GitoxideLabs/gitoxide · error · anyhow::Error

`--update-head` cannot be used with `--in-memory` - cannot…

Error message

`--update-head` cannot be used with `--in-memory` - cannot set head to nothing

What it means

The `gix repo merge tree` CLI command refuses the combination of `--update-head` and `--in-memory`. `--update-head` moves the current branch ref to the merge result commit, but an in-memory merge produces no persisted commit, so there would be no head to update. The command fails fast with this message before doing any work.

Solutions

  1. Drop `--in-memory` so the merge writes a commit that `--update-head` can point the branch at.
  2. Drop `--update-head` if you only want the computed tree/merge result without moving the branch.
  3. If you need in-memory computation plus a head update, write the commit yourself from the resulting tree id, then update the ref with `gix ref` APIs.

Example fix

// before
gix repo merge tree --update-head --in-memory <base> <ours> <theirs>
// after
# either drop --in-memory
gix repo merge tree --update-head --message "merge" <base> <ours> <theirs>
# or drop --update-head
gix repo merge tree --in-memory <base> <ours> <theirs>
Defensive patterns

Strategy: validation

Validate before calling

if update_head && in_memory {
    return Err(anyhow::anyhow!("--update-head and --in-memory are mutually exclusive"));
}

Try / catch

match run_merge_tree(opts) {
    Err(e) if e.to_string().contains("--update-head`") => eprintln!("pick either --update-head or --in-memory"),
    r => r?,
}

Prevention

When it happens

Trigger: Running `gix repo merge tree` (gitoxide-core repository::merge::tree `tree()`) with both `update_head == true` and `in_memory == true` set in Options.

Common situations: Scripting a merge whose result should land on the current branch while also passing `--in-memory` (e.g. copied flags from an earlier dry-run invocation); wrapping the CLI in automation that always enables in-memory mode for speed.

Understand the failure class

Background: "mutually exclusive" flag errors: what "can't supply both nx and xx", "--raw is not compatible with -i" and "cannot be used with" mean, and how to fix them — this error's family across 29 libraries.

Related errors

AI-assisted analysis of GitoxideLabs/gitoxide@e73179060b (2026-09-08). Data as JSON: /api/errors/09fad30bf77648b5. Report an issue: GitHub.

Appendix: source

Thrown at gitoxide-core/src/repository/merge/tree.rs:48

        err: &mut dyn std::io::Write,
        base: BString,
        ours: BString,
        theirs: BString,
        Options {
            format,
            file_favor,
            tree_favor,
            in_memory,
            debug,
            message,
            update_head,
        }: Options,
    ) -> anyhow::Result<()> {
        if format != OutputFormat::Human {
            bail!("JSON output isn't implemented yet");
        }
        if update_head && in_memory {
            bail!("`--update-head` cannot be used with `--in-memory` - cannot set head to nothing");
        }
        if update_head && message.is_none() {
            bail!("`--update-head` requires `--message`");
        }
        repo.object_cache_size_if_unset(repo.compute_object_cache_size_for_tree_diffs(&**repo.index_or_empty()?));
        if in_memory || message.is_some() {
            repo.objects.enable_object_memory();
        }
        let (base_ref, base_id) = refname_and_tree(&repo, base)?;
        let (ours_ref, ours_id) = refname_and_tree(&repo, ours)?;
        let (theirs_ref, theirs_id) = refname_and_tree(&repo, theirs)?;

        let options = repo
            .tree_merge_options()?
            .with_file_favor(file_favor)
            .with_tree_favor(tree_favor);
        let base_id_str = base_id.to_string();
        let ours_id_str = ours_id.to_string();

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