Provides TestResult type that can be used in tests to avoid
unwraps 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:
```rust
fn it_works() { // ... std::fs::File::open("this-file-does-not-exist").unwrap(); // ... } ```
Improved version of this code uses Result and the ? operator:
```rust
fn it_works() -> Result<(), Box
Running the following code with RUST_BACKTRACE=1 cargo test shows
the following stacktrace:
text
---- 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:
```rust
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:
text
---- tests::it_works stdout ----
thread 'tests::it_works' panicked at 'error: std::io::error::Error - No such file or directory (os error 2)', src/lib.rs:53:9
... ^^^^^^^^^^^^^^^
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.
Note that the error location is now in the backtrace and also in the test failure message. This means that we don't even need the backtrace to know where the error happened.
The advantages of using TestResult:
- exact failure line is present in the test failure and the backtrace,
- the underlying error type and message are present in the test failure,
- the signature of the test result is simpler.