Function save 

pub fn save(
    ctx: &Ctx,
    step: &str,
    snapshot: &Snapshot,
    run: &mut Runner<'_>,
) -> Result<PathBuf, RkError>

Read and save the before-state of step into snapshot, returning where it went.

Errors

An observation-incomplete refusal where a read fails for a reason other than a definite not-found, and the snapshot’s own write failures.