merge doesn't seem to work

        let reference = self.repo.resolve_reference_from_short_name("FETCH_HEAD")?;
        let oid = match reference.target() {
            Some(oid) => oid,
            None => return Err(Error::from_str("No oid found for FETCH_HEAD")),
        };
        let fetch_head = self.repo.find_annotated_commit(oid)?;
        let checkout = &mut CheckoutBuilder::new();
        checkout.safe();
        let opts = &mut MergeOptions::new();
        opts.fail_on_conflict(true);
        self.repo.merge(&[&fetch_head], Some(opts), Some(checkout));
        let r: (MergeAnalysis, MergePreference) = self.repo.merge_analysis(&[&fetch_head])?;
        println!("Merge Analysis Empty: {:?}", r.0.is_empty());
        println!("Index Empty: {:?}", self.repo.index()?.is_empty());
        Ok(())

I'm trying to use merge after fetch, but this doesn't have any effect on my repo and index. The index is empty, can anyone explain how to use it?

No repository file or test is named. Start with Repository::merge and Repository::merge_analysis in the linked git2 documentation, then reproduce the FETCH_HEAD sequence and inspect the returned analysis, index, and checkout state. Done means explaining or correcting why the merge has no visible effect.