src/doc/faq.md

% 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.org 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 project 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][1] and Ruby’s [bundler][2] 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.

[1]: https://www.npmjs.org
[2]: https://bundler.io

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)?

Yes!

Cargo handles compiling Rust code, but we know that many Rust projects
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](build-script.html)
(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 `make` (or `ninja`, or ...)

Indeed. While we intend Cargo to be useful as a standalone way to
compile Rust projects 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 projects or cross-compilation?

Rust itself provides facilities for configuring sections of code based
on the platform. Cargo also supports [platform-specific
dependencies][target-deps], and we plan to support more per-platform
configuration in `Cargo.toml` in the future.

[target-deps]: manifest.html#the-dependencies-section

In the longer-term, we’re looking at ways to conveniently cross-compile
projects using Cargo.

# Does Cargo support environments, like `production` or `test`?

We support environments through the use of [profiles][profile] to support:

[profile]: manifest.html#the-profile-sections

* environment-specific flags (like `-g --opt-level=0` for development
  and `--opt-level=3` for production).
* environment-specific dependencies (like `hamcrest` for test assertions).
* environment-specific `#[cfg]`
* a `cargo test` command

# Does Cargo work on Windows?

Yes!

All commits to Cargo are required to pass the local test suite on Windows.
If, however, you find a Windows issue, we consider it a bug, so [please file an
issue][3].

[3]: https://github.com/rust-lang/cargo/issues

# Why do binaries have `Cargo.lock` in version control, but not libraries?

The purpose of a `Cargo.lock` is to describe the state of the world at the time
of a successful build. It is then used to provide deterministic builds across
whatever machine is building the project by ensuring that the exact same
dependencies are being compiled.

This property is most desirable from applications and projects which are at the
very end of the dependency chain (binaries). As a result, it is recommended that
all binaries check in their `Cargo.lock`.

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 all libraries were to check in their `Cargo.lock`, then
multiple copies of the library would be used, and perhaps even a version
conflict.

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
of `*` 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.”

# Why `Cargo.toml`?

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
capital-`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 `Makefile` and
`Cargo.toml` are placed together. The trailing `.toml` was chosen to emphasize
the fact that the file is in the [TOML configuration
format](https://github.com/toml-lang/toml).

Cargo does not allow other names such as `cargo.toml` or `Cargofile` to
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.

[crates.io]: https://crates.io/

# 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 comes 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 `Cargo.lock` existing
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.

For more information about vendoring, see documentation on [source
replacement][replace].

[replace]: source-replacement.html