different commit whose file was longer; line-ending or formatting changes that shift the recorded start position past the current end of file.

Common situations: Iterating on a reverting test: edit source, forget to rebuild, run addr2line on the stale binary and stale source map; CI caching old artifacts while sources advance; checking out a newer commit but reusing out/debug/*-source_map.json.

Related errors

• Could not get homedir
• Failed to run forc plugins
• InvalidData
• failed to parse manifest: {}.
• dependency of {:?} named {:?} is invalid: {}

AI-assisted analysis of FuelLabs/sway@47e5e902fa (2026-08-16). Data as JSON: /api/errors/3c930412a5ecd283. Report an issue: GitHub.