Frequently Asked Questions
Is the plan to use GitHub as a package repository?
No. The plan for Cargo is to use crates.io, like npm or Rubygems do with npmjs.com and rubygems.org.
We plan to support git repositories as a source of packages forever, because they can be used for early development and temporary patches, even when people use the registry as the primary source of packages.
Why build crates.io rather than use GitHub as a registry?
We think that it’s very important to support multiple ways to download packages, including downloading from GitHub and copying packages into your package itself.
That said, we think that crates.io offers a number of important benefits, and will likely become the primary way that people download packages in Cargo.
For precedent, both Node.js’s npm and Ruby’s bundler support both a central registry model as well as a Git-based model, and most packages are downloaded through the registry in those ecosystems, with an important minority of packages making use of git-based packages.
Some of the advantages that make a central registry popular in other languages include:
- Discoverability. A central registry provides an easy place to look for existing packages. Combined with tagging, this also makes it possible for a registry to provide ecosystem-wide information, such as a list of the most popular or most-depended-on packages.
- Speed. A central registry makes it possible to easily fetch just the metadata for packages quickly and efficiently, and then to efficiently download just the published package, and not other bloat that happens to exist in the repository. This adds up to a significant improvement in the speed of dependency resolution and fetching. As dependency graphs scale up, downloading all of the git repositories bogs down fast. Also remember that not everybody has a high-speed, low-latency Internet connection.
Will Cargo work with C code (or other languages)?
Cargo handles compiling Rust code, but we know that many Rust packages link against C code. We also know that there are decades of tooling built up around compiling languages other than Rust.
Our solution: Cargo allows a package to specify a script
(written in Rust) to run before invoking
rustc. Rust is leveraged to
implement platform-specific configuration and refactor out common build
functionality among packages.
Can Cargo be used inside of
ninja, or …)
Indeed. While we intend Cargo to be useful as a standalone way to compile Rust packages at the top-level, we know that some people will want to invoke Cargo from other build tools.
We have designed Cargo to work well in those contexts, paying attention to things like error codes and machine-readable output modes. We still have some work to do on those fronts, but using Cargo in the context of conventional scripts is something we designed for from the beginning and will continue to prioritize.
Does Cargo handle multi-platform packages or cross-compilation?
Rust itself provides facilities for configuring sections of code based
on the platform. Cargo also supports platform-specific
dependencies, and we plan to support more per-platform
Cargo.toml in the future.
In the longer-term, we’re looking at ways to conveniently cross-compile packages using Cargo.
Does Cargo support environments, like
We support environments through the use of profiles to support:
- environment-specific flags (like
-g --opt-level=0for development and
- environment-specific dependencies (like
hamcrestfor test assertions).
Does Cargo work on Windows?
All commits to Cargo are required to pass the local test suite on Windows. If you encounter an issue while running on Windows, we consider it a bug, so please file an issue.
Why do binaries have
Cargo.lock in version control, but not libraries?
The purpose of a
Cargo.lock lockfile is to describe the state of the world at
the time of a successful build. Cargo uses the lockfile to provide
deterministic builds on different times and different systems, by ensuring that
the exact same dependencies and versions are used as when the
was originally generated.
This property is most desirable from applications and packages which are at the
very end of the dependency chain (binaries). As a result, it is recommended that
all binaries check in their
For libraries the situation is somewhat different. A library is not only used by
the library developers, but also any downstream consumers of the library. Users
dependent on the library will not inspect the library’s
Cargo.lock (even if it
exists). This is precisely because a library should not be deterministically
recompiled for all users of the library.
If a library ends up being used transitively by several dependencies, it’s
likely that just a single copy of the library is desired (based on semver
compatibility). If Cargo used all of the dependencies’
then multiple copies of the library could be used, and perhaps even a version
In other words, libraries specify SemVer requirements for their dependencies but cannot see the full picture. Only end products like binaries have a full picture to decide what versions of dependencies should be used.
Can libraries use
* as a version for their dependencies?
As of January 22nd, 2016, crates.io rejects all packages (not just libraries) with wildcard dependency constraints.
While libraries can, strictly speaking, they should not. A version requirement
* says “This will work with every version ever”, which is never going
to be true. Libraries should always specify the range that they do work with,
even if it’s something as general as “every 1.x.y version”.
