Writing Tests

The following focuses on writing an integration test. However, writing unit tests is also encouraged!

Testsuite

Cargo has a wide variety of integration tests that execute the cargo binary and verify its behavior, located in the testsuite directory. The support crate and snapbox contain many helpers to make this process easy.

There are two styles of tests that can roughly be categorized as

  • functional tests
    • The fixture is programmatically defined
    • The assertions are regular string comparisons
    • Easier to share in an issue as a code block is completely self-contained
    • More resilient to insignificant changes though ui tests are easy to update when a change does occur
  • ui tests
    • The fixture is file-based
    • The assertions use file-backed snapshots that can be updated with an env variable
    • Easier to review the expected behavior of the command as more details are included
    • Easier to get up and running from an existing project
    • Easier to reason about as everything is just files in the repo

These tests typically work by creating a temporary "project" with a Cargo.toml file, executing the cargo binary process, and checking the stdout and stderr output against the expected output.

Functional Tests

Generally, a functional test will be placed in tests/testsuite/<command>.rs and will look roughly like:

#[cargo_test]
fn <description>() {
    let p = project()
        .file("src/main.rs", r#"fn main() { println!("hi!"); }"#)
        .build();

    p.cargo("run --bin foo")
        .with_stderr(
            "\
    [COMPILING] foo [..]
    [FINISHED] [..]
    [RUNNING] `target/debug/foo`
    ",
        )
        .with_stdout("hi!")
        .run();
    }
}

The #[cargo_test] attribute is used in place of #[test] to inject some setup code.

ProjectBuilder via project():

  • Each project is in a separate directory in the sandbox
  • If you do not specify a Cargo.toml manifest using file(), one is automatically created with a project name of foo using basic_manifest().

Execs via p.cargo(...):

  • This executes the command and evaluates different assertions
    • See support::compare for an explanation of the string pattern matching. Patterns are used to make it easier to match against the expected output.

#[cargo_test] attribute

The #[cargo_test] attribute injects code which does some setup before starting the test. It will create a filesystem "sandbox" under the "cargo integration test" directory for each test, such as /path/to/cargo/target/tmp/cit/t123/. The sandbox will contain a home directory that will be used instead of your normal home directory.

The #[cargo_test] attribute takes several options that will affect how the test is generated. They are listed in parentheses separated with commas, such as:

#[cargo_test(nightly, reason = "-Zfoo is unstable")]

The options it supports are:

  • nightly — This will cause the test to be ignored if not running on the nightly toolchain. This is useful for tests that use unstable options in rustc or rustdoc. These tests are run in Cargo's CI, but are disabled in rust-lang/rust's CI due to the difficulty of updating both repos simultaneously. A reason field is required to explain why it is nightly-only.
  • build_std_real — This is a "real" -Zbuild-std test (in the build_std integration test). This only runs on nightly, and only if the environment variable CARGO_RUN_BUILD_STD_TESTS is set (these tests on run on Linux).
  • build_std_mock — This is a "mock" -Zbuild-std test (which uses a mock standard library). This only runs on nightly, and is disabled for windows-gnu.
  • requires_ — This indicates a command that is required to be installed to be run. For example, requires_rustfmt means the test will only run if the executable rustfmt is installed. These tests are always run on CI. This is mainly used to avoid requiring contributors from having every dependency installed.
  • >=1.64 — This indicates that the test will only run with the given version of rustc or newer. This can be used when a new rustc feature has been stabilized that the test depends on. If this is specified, a reason is required to explain why it is being checked.

Testing Nightly Features

If you are testing a Cargo feature that only works on "nightly" Cargo, then you need to call masquerade_as_nightly_cargo on the process builder and pass the name of the feature as the reason, like this:

p.cargo("build").masquerade_as_nightly_cargo(&["print-im-a-teapot"])

If you are testing a feature that only works on nightly rustc (such as benchmarks), then you should use the nightly option of the cargo_test attribute, like this:

#[cargo_test(nightly, reason = "-Zfoo is unstable")]

This will cause the test to be ignored if not running on the nightly toolchain.

