Function copy 

pub fn copy(
    worktree: &Path,
    project: &Path,
    checkout: &Path,
    patterns: &[String],
) -> Result<Copied, String>

Copies the files matched names from checkout (the project’s directory there) to the same paths under project (its directory in the worktree worktree): new ones, and again those whose content changed. Registers each copied secret file’s values for masking.

Errors

The record of copies could not be written.