The Manifest Format - Cargo Documentation

The `[package]` Section

The first section in a Cargo.toml is [package].

[package]
name = "hello_world" # the name of the package
version = "0.1.0"    # the current version, obeying semver
authors = ["you@example.com"]

All three of these fields are mandatory. Cargo bakes in the concept of Semantic Versioning, so make sure you follow some basic rules:

Before you reach 1.0.0, anything goes.
After 1.0.0, only make breaking changes when you increment the major version. In Rust, breaking changes include adding fields to structs or variants to enums. Don't break the build.
After 1.0.0, don't add any new public API (no new pub anything) in tiny versions. Always increment the minor version if you add any new pub structs, traits, fields, types, functions, methods or anything else.
Use version numbers with three numeric parts such as 1.0.0 rather than 1.0.

For more on versions, see this documentation.

The `build` Field (optional)

This field specifies a file in the repository which is a build script for building native code. More information can be found in the build script guide.

[package]
# ...
build = "build.rs"

The `exclude` and `include` Fields (optional)

You can explicitly specify to Cargo that a set of globs should be ignored or included for the purposes of packaging and rebuilding a package. The globs specified in the exclude field identify a set of files that are not included when a package is published as well as ignored for the purposes of detecting when to rebuild a package, and the globs in include specify files that are explicitly included.

If a VCS is being used for a package, the exclude field will be seeded with the VCS's ignore settings (.gitignore for git for example).

[package]
# ...
exclude = ["build/**/*.o", "doc/**/*.html"]

[package]
# ...
include = ["src/**/*", "Cargo.toml"]

The options are mutually exclusive: setting include will override an exclude. Note that include must be an exhaustive list of files as otherwise necessary source files may not be included.

Package metadata

There are a number of optional metadata fields also accepted under the [package] section:

[package]
# ...

# A short blurb about the package. This is not rendered in any format when
# uploaded to crates.io (aka this is not markdown)
description = "..."

# These URLs point to more information about the repository
documentation = "..."
homepage = "..."
repository = "..."

# This points to a file in the repository (relative to this Cargo.toml). The
# contents of this file are stored and indexed in the registry.
readme = "..."

# This is a small list of keywords used to categorize and search for this
# package.
keywords = ["...", "..."]

# This is a string description of the license for this package. Currently
# crates.io will validate the license provided against a whitelist of known
# license identifiers from http://spdx.org/licenses/. Multiple licenses can
# be separated with a `/`
license = "..."

# If a project is using a nonstandard license, then this key may be specified in
# lieu of the above key and must point to a file relative to this manifest
# (similar to the readme key)
license-file = "..."

The crates.io registry will render the description, display the license, link to the three URLs and categorize by the keywords. These keys provide useful information to users of the registry and also influence the search ranking of a crate. It is highly discouraged to omit everything in a published crate.

The `[dependencies]` Section

You list dependencies using keys inside of the [dependencies] section. For example, if you wanted to depend on hammer, color, and geometry:

[package]
# ...

[dependencies]
hammer = { version = "0.5.0", git = "https://github.com/wycats/hammer.rs" }
color = { git = "https://github.com/bjz/color-rs" }
geometry = { path = "crates/geometry" }

You can specify the source of a dependency in a few ways:

git = "<git-url>": A git repository with a Cargo.toml inside it (not necessarily at the root). The rev, tag, and branch options are also recognized to use something other than the master branch.
path = "<relative-path>": A path relative to the current Cargo.toml pointing to another directory with a Cargo.toml and an associated package.
If path and git are omitted, then a dependencies will come from crates.io and use the version key to indicate the version requirement.

Dependencies from crates.io can also use a shorthand where just the version requirement is specified:

[dependencies]
hammer = "0.5.0"
color = "> 0.6.0, < 0.8.0"

The syntax of the requirement strings is described in the crates.io guide.

Platform-specific dependencies take the same format, but are listed under the target.$triple section:

[target.x86_64-pc-windows-gnu.dependencies]
winhttp = "0.4.0"

[target.i686-unknown-linux-gnu.dependencies]
openssl = "1.0.1"
native = { path = "native/i686" }

[target.x86_64-unknown-linux-gnu.dependencies]
openssl = "1.0.1"
native = { path = "native/x86_64" }

If you're using a custom target specification, quote the full path and file name:

[target."x86_64/windows.json".dependencies]
winhttp = "0.4.0"

[target."i686/linux.json".dependencies]
openssl = "1.0.1"
native = { path = "native/i686" }

[target."x86_64/linux.json".dependencies]
openssl = "1.0.1"
native = { path = "native/x86_64" }

The `[profile.*]` Sections

Cargo supports custom configuration of how rustc is invoked through profiles at the top level. Any manifest may declare a profile, but only the top level project's profiles are actually read. All dependencies' profiles will be overridden. This is done so the top-level project has control over how its dependencies are compiled.