Specifying Dependencies

You should not write any tests that use the network such as contacting crates.io. Typically, simple path dependencies are the easiest way to add a dependency. Example:

let p = project()
    .file("Cargo.toml", r#"
        [package]
        name = "foo"
        version = "1.0.0"

        [dependencies]
        bar = {path = "bar"}
    "#)
    .file("src/lib.rs", "extern crate bar;")
    .file("bar/Cargo.toml", &basic_manifest("bar", "1.0.0"))
    .file("bar/src/lib.rs", "")
    .build();

If you need to test with registry dependencies, see support::registry::Package for creating packages you can depend on.

If you need to test git dependencies, see support::git to create a git dependency.

UI Tests

UI Tests are a bit more spread out and generally look like:

tests/testsuite/<command>/mod.rs:

mod <case>;

tests/testsuite/<command>/<case>/mod.rs:

use cargo_test_support::prelude::*;
use cargo_test_support::compare::assert_ui;
use cargo_test_support::Project;
use cargo_test_support::curr_dir;

#[cargo_test]
fn <name>() {
    let project = Project::from_template(curr_dir!().join("in"));
    let project_root = project.root();
    let cwd = &project_root;

    snapbox::cmd::Command::cargo_ui()
        .arg("run")
        .arg_line("--bin foo")
        .current_dir(cwd)
        .assert()
        .success()
        .stdout_matches_path(curr_dir!().join("stdout.log"))
        .stderr_matches_path(curr_dir!().join("stderr.log"));

    assert_ui().subset_matches(curr_dir!().join("out"), &project_root);
}

Then populate

  • tests/testsuite/<command>/<case>/in with the project's directory structure
  • tests/testsuite/<command>/<case>/out with the files you want verified
  • tests/testsuite/<command>/<case>/stdout.log with nothing
  • tests/testsuite/<command>/<case>/stderr.log with nothing

#[cargo_test]:

  • This is used in place of #[test]
  • This attribute injects code which does some setup before starting the test, creating a filesystem "sandbox" under the "cargo integration test" directory for each test such as /path/to/cargo/target/cit/t123/
  • The sandbox will contain a home directory that will be used instead of your normal home directory

Project:

  • The project is copied from a directory in the repo
  • Each project is in a separate directory in the sandbox

Command via Command::cargo_ui():

  • Set up and run a command.

OutputAssert via Command::assert():

  • Perform assertions on the result of the Command

Assert via assert_ui():

  • Verify the command modified the file system as expected

Updating Snapshots

The project, stdout, and stderr snapshots can be updated by running with the SNAPSHOTS=overwrite environment variable, like:

$ SNAPSHOTS=overwrite cargo test

Be sure to check the snapshots to make sure they make sense.

Testing Nightly Features

If you are testing a Cargo feature that only works on "nightly" Cargo, then you need to call masquerade_as_nightly_cargo on the process builder and pass the name of the feature as the reason, like this:

    snapbox::cmd::Command::cargo()
        .masquerade_as_nightly_cargo(&["print-im-a-teapot"])

If you are testing a feature that only works on nightly rustc (such as benchmarks), then you should use the nightly option of the cargo_test attribute, like this:

#[cargo_test(nightly, reason = "-Zfoo is unstable")]

This will cause the test to be ignored if not running on the nightly toolchain.

Platform-specific Notes

When checking output, use / for paths even on Windows: the actual output of \ on Windows will be replaced with /.

Be careful when executing binaries on Windows. You should not rename, delete, or overwrite a binary immediately after running it. Under some conditions Windows will fail with errors like "directory not empty" or "failed to remove" or "access is denied".

Debugging tests

In some cases, you may need to dig into a test that is not working as you expect, or you just generally want to experiment within the sandbox environment. The general process is:

  1. Build the sandbox for the test you want to investigate. For example:

    cargo test --test testsuite -- features2::inactivate_targets.

  2. In another terminal, head into the sandbox directory to inspect the files and run cargo directly.

    1. The sandbox directories start with t0 for the first test.

      cd target/tmp/cit/t0

    2. Set up the environment so that the sandbox configuration takes effect:

      export CARGO_HOME=$(pwd)/home/.cargo

    3. Most tests create a foo project, so head into that:

      cd foo

  3. Run whatever cargo command you want. See Running Cargo for more details on running the correct cargo process. Some examples:

    • /path/to/my/cargo/target/debug/cargo check
    • Using a debugger like lldb or gdb:
      1. lldb /path/to/my/cargo/target/debug/cargo
      2. Set a breakpoint, for example: b generate_targets
      3. Run with arguments: r check