#ninja #build-system #tooling

bin+lib ninjars

A rust implementation of the ninja build system. This is the main application. This one is built on the theory in Build Systems a la Carte.

1 unstable release

0.1.0 Aug 29, 2020

#852 in Development tools




ninja-rs (Like ninjas, but pronounced with an intrusive "r" ;-)) is a clone of the ninja build system.


ninja-rs was created for several different reasons:

  1. Education - Build Systems a la Carte was a hugely influential paper for me as it formalized and distilled build systems. This was an attempt to implement a real build system using the ideas in that paper. Read the notes on design below for more about this. Evan Martin, the original author of ninja, also wishes they had this paper around.

  2. An exercise in using Rust outside of my day job for a real project.

  3. I had never written a proper parser before.

ninja is a fairly simple build system (it was explicitly designed to be the "assembly language" of build systems), so I figured it would be a good place to start. It is relatively small, and the original code is quite readable. It doesn't concern itself with any networking or packaging. As Evan Martin says:

Ninja is pretty easy to implement for the fun 20% of it and the remaining 80% is "just" some fiddly details.

It is a work in progress. As my understanding of Ninja and Rust evolved, various things changed and continue to change.

Feature complete-ness

At this point the parser is fairly feature complete and ninja-rs is capable of building the simple CMake hello-world in this repository. It has a long way to go to fully support everything ninja supports.

  • Working parser and topological sort based builder
  • mtime based rebuilding
  • Basic command-line compatibility with Ninja
  • Implicit and ordered dependencies
  • Variables and scoping
  • Handling failed commands correctly
  • Pools
  • Path canonicalization
  • Windows support (Nothing intentionally stopping it, but not tested either.)
  • Ninja log
  • build file regeneration
  • C compiler include parsing (-M for GCC/clang, /showIncludes for MSVC) and dependency log
  • Dynamic dependencies
  • Better pretty-printing
  • Extra tools
  • Provide releases
  • Make ninjars available as a cargo install-able command.


Make sure you have at least Rust 1.46-nightly (that is the only one I've tested with). Clone this repository, then:

cargo build --release -p ninjars --bin ninja

This will result in a binary ./target/release/ninja which is statically linked and can be copied wherever you need.


Contributions are welcome. Please understand that I have limited time to look at pull requests, so I may not reply promptly. For bug fixes, simply submit a PR, ideally with a regression test for that bug.

If you are considering implementing a major feature, please file an issue discussing the feature and how you intend to implement it. There may be multiple solutions and trade-offs. A thousand-line PR with a major feature is likely to get rejected if you do not discuss it first.


Running this in the workspace directory will run all tests. Please make sure all tests pass when you submit a PR. Please add a test for any new code introduced. PRs with tests are easier to review and accept.

RUST_BACKTRACE=full cargo test


Run cargo fmt over the code before submitting.


The DESIGN.md and parse/DESIGN.md documents were written while I was thinking of various things and are not meant to make sense to anyone else. This section summarizes the actual design.

This design falls out of the following goals:

Try to stick to the primitives of the Build Systems a la Carte paper

Most production build systems have been around before this paper, and most of them tend not to have strict boundaries between the various stages. I've tried to make the primitives shine through when possible. For example, the rebuilder and scheduler very explicitly use graphs, talk in terms of Keys and Tasks and try to avoid direct I/O (or thinking in terms of real-world things like unix times) as much as possible. There are of course limitations on this, since Rust doesn't have the full generic/higher-kinded types support that Haskell does. Writing excessively generic code in Rust can get old quickly if one has to start putting trait bounds or generics everywhere. There is a first stage that parses ninja files and translates these to a build description. This is then transformed to a set of Keys and Tasks as in the paper. Then, there is an implementation of the Scheduler (topological) and Rebuilder (mtime based) that implements ninja semantics.

In some sense, this is almost a compiler pipeline for the ninja language, but the final stage is an interpreter for the action graph instead of a code generator.

Try to have usable stand-alone crates

The implementation is a collection of crates. The parser can be used by itself. The lexer preserves relatively complete information about tokens (unlike ninja), which could allow things like a ninja-fmt. I originally envisioned the parser yielding per-file ASTs, but the way ninja handles scoping across include and subninja rules, makes this intractable. Specifically, variables in the included file are evaluated immediately within the current environment, so this cannot be parallelized and a per-file AST is meaningless as soon as inclusions happen.

The main ninja crate simply assembles all these pieces together. Theoretically, one could use the tasks and build crates to create another build system. There are a bunch of unclean API boundaries right now, but those are amenable to cleaning up.

Enable optimizing for performance

I started off really focusing on trying to avoid allocations and copies, but this quickly got intractable with Rust's strong ownership requirements. So I've currently gone to the other extreme, with liberal uses of clone(). The lexer is still "zero-copy", dishing out references to a single &[u8]. Canonicalization and string-interning can take care of the paths and it should be possible to go from the lexer to the task description without copying, as long as one is willing to propagate more lifetimes around. Of course, variable evaluation will always require allocation of new bytes.

It isn't clear yet whether this can achieve ninja's levels of performance while maintaining readability.

Be readable/documented

This generally falls out of sticking to the primitives, but I've also called out bits and pieces more obviously, when it isn't clear how to translate something from "how does ninja do it" to "how do I express this in the primitives from the paper". A particular instance is the rebuilder's handling of phony rules and dirtiness. Similarly, the scheduler does a very intentional depth-first topological sort, unlike ninja, where something similar is done but the graph building is coupled with the parsing and handling dynamic dependencies.

Be testable

Ninja doesn't really have a spec. It has a manual that explains behavior broadly, but the original implementation is the only source of truth. This means a lot of the canonical behavior can only be determined by running specific build files through ninja. I've tried to add tests for as much of the code base as possible. Particularly, the parser has a bunch of acceptance tests to ensure compliance with ninja and also to act as regression tests. These acceptance tests are just .ninja files, which can be quickly run with ninja to determine if they are following "the spec".


~99K SLoC