Having reproducible tests helps debugging problems that have probabilistic nature. This library provides
a random numbers generator DevRng compatible with rand crate (it implements Rng, RngCore,
SeedableRng traits). When generator is constructed, its seed is printed to stdout. You can override a
seed by setting RUST_TESTS_SEED env variable. Same seed leads to same randomness generated across all
platforms.
Reproducible source of randomness can be added in one line:
use rand::Rng;
use rand_dev::DevRng;
#[test]
fn it_works() {
let mut rng = DevRng::new();
assert!(rng.gen_range(0..=10) < 10);
}Then if test fails, you can observe seed of randomness generator in stdout:
$ cargo test
Finished test [unoptimized + debuginfo] target(s) in 0.00s
Running unittests (target/debug/deps/simple_usage-592d47155d40d1f7)
running 1 test
test tests::it_works ... FAILED
failures:
---- tests::it_works stdout ----
RUST_TESTS_SEED=cab4ab5c8471fa03691bb86d96c2febeb9b1099a78d164e8addbe7f83d107c78
thread 'tests::it_works' panicked at 'assertion failed: rng.gen_range(0..=10) < 10', src/lib.rs:9:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
failures:
tests::it_works
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
Now you can fix the seed by setting env variable to reproduce and debug a failing test:
$ export RUST_TESTS_SEED=cab4ab5c8471fa03691bb86d96c2febeb9b1099a78d164e8addbe7f83d107c78
$ cargo test
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