cross-rs/cross · error

unix paths cannot handle windows prefix

Error message

unix paths cannot handle windows prefix {prefix:?}.

What it means

`as_posix_relative` converts a Path to a POSIX-style relative string, but it cannot represent Windows path prefixes (drive letters like `C:` or UNC `\\server\share`). If the path's components include a `Component::Prefix`, it bails because such a path has no valid POSIX relative form.

Solutions

1. Pass a relative path (relative to the working directory) instead of an absolute Windows path.
2. Use a forward-slash path without the drive prefix, e.g. `target` instead of `C:\project\target`.
3. Run the invocation from a Unix-style environment (WSL) where paths have no Windows prefix.

Example fix

// before (cross config)
[target.x86_64-pc-windows-gnu]
# path passed: C:\project\target
// after
# run from project root and pass relative path: target

function isAbsolutePathWithPrefix(p) {
  return /^[A-Za-z]:[\\/]/.test(p) || /^\\\\[^\\]/.test(p); // C:\ or \\server\share
}
// before calling as_posix_relative, ensure the path has no drive/UNC prefix

try {
  return path.asPosixRelative(p);
} catch (e) {
  if (String(e.message).includes('windows prefix')) {
    throw new Error(`Path ${p} must be relative and free of drive/UNC prefixes`);
  }
  throw e;
}

When it happens

Trigger: Calling `as_posix_relative` on an absolute Windows path (e.g. `C:\Users\me\project` or a UNC path) that was passed to code expecting a relative Unix-style path.

Common situations: Running cross on Windows or with Windows-originated paths (CARGO_TARGET_DIR, config paths) where a relative path is required inside the container; hardcoded `C:\...` paths in cross config files.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

Related errors

• unable to find provided nix-store directory
• Could not get UserName.
• workspace_root can't end in `..`
• source is pointing to a directory instead of its contents
• artifact path does not start with , skipping

AI-assisted analysis of cross-rs/cross@8c1a8aa4b6 (2026-09-13). Data as JSON: /api/errors/4feba72572cb2c6e. Report an issue: GitHub.

Appendix: source

Thrown at src/file.rs:95

    }
}

impl PathExt for Path {
    fn as_posix_relative(&self) -> Result<String> {
        if cfg!(target_os = "windows") {
            let push = |p: &mut String, c: &str| {
                if !p.is_empty() && p != "/" {
                    p.push('/');
                }
                p.push_str(c);
            };

            // iterate over components to join them
            let mut output = String::new();
            for component in self.components() {
                match component {
                    Component::Prefix(prefix) => {
                        eyre::bail!("unix paths cannot handle windows prefix {prefix:?}.")
                    }
                    Component::RootDir => output = "/".to_owned(),
                    Component::CurDir => push(&mut output, "."),
                    Component::ParentDir => push(&mut output, ".."),
                    Component::Normal(path) => push(&mut output, path.to_utf8()?),
                }
            }
            Ok(output)
        } else {
            self.to_utf8().map(|x| x.to_owned())
        }
    }

    #[cfg(not(target_family = "windows"))]
    fn as_posix_absolute(&self) -> Result<String> {
        absolute_path(self)?.to_utf8().map(ToOwned::to_owned)
    }

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