Function patch 

pub fn patch(cfg: &Config, opts: PatchOpts) -> Result<PatchReport, PkgError>

Create (or, with force, rebuild) the patch_dir of one dep from the commit its pin resolves to, and record that commit as patch_base in the lockfile.

The whole package root is copied, so types/ and other siblings of the entry come along. From then on install resolves the dep from the patch for as long as the pin keeps resolving to patch_base; edit the files in place and commit them with the project. When the pin moves, install falls back to upstream with a warning, and patch --force rebuilds the directory from the new commit (discarding local edits — re-apply them from your VCS history).

Errors

• PkgError::UnknownDep when name is not in [deps].
• PkgError::Validation when the dep has no patch_dir, or the directory exists and force is not set.
• PkgError::Fetch wrapping the fetcher error.
• PkgError::Io for the copy / lockfile write.