There are five currently supported profile names, all of which have the same configuration available to them. Listed below is the configuration available, along with the defaults for each profile.

# The development profile, used for `cargo build`
[profile.dev]
opt-level = 0  # Controls the --opt-level the compiler builds with
debug = true   # Controls whether the compiler passes `-g`
rpath = false  # Controls whether the compiler passes `-C rpath`
lto = false    # Controls `-C lto` for binaries and staticlibs
debug-assertions = true  # Controls whether debug assertions are enabled
codegen-units = 1 # Controls whether the compiler passes `-C codegen-units`
                  # `codegen-units` is ignored when `lto = true`

# The release profile, used for `cargo build --release`
[profile.release]
opt-level = 3
debug = false
rpath = false
lto = false
debug-assertions = false
codegen-units = 1

# The testing profile, used for `cargo test`
[profile.test]
opt-level = 0
debug = true
rpath = false
lto = false
debug-assertions = true
codegen-units = 1

# The benchmarking profile, used for `cargo bench`
[profile.bench]
opt-level = 3
debug = false
rpath = false
lto = false
debug-assertions = false
codegen-units = 1

# The documentation profile, used for `cargo doc`
[profile.doc]
opt-level = 0
debug = true
rpath = false
lto = false
debug-assertions = true
codegen-units = 1

The `[features]` Section

Cargo supports features to allow expression of:

Conditional compilation options (usable through cfg attributes);
Optional dependencies, which enhance a package, but are not required;
Clusters of optional dependencies, such as "postgres", that would include the postgres package, the postgres-macros package, and possibly other packages (such as development-time mocking libraries, debugging tools, etc.)

A feature of a package is either an optional dependency, or a set of other features. The format for specifying features is:

[package]
name = "awesome"

[features]
# The "default" set of optional packages. Most people will
# want to use these packages, but they are strictly optional.
# Note that `session` is not a package but rather another
# feature listed in this manifest.
default = ["jquery", "uglifier", "session"]

# A feature with no dependencies is used mainly for conditional
# compilation, like `#[cfg(feature = "go-faster")]`.
go-faster = []

# The "secure-password" feature depends on the bcrypt package.
# This aliasing will allow people to talk about the feature in
# a higher-level way and allow this package to add more
# requirements to the feature in the future.
secure-password = ["bcrypt"]

# Features can be used to reexport features of other packages.
# The `session` feature of package `awesome` will ensure that the
# `session` feature of the package `cookie` is also enabled.
session = ["cookie/session"]

[dependencies]
# These packages are mandatory and form the core of this
# package's distribution
cookie = "1.2.0"
oauth = "1.1.0"
route-recognizer = "=2.1.0"

# A list of all of the optional dependencies, some of which
# are included in the above "features". They can be opted
# into by apps.
jquery = { version = "1.0.2", optional = true }
uglifier = { version = "1.5.3", optional = true }
bcrypt = { version = "*", optional = true }
civet = { version = "*", optional = true }

To use the package awesome:

[dependencies]
awesome = { version = "1.3.5", features = ["secure-password", "civet"] }

# do not include the default features, and optionally
# cherry-pick individual features
default-features = false

Rules

The usage of features is subject to a few rules:

Feature names must not conflict with other package names in the manifest. This is because they are opted into via features = [...], which only has a single namespace
With the exception of the default feature, all features are opt-in. To opt out of the default feature, use default-features = false and cherry-pick individual features.
Feature groups are not allowed to cyclicly depend on one another.
Dev-dependencies cannot be optional
Features groups can only reference optional dependencies
When a feature is selected, Cargo will call rustc with --cfg feature="${feature_name}". If a feature group is included, it and all of its individual features will be included. This can be tested in code via #[cfg(feature = "foo")]

Note that it is explicitly allowed for features to not actually activate any optional dependencies. This allows packages to internally enable/disable features without requiring a new dependency.