As one of the most frequent interactions with Cargo, the question of why the
configuration file is named
Cargo.toml arises from time to time. The leading
C was chosen to ensure that the manifest was grouped with other
similar configuration files in directory listings. Sorting files often puts
capital letters before lowercase letters, ensuring files like
Cargo.toml are placed together. The trailing
.toml was chosen to emphasize
the fact that the file is in the TOML configuration
Cargo does not allow other names such as
emphasize the ease of how a Cargo repository can be identified. An option of
many possible names has historically led to confusion where one case was handled
but others were accidentally forgotten.
How can Cargo work offline?
Cargo is often used in situations with limited or no network access such as airplanes, CI environments, or embedded in large production deployments. Users are often surprised when Cargo attempts to fetch resources from the network, and hence the request for Cargo to work offline comes up frequently.
Cargo, at its heart, will not attempt to access the network unless told to do so. That is, if no crates come from crates.io, a git repository, or some other network location, Cargo will never attempt to make a network connection. As a result, if Cargo attempts to touch the network, then it’s because it needs to fetch a required resource.
Cargo is also quite aggressive about caching information to minimize the amount
of network activity. It will guarantee, for example, that if
cargo build (or
an equivalent) is run to completion then the next
cargo build is guaranteed to
not touch the network so long as
Cargo.toml has not been modified in the
meantime. This avoidance of the network boils down to a
and a populated cache of the crates reflected in the lock file. If either of
these components are missing, then they’re required for the build to succeed and
must be fetched remotely.
As of Rust 1.11.0, Cargo understands a new flag,
--frozen, which is an
assertion that it shouldn’t touch the network. When passed, Cargo will
immediately return an error if it would otherwise attempt a network request.
The error should include contextual information about why the network request is
being made in the first place to help debug as well. Note that this flag does
not change the behavior of Cargo, it simply asserts that Cargo shouldn’t touch
the network as a previous command has been run to ensure that network activity
shouldn’t be necessary.
--offline flag was added in Rust 1.36.0. This flag tells Cargo to not
access the network, and try to proceed with available cached data if possible.
You can use
cargo fetch in one project to download dependencies before
going offline, and then use those same dependencies in another project with
--offline flag (or configuration value).
For more information about vendoring, see documentation on source replacement.
Why is Cargo rebuilding my code?
Cargo is responsible for incrementally compiling crates in your project. This
means that if you type
cargo build twice the second one shouldn’t rebuild your
crates.io dependencies, for example. Nevertheless bugs arise and Cargo can
sometimes rebuild code when you’re not expecting it!
We’ve long wanted to provide better diagnostics about
this but unfortunately haven’t
been able to make progress on that issue in quite some time. In the meantime,
however, you can debug a rebuild at least a little by setting the
$ CARGO_LOG=cargo::core::compiler::fingerprint=info cargo build
This will cause Cargo to print out a lot of information about diagnostics and
rebuilding. This can often contain clues as to why your project is getting
rebuilt, although you’ll often need to connect some dots yourself since this
output isn’t super easy to read just yet. Note that the
CARGO_LOG needs to be
set for the command that rebuilds when you think it should not. Unfortunately
Cargo has no way right now of after-the-fact debugging “why was that rebuilt?”
Some issues we’ve seen historically which can cause crates to get rebuilt are:
A build script prints
foois a file that doesn’t exist and nothing generates it. In this case Cargo will keep running the build script thinking it will generate the file but nothing ever does. The fix is to avoid printing
rerun-if-changedin this scenario.
Two successive Cargo builds may differ in the set of features enabled for some dependencies. For example if the first build command builds the whole workspace and the second command builds only one crate, this may cause a dependency on crates.io to have a different set of features enabled, causing it and everything that depends on it to get rebuilt. There’s unfortunately not really a great fix for this, although if possible it’s best to have the set of features enabled on a crate constant regardless of what you’re building in your workspace.
Some filesystems exhibit unusual behavior around timestamps. Cargo primarily uses timestamps on files to govern whether rebuilding needs to happen, but if you’re using a nonstandard filesystem it may be affecting the timestamps somehow (e.g. truncating them, causing them to drift, etc). In this scenario, feel free to open an issue and we can see if we can accommodate the filesystem somehow.
A concurrent build process is either deleting artifacts or modifying files. Sometimes you might have a background process that either tries to build or check your project. These background processes might surprisingly delete some build artifacts or touch files (or maybe just by accident), which can cause rebuilds to look spurious! The best fix here would be to wrangle the background process to avoid clashing with your work.
If after trying to debug your issue, however, you’re still running into problems then feel free to open an issue!