Ohm

An experimental fork of the Rust toolchain, designed for fast, highly parallel local development.

Comparing Ohm 1.99.0-27 to upstream Rust, end-to-end development is around 1.82× faster:

Scope	Rust 1.99.0	Ohm	Time saved
Ruff session	401.80 s	221.08 s	44.98%
uv session	137.37 s	78.94 s	42.54%
Codex session	511.62 s	268.48 s	47.52%
ty session	283.31 s	164.41 s	41.97%
All sessions	1,334.09 s	732.91 s	45.06%
cargo check	142.39 s	47.26 s	66.81%
cargo test, cargo nextest, just test	760.43 s	516.41 s	32.09%
cargo clippy	388.13 s	140.64 s	63.76%
cargo fmt	13.77 s	8.62 s	37.43%
Other commands	27.38 s	17.47 s	36.19%
All commands	1,332.11 s	730.40 s	45.17%

And disk usage (on APFS) decreases by around 55%:

Scope	Rust 1.99.0	Ohm 1.99.0-11	Space saved
Ruff/ty session	5.82 GiB	3.49 GiB	40.06%
Ruff/ty, 2 worktrees	10.07 GiB	5.91 GiB	41.25%
Ruff/ty, 10 worktrees	52.98 GiB	26.46 GiB	50.05%
uv session	12.09 GiB	7.76 GiB	35.83%
uv, 2 worktrees	16.25 GiB	8.55 GiB	47.37%
uv, 10 worktrees	79.41 GiB	32.31 GiB	59.31%

Compared to upstream, Ohm:

• Enables (by default) existing experimental features (e.g., parallel compiler frontends).
• Implements experimental, performance-oriented features proposed upstream (e.g., Relink, don't rebuild).
• Fixes various performance bugs and regressions (e.g., around target/ size increases).
• Implements additional optimizations identified by Codex (e.g., reusing build artifacts between cargo build and cargo check, avoiding rustfmt on unchanged files).

Warning: Ohm includes backported and experimental Rust, Cargo, and Clippy optimizations that may be unsound. This README is human-edited, but all code changes, PR summaries, and additional documentation were authored entirely by GPT in Codex.

Installation

Ohm supports Apple Silicon macOS and x86_64 Linux. Install rustup, then install the latest Ohm release with our standalone installer:

curl --proto '=https' --tlsv1.2 -LsSf \
  https://labs.astral.sh/ohm/install.sh | sh

Ohm's installer verifies its embedded archive checksum, installs the complete Rust toolchain, and registers an exact version and a moving ohm alias with rustup, enabling cargo +ohm invocations (e.g., cargo +ohm check).

To install an exact version:

curl --proto '=https' --tlsv1.2 -LsSf \
  https://labs.astral.sh/ohm/releases/ohm-1.99.0-9/install.sh | sh

Each Ohm release targets a specific upstream Rust release. When a Rust version becomes available, we replay the optimizations that remain relevant and drop those that have already landed upstream. Multiple Ohm revisions may target the same Rust release; for example, ohm-1.97.1-20 and ohm-1.97.1-21.

Usage

Use rustup's +toolchain syntax to select Ohm:

cargo +ohm test
cargo +ohm check
cargo +ohm clippy
cargo +ohm fmt

Exact toolchain names have the form ohm-<rust-version>-<ohm-revision>. For example, cargo +ohm-1.99.0-9 test selects that installed revision.

Ohm enables dependency-aware rebuild reduction, cached relinking, parallel compiler frontends, and reuse of previously built registry and workspace dependencies by default. Opt out with -Zohm-defaults=no:

cargo +ohm -Zohm-defaults=no test

Run Ohm's garbage collector from any directory to clean up unused build artifacts in idle worktrees and shared caches.

cargo +ohm gc

AGENTS.md

To teach Codex about Ohm, consider adding the following to your ~/.codex/AGENTS.md:

## Ohm Rust toolchain

- Use `cargo +ohm <command>` for all local Cargo commands, including `check`, `test`, `clippy`,
  `fmt`, `metadata`, and external subcommands. Use `rustc +ohm` for direct compiler calls and
  `RUSTUP_TOOLCHAIN=ohm just <recipe>` for task runners.
- Ohm automatically enables `-Zrelink-only`, `-Zpublic-api-hash`, and `-Zthreads=8` when compiling.
  Cargo supplies these flags for you. Direct `rustc +ohm` uses its own defaults.
- Keep a stable, distinct `CARGO_TARGET_DIR` for each worktree. Set `CARGO_BUILD_BUILD_DIR` to a
  separate shared path for all trusted local projects and worktrees using the same Ohm toolchain.
  Keep the shared build directory separate from every worktree target directory, and do not delete
  it when cleaning up an individual project or worktree.
- Create worktrees for local Rust development from an existing Cargo workspace with
  `cargo +ohm worktree add -b <branch> <path> <base>` to seed reusable build state automatically. If
  using custom target directories, pass the new worktree's target directory via `--target-dir`.
- Set `CARGO_UNSTABLE_OHM_PROC_MACRO_TRUST=all` to maximize authenticated registry procedural-macro
  reuse across worktrees. On `cargo +ohm worktree add`, this also opts into importing snapshots
  containing local procedural macros. This opt-in is potentially unsound; avoid it for CI, releases,
  etc.
- Set `CARGO_UNSTABLE_OHM_NATIVE_TOOL_TRUST=all` to maximize reuse of authenticated registry build
  scripts and native-tool helpers across worktrees. This opt-in is potentially unsound; avoid it for
  CI, releases, etc.
- Use `cargo +ohm -Zohm-defaults=no <command>` to disable Ohm's automatic experimental defaults. Use
  plain `cargo <command>` and the repository's normal toolchain for CI and production builds.