Usage In End Products

One major use-case for this feature is specifying optional features in end-products. For example, the Servo project may want to include optional features that people can enable or disable when they build it.

In that case, Servo will describe features in its Cargo.toml and they can be enabled using command-line flags:

$ cargo build --release --features "shumway pdf"

Default features could be excluded using --no-default-features.

Usage In Packages

In most cases, the concept of "optional dependency" in a library is best expressed as a separate package that the top-level application depends on.

However, high-level packages, like Iron or Piston, may want the ability to curate a number of packages for easy installation. The current Cargo system allows them to curate a number of mandatory dependencies into a single package for easy installation.

In some cases, packages may want to provide additional curation for optional dependencies:

Grouping a number of low-level optional dependencies together into a single high-level "feature".
Specifying packages that are recommended (or suggested) to be included by users of the package.
Including a feature (like secure-password in the motivating example) that will only work if an optional dependency is available, and would be difficult to implement as a separate package. For example, it may be overly difficult to design an IO package to be completely decoupled from OpenSSL, with opt-in via the inclusion of a separate package.

In almost all cases, it is an antipattern to use these features outside of high-level packages that are designed for curation. If a feature is optional, it can almost certainly be expressed as a separate package.

The `[dev-dependencies]` Section

The format of this section is equivalent to [dependencies]. Dev-dependencies are not used when compiling a package for building, but are used for compiling tests and benchmarks.

These dependencies are not propagated to other packages which depend on this package.

The Project Layout

If your project is an executable, name the main source file src/main.rs. If it is a library, name the main source file src/lib.rs.

Cargo will also treat any files located in src/bin/*.rs as executables.

Your project can optionally contain folders named examples, tests, and benches, which Cargo will treat as containing example executable files, integration tests, and benchmarks respectively.

▾ src/          # directory containing source files
  lib.rs        # the main entry point for libraries and packages
  main.rs       # the main entry point for projects producing executables
  ▾ bin/        # (optional) directory containing additional executables
    *.rs
▾ examples/     # (optional) examples
  *.rs
▾ tests/        # (optional) integration tests
  *.rs
▾ benches/      # (optional) benchmarks
  *.rs

Examples

Files located under examples are example uses of the functionality provided by the library. When compiled, they are placed in the target/examples directory.

They must compile as executables (with a main() function) and load in the library by using extern crate <library-name>. They are compiled when you run your tests to protect them from bitrotting.

Tests

When you run cargo test, Cargo will:

Compile your library's unit tests, which are in files reachable from lib.rs. Any sections marked with #[cfg(test)] will be included.
Compile your library’s documentation tests, which are embedded inside of documentation blocks.
Compile your library's integration tests, which are located in tests. Files in tests load in your library by using extern crate <library-name> like any other code that depends on it.
Compile your library's examples.

Configuring a target

All of the [[bin]], [lib], [[bench]], and [[test]] sections support similar configuration for specifying how a target should be built. The example below uses [lib], but it also applies to all other sections as well. All values listed are the defaults for that option unless otherwise specified.

[package]
# ...

[lib]
# The name of a target is the name of the library that will be generated. This
# is defaulted to the name of the package or project.
name = "foo"

# This field points at where the crate is located, relative to the Cargo.toml.
path = "src/lib.rs"

# A flag for enabling unit tests for this target. This is used by `cargo test`.
test = true

# A flag for enabling documentation tests for this target. This is only
# relevant for libraries, it has no effect on other sections. This is used by
# `cargo test`.
doctest = true

# A flag for enabling benchmarks for this target. This is used by `cargo bench`.
bench = true

# A flag for enabling documentation of this target. This is used by `cargo doc`.
doc = true

# If the target is meant to be a compiler plugin, this field must be set to true
# for cargo to correctly compile it and make it available for all dependencies.
plugin = false

# If set to false, `cargo test` will omit the --test flag to rustc, which stops
# it from generating a test harness. This is useful when the binary being built
# manages the test runner itself.
harness = true

Building Dynamic or Static Libraries

If your project produces a library, you can specify which kind of library to build by explicitly listing the library in your Cargo.toml:

# ...

[lib]
name = "..."
# this could be "staticlib" as well
crate-type = ["dylib"]

The available options are dylib, rlib, and staticlib. You should only use this option in a project. Cargo will always compile packages (dependencies) based on the requirements of the project that includes them.

CARGO

The Manifest Format - Cargo Documentation