Cargo Targets

Cargo packages consist of targets which correspond to source files which can be compiled into a crate. Packages can have library, binary, example, test, and benchmark targets. The list of targets can be configured in the Cargo.toml manifest, often inferred automatically by the directory layout of the source files.

See Configuring a target below for details on configuring the settings for a target.

Library

The library target defines a "library" that can be used and linked by other libraries and executables. The filename defaults to src/lib.rs, and the name of the library defaults to the name of the package. A package can have only one library. The settings for the library can be customized in the [lib] table in Cargo.toml.

# Example of customizing the library in Cargo.toml.
[lib]
crate-type = ["cdylib"]
bench = false

Binaries

Binary targets are executables programs that can be run after being compiled. The default binary filename is src/main.rs, which defaults to the name of the package. Additional binaries are stored in the src/bin/ directory. The settings for each binary can be customized in the [[bin]] tables in Cargo.toml.

Binaries can use the public API of the package's library. They are also linked with the [dependencies] defined in Cargo.toml.

You can run individual binaries with the cargo run command with the --bin <bin-name> option. cargo install can be used to copy the executable to a common location.

# Example of customizing binaries in Cargo.toml.
[[bin]]
name = "cool-tool"
test = false
bench = false

[[bin]]
name = "frobnicator"
required-features = ["frobnicate"]

Examples

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

Examples can use the public API of the package's library. They are also linked with the [dependencies] and [dev-dependencies] defined in Cargo.toml.

By default, examples are executable binaries (with a main() function). You can specify the crate-type field to make an example be compiled as a library:

[[example]]
name = "foo"
crate-type = ["staticlib"]

You can run individual executable examples with the cargo run command with the --example <example-name> option. Library examples can be built with cargo build with the --example <example-name> option. cargo install with the --example <example-name> option can be used to copy executable binaries to a common location. Examples are compiled by cargo test by default to protect them from bit-rotting. Set the test field to true if you have #[test] functions in the example that you want to run with cargo test.

Tests

There are two styles of tests within a Cargo project:

  • Unit tests which are functions marked with the #[test] attribute located within your library or binaries (or any target enabled with the test field). These tests have access to private APIs located within the target they are defined in.
  • Integration tests which is a separate executable binary, also containing #[test] functions, which is linked with the project's library and has access to its public API.

Tests are run with the cargo test command. By default, Cargo and rustc use the libtest harness which is responsible for collecting functions annotated with the #[test] attribute and executing them in parallel, reporting the success and failure of each test. See the harness field if you want to use a different harness or test strategy.

Integration tests

Files located under the tests directory are integration tests. When you run cargo test, Cargo will compile each of these files as a separate crate, and execute them.

Integration tests can use the public API of the package's library. They are also linked with the [dependencies] and [dev-dependencies] defined in Cargo.toml.

If you want to share code among multiple integration tests, you can place it in a separate module such as tests/common/mod.rs and then put mod common; in each test to import it.

Each integration test results in a separate executable binary, and cargo test will run them serially. In some cases this can be inefficient, as it can take longer to compile, and may not make full use of multiple CPUs when running the tests. If you have a lot of integration tests, you may want to consider creating a single integration test, and split the tests into multiple modules. The libtest harness will automatically find all of the #[test] annotated functions and run them in parallel. You can pass module names to cargo test to only run the tests within that module.

Benchmarks

Benchmarks provide a way to test the performance of your code using the cargo bench command. They follow the same structure as tests, with each benchmark function annotated with the #[bench] attribute. Similarly to tests:

  • Benchmarks are placed in the benches directory.
  • Benchmark functions defined in libraries and binaries have access to the private API within the target they are defined in. Benchmarks in the benches directory may use the public API.
  • The bench field can be used to define which targets are benchmarked by default.
  • The harness field can be used to disable the built-in harness.

Note: The #[bench] attribute is currently unstable and only available on the nightly channel. There are some packages available on crates.io that may help with running benchmarks on the stable channel, such as Criterion.

Configuring a target

All of the [lib], [[bin]], [[example]], [[test]], and [[bench]] sections in Cargo.toml support similar configuration for specifying how a target should be built. The double-bracket sections like [[bin]] are array-of-table of TOML, which means you can write more than one [[bin]] section to make several executables in your crate. You can only specify one library, so [lib] is a normal TOML table.

The following is an overview of the TOML settings for each target, with each field described in detail below.