Philosophy

Rust's compile times and disk usage are emerging as bottlenecks in day-to-day development, especially as agents enable us to parallelize across (dozens of) worktrees.

Ohm enables experimental optimizations to improve Rust's iteration speed and throughput. This is a deliberate tradeoff: we accept the risks of experimental implementations in local development, where the blast radius of bugs is comparatively limited and the benefits of faster iteration are substantial (while continuing to use the official toolchain for production builds).

Ohm is authored entirely by the GPT model series in Codex. Agents lower the cost of implementing, testing, and refining ideas. When an optimization proves useful, we can propose it upstream. To keep that path open, Ohm avoids fundamental changes to the Rust toolchain, such as introducing a stateful development server.

For details on the optimizations Ohm enables and their measured impact, see Optimizations.

Ohm's patches are maintained in the following repositories:

• ohm-rust: Rust compiler and standard library.
• ohm-cargo: Cargo.
• ohm-rust-clippy: Clippy.
• ohm-rustfmt: rustfmt.

Benchmarks

See the benchmark guide for details.

Project	Workflow	Rust 1.99.0	Ohm 1.99.0-27	Time saved
Ruff/ty	Edit → focused test	13.87 s	14.84 s	−7.01%
Ruff/ty	Edit dependency → downstream test	13.99 s	12.24 s	12.52%
Ruff/ty	Edit → check → test → check	20.45 s	22.35 s	−9.27%
Ruff/ty	Check → build → check	76.39 s	50.36 s	34.08%
Ruff/ty	Edit → fmt → clippy → test	27.03 s	22.85 s	15.46%
uv	Edit → focused test	19.13 s	8.90 s	53.48%
uv	Edit dependency → downstream test	20.94 s	2.39 s	88.60%
uv	Edit → check → test → check	21.67 s	11.10 s	48.78%
uv	Check → build → check	116.28 s	60.78 s	47.73%
uv	Edit → fmt → clippy → test	24.77 s	12.85 s	48.12%
Codex	Edit → focused test	27.06 s	25.63 s	5.31%
Codex	Edit dependency → downstream test	50.67 s	3.55 s	92.99%
Codex	Edit → check → test → check	33.48 s	31.97 s	4.51%
Codex	Check → build → check	156.80 s	109.03 s	30.46%
Codex	Edit → fmt → clippy → test	36.63 s	31.29 s	14.57%

Direct edits modify the crate being checked, linted, or tested:

Project	Workflow	Edited crate	Rust 1.99.0	Ohm 1.99.0-27	Speedup
uv	cargo check	uv	1.99 s	1.77 s	1.12×
uv	cargo clippy	uv	3.29 s	2.52 s	1.31×
uv	cargo test --test it	uv	18.07 s	7.96 s	2.27×
Codex	cargo check	codex-core	4.43 s	4.12 s	1.08×
Codex	just clippy	codex-core	19.98 s	13.24 s	1.51×
Codex	just test	codex-core	27.81 s	26.60 s	1.05×

Indirect edits modify a dependency's implementation without changing its public API, so Ohm can avoid rebuilding unchanged dependents:

Project	Workflow	Edited dependency	Rust 1.99.0	Ohm 1.99.0-27	Speedup
uv	cargo check	uv-normalize	8.79 s	0.50 s	17.51×
uv	cargo clippy	uv-normalize	12.94 s	0.62 s	21.02×
uv	cargo test --test it	uv-normalize	19.53 s	2.21 s	8.84×
Codex	cargo check	codex-utils-string	18.03 s	0.88 s	20.51×
Codex	just clippy	codex-utils-string	40.32 s	1.13 s	35.65×
Codex	just test	codex-utils-string	50.62 s	3.03 s	16.70×

Other common cases include build-to-check reuse, formatting, and worktree startup:

Project	Workflow	Package	Rust 1.99.0	Ohm	Speedup
uv	Check after build	uv	59.92 s	11.91 s	5.03×
uv	Shared-worktree build	uv-normalize	6.75 s	0.73 s	9.24×
Codex	Check after build	codex-core	106.43 s	27.54 s	3.86×
Codex	No-op formatting	Workspace	9.90 s	0.17 s	57.71×
Codex	Formatting after an edit	codex-core	9.53 s	0.28 s	34.21×
Codex	Shared-worktree build	codex-utils-string	2.74 s	0.66 s	4.16×
ty	Worktree startup (ext4)	ty_python_semantic	92.94 s	53.59 s	1.73×
ty	Worktree startup (Btrfs)	ty_python_semantic	98.19 s	51.38 s	1.91×
Ruff	Doctests	Selected crates	13.564 s	1.740 s	7.80×

License

Ohm is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in Ohm by you, as defined in the Apache-2.0 license, shall be dually licensed as above, without any additional terms or conditions.