tryexpand

Similar to trybuild, but allows you to test how declarative or procedural macros are expanded.

Documentation

Please refer to the documentation on docs.rs.

Requirements

tryexpand requires cargo-expand to be installed.

Usage

Installation

Add tryexpand to your project as a dev-dependency by running

cargo install --dev tryexpand

Writing tests

Then under your crate's tests/ directory, create tests.rs file containing calls to tryexpand::expand() and populate the tests/expand/pass/, tests/expand/checked_pass/ and tests/expand/fail/ directories with corresponding Rust source files under test.

Pass

The base of each tryexpand test suite is the tryexpand::expand() function, which you pass a list of file paths (or glob patterns) to:

#[test]
pub fn pass() {
    tryexpand::expand(
        ["tests/expand/pass/*.rs"]
    ).expect_pass();

    // or its short-hand (by default `.expect_pass()` is implied):

    tryexpand::expand(
        ["tests/expand/pass/*.rs"]
    );
}

By default tryexpand::expand() assert matched test files to expand successfully.

Fail

If instead you want to write tests for macro expansion diagnostics, then will have to add a call to .expect_fail():

#[test]
pub fn fail() {
    tryexpand::expand(
        ["tests/expand/fail/*.rs"]
    ).expect_fail();
}

CLI arguments

Additionally you can specify arguments to pass to cargo expand:

#[test]
tryexpand::expand(
    // ...
)
// ...
.args(["--features", "test-feature"])
.expect_pass();

CLI env vars

as well as environment variables to set for cargo expand:

tryexpand::expand(
    // ...
)
// ...
.envs([("MY_ENV", "my env var value")])
.expect_pass();

cargo check

You can also make tryexpand type-check the expanded code for you (i.e. cargo check):

tryexpand::expand(
    // ...
)
// ...
.and_check()
.expect_pass();

cargo run

Or you can make tryexpand run the expanded code for you (i.e. cargo run):

tryexpand::expand(
    // ...
)
// ...
.and_run()
.expect_pass();

cargo test

Or you can make tryexpand run the expanded code's included unit tests (if there are any) for you (i.e. cargo test):

tryexpand::expand(
    // ...
)
// ...
.and_run_tests()
.expect_pass();

Running tests

The test can be run with:

cargo test

While it is possible to run parallel tests it is recommended to run them serially:

cargo test -- --test-threads=1

For debugging purposes you may want to see the output for all tests, not just the failing ones:

cargo test -- --no-capture

Each tryexpand test will invoke the cargo expand command (as well as any of the optional follow-up commands: cargo check, cargo run, cargo test) on each of the source files that matches the glob pattern and will compare the expansion result with the corresponding *.out.rs, *.out.txt or *.err.txt snapshot files.

If the environment variable TRYEXPAND=overwrite is provided (e.g. $ TRYEXPAND=overwrite cargo test), then snapshot files will be created, or overwritten, if one already exists. Snapshot files should get checked into version control.

Hand-writing snapshot files is not recommended.

Performance considerations

When working with multiple expansion test files, it is recommended to specify wildcard (*.rs) instead of doing a multiple calls to the expand functions for individual files.

Usage of wildcards for multiple files will group them under a single temporary crate for which dependencies will be built a single time. In contrast, calling expand functions for each source file will create multiple temporary crates and that will reduce performance as dependencies will be build for each of the temporary crates.

More info on how glob patterns work.

See tests/macro-tests and tests/proc-macro-tests as a reference.

Reliability considerations

Since each rustc/cargo release might make changes to the emitted diagnostics it is recommended to run tryexpand tests using a pinned toolchain, e.g.:

cargo +1.76.0 test <OPTIONS>

Debugging

For each expand()-like method call within your tests a temporary and uniquely named Rust project will get generated within $CARGO_TARGET_DIR/target/tests/. By default these projects will get deleted upon test completion (regardless of the outcome). If you wish to take a look at the actual code/projects being expanded you can provide TRYEXPAND_KEEP_ARTIFACTS=1 (e.g. $ TRYEXPAND_KEEP_ARTIFACTS=1 cargo test) and tryexpand will skip the cleanup.

Contributing

Please read CONTRIBUTING.md for details on our code of conduct,
and the process for submitting pull requests to us.

Versioning

We use SemVer for versioning. For the versions available, see the tags on this repository.

License

This project is licensed under the MIT or Apache-2.0 – see the LICENSE-MIT.md/LICENSE-APACHE.md files for details.

Provenance

The tryexpand crate originated as a fork of eupn's macrotest (crates.io).