[lib]
name = "foo"           # The name of the target.
path = "src/lib.rs"    # The source file of the target.
test = true            # Is tested by default.
doctest = true         # Documentation examples are tested by default.
bench = true           # Is benchmarked by default.
doc = true             # Is documented by default.
plugin = false         # Used as a compiler plugin (deprecated).
proc-macro = false     # Set to `true` for a proc-macro library.
harness = true         # Use libtest harness.
edition = "2015"       # The edition of the target.
crate-type = ["lib"]   # The crate types to generate.
required-features = [] # Features required to build this target (N/A for lib).

The name field

The name field specifies the name of the target, which corresponds to the filename of the artifact that will be generated. For a library, this is the crate name that dependencies will use to reference it.

For the [lib] and the default binary (src/main.rs), this defaults to the name of the package, with any dashes replaced with underscores. For other auto discovered targets, it defaults to the directory or file name.

This is required for all targets except [lib].

The path field

The path field specifies where the source for the crate is located, relative to the Cargo.toml file.

If not specified, the inferred path is used based on the target name.

The test field

The test field indicates whether or not the target is tested by default by cargo test. The default is true for lib, bins, and tests.

Note: Examples are built by cargo test by default to ensure they continue to compile, but they are not tested by default. Setting test = true for an example will also build it as a test and run any #[test] functions defined in the example.

The doctest field

The doctest field indicates whether or not documentation examples are tested by default by cargo test. This is only relevant for libraries, it has no effect on other sections. The default is true for the library.

The bench field

The bench field indicates whether or not the target is benchmarked by default by cargo bench. The default is true for lib, bins, and benchmarks.

The doc field

The doc field indicates whether or not the target is included in the documentation generated by cargo doc by default. The default is true for libraries and binaries.

Note: The binary will be skipped if its name is the same as the lib target.

The plugin field

This field is used for rustc plugins, which are being deprecated.

The proc-macro field

The proc-macro field indicates that the library is a procedural macro (reference). This is only valid for the [lib] target.

The harness field

The harness field indicates that the --test flag will be passed to rustc which will automatically include the libtest library which is the driver for collecting and running tests marked with the #[test] attribute or benchmarks with the #[bench] attribute. The default is true for all targets.

If set to false, then you are responsible for defining a main() function to run tests and benchmarks.

Tests have the cfg(test) conditional expression enabled whether or not the harness is enabled.

The edition field

The edition field defines the Rust edition the target will use. If not specified, it defaults to the edition field for the [package]. This field should usually not be set, and is only intended for advanced scenarios such as incrementally transitioning a large package to a new edition.

The crate-type field

The crate-type field defines the crate types that will be generated by the target. It is an array of strings, allowing you to specify multiple crate types for a single target. This can only be specified for libraries and examples. Binaries, tests, and benchmarks are always the "bin" crate type. The defaults are:

TargetCrate Type
Normal library"lib"
Proc-macro library"proc-macro"
Example"bin"

The available options are bin, lib, rlib, dylib, cdylib, staticlib, and proc-macro. You can read more about the different crate types in the Rust Reference Manual.

The required-features field

The required-features field specifies which features the target needs in order to be built. If any of the required features are not enabled, the target will be skipped. This is only relevant for the [[bin]], [[bench]], [[test]], and [[example]] sections, it has no effect on [lib].

[features]
# ...
postgres = []
sqlite = []
tools = []

[[bin]]
name = "my-pg-tool"
required-features = ["postgres", "tools"]

Target auto-discovery

By default, Cargo automatically determines the targets to build based on the layout of the files on the filesystem. The target configuration tables, such as [lib], [[bin]], [[test]], [[bench]], or [[example]], can be used to add additional targets that don't follow the standard directory layout.

The automatic target discovery can be disabled so that only manually configured targets will be built. Setting the keys autobins, autoexamples, autotests, or autobenches to false in the [package] section will disable auto-discovery of the corresponding target type.

[package]
# ...
autobins = false
autoexamples = false
autotests = false
autobenches = false

Disabling automatic discovery should only be needed for specialized situations. For example, if you have a library where you want a module named bin, this would present a problem because Cargo would usually attempt to compile anything in the bin directory as an executable. Here is a sample layout of this scenario:

├── Cargo.toml
└── src
    ├── lib.rs
    └── bin
        └── mod.rs

To prevent Cargo from inferring src/bin/mod.rs as an executable, set autobins = false in Cargo.toml to disable auto-discovery:

[package]
# …
autobins = false

Note: For packages with the 2015 edition, the default for auto-discovery is false if at least one target is manually defined in Cargo.toml. Beginning with the 2018 edition, the default is always true.