UPSTREAM PR #2409: [WIP] Add functionality to execute Rust closures on directories

[loci-dev]

Note

Source pull request: GitoxideLabs/gitoxide#2409

Thanks for asking me to work on this. I will get started on it and keep this PR's description up to date as I form a plan and make progress.

Original prompt

Add a way to execute Rust closures on a given directory, fully integrated into the existing ways of creating fixtures with scripts. The key here is to let the caller provide a version number (simple integer), that needs to be incremented manually to allow caching.
With it, it should be possible to make a fixture something like this:

let dir = gix_testtools::rust_fixture_read_only(1, |dir| { …create the fixture in Rust here })?;

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[loci-review]

Overview

Analysis of 26,353 functions across two Gitoxide binaries reveals stable performance between versions. 53 functions modified, 152 added, 152 removed, and 25,996 unchanged. Power consumption shows marginal improvements: target.aarch64-unknown-linux-gnu.release.gix decreased 0.011% (790,293 → 790,203 nanojoules), and target.aarch64-unknown-linux-gnu.release.ein decreased 0.005% (294,086 → 294,072 nanojoules).

Function Analysis

Only one function met the analysis threshold: parse_from (clap_builder CLI parser) in the ein binary. Response time improved from 65,069,468ns to 65,060,176ns (-9,292ns, -0.014%), and throughput time improved from 148.76ns to 145.77ns (-2.99ns, -2.01%). This one-time initialization function is non-critical, called once per program invocation. No source code changes were detected in the repository; improvements are attributed to compiler optimization variations. Performance-critical areas identified in project insights—object database operations, pack file handling, parallel processing, and diffing algorithms—show no significant changes, maintaining stable performance for core Git operations.

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