Provides TestResult
type that can be used in tests to avoid
unwrap
s but at the same time to have precise stacktraces with the
point of failure clearly written.
Consider the following code. It uses unwrap
so the test failure
stacktrace will informative. Unfortunately it's not as concise as it
could be:
#[test]
fn it_works() {
// ...
std::fs::File::open("this-file-does-not-exist").unwrap();
// ...
}
Improved version of this code uses Result
and the ?
operator:
#[test]
fn it_works() -> Result<(), Box<dyn std::error::Error>> {
// ...
std::fs::File::open("this-file-does-not-exist")?;
// ...
Ok(())
}
Running the following code with RUST_BACKTRACE=1 cargo test
shows
the following stacktrace:
---- tests::it_works stdout ----
thread 'tests::it_works' panicked at 'assertion failed: `(left == right)`
left: `1`,
...
4: test::assert_test_result
at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/test/src/lib.rs:184:5
5: testresult::tests::it_works::{{closure}}
at ./src/lib.rs:52:5
6: core::ops::function::FnOnce::call_once
at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/ops/function.rs:248:5
...
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
Unfortunately even though the test function location is recorded, the exact line where the test failure occurred is not present in the backtrace.
Let's adjust the test result type to use TestResult
. This is the
only change compared to previous example:
#[test]
fn it_works() -> TestResult {
// ...
std::fs::File::open("this-file-does-not-exist")?;
// ...
Ok(())
}
Running it again with RUST_BACKTRACE=1 cargo test
shows more details:
---- tests::it_works stdout ----
thread 'tests::it_works' panicked at 'Error: No such file or directory (os error 2)', src/lib.rs:10:9
...
3: <core::result::Result<T,F> as core::ops::try_trait::FromResidual<core::result::Result<core::convert::Infallible,E>>>::from_residual
at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/result.rs:2125:27
4: testresult::tests::it_works
at ./src/lib.rs:53:9 // <-----
5: testresult::tests::it_works::{{closure}}
at ./src/lib.rs:52:5
6: core::ops::function::FnOnce::call_once
at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/ops/function.rs:248:5
...
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
The advantages of using TestResult
:
- exact failure line is present in the backtrace,
- the underlying error message is present in the test failure,
- the signature of the test result is simpler.