Asserts are nearly undebuggable without a backtrace to the actual panic

[Dr-Emann]

Cucumber-rs intentionally installs a panic handler that does nothing. This makes debugging asserts very difficult if there are multiple possible assert locations, as we're given only the panic message.

e.g.

Rust code

use cucumber::{when, World};

#[derive(Debug, Default, World)]
pub struct PanicWorld;

fn panic_on_5(n: u32) {
    assert_ne!(n, 5);
}

#[when("a panic is raised")]
fn panic_fn(_world: &mut PanicWorld) {
    panic_on_5(1);
    panic_on_5(2);
    panic_on_5(3);
    panic_on_5(4);
    panic_on_5(5);
    panic_on_5(6);
    panic_on_5(7);
    panic_on_5(8);
}

fn main() {
    futures::executor::block_on(PanicWorld::run(
        "tests/spec.feature",
    ));
}

Feature File:

Feature: Panics
  Scenario: Debugging without location info is hard
    When a panic is raised
    Then the output should at least include the line number that failed

Command:

cargo test --test example -- -vvvvvvvv

Expected Behavior:

I expect to be able to get any information on where a failure occurred, ideally with a full backtrace (as an option, it would be okay to only include the panic location by default, but it should be possible to get a full backtrace)

Actual Output:

The output contains the panic message, but no information about where the original panic occurred. The actual panic backtrace simply points to the location of the call to run_and_exit (also: should the tests failing really be a panic or just a non-zero exit code and message?).

Feature: Panics
  Scenario: Debugging without location info is hard
   ✘  When a panic is raised
      Step failed:
      Defined: tests/spec.feature:3:5
      Matched: tests/example.rs:10:1
      Step panicked. Captured output: assertion `left != right` failed
        left: 5
       right: 5
      PanicWorld
[Summary]
1 feature
1 scenario (1 failed)
1 step (1 failed)
thread 'main' panicked at /Users/<user>/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cucumber-0.21.1/src/cucumber.rs:1287:13:
1 step failed
stack backtrace:
   0: rust_begin_unwind
             at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/9fc6b43126469e3858e2fe86cafb4f0fd5068869/library/core/src/panicking.rs:76:14
   2: core::panicking::panic_display
             at /Users/<user>/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/panicking.rs:269:5
   3: cucumber::cucumber::Cucumber<W,P,I,R,Wr,Cli>::filter_run_and_exit::{{closure}}::panic_cold_display
             at /Users/<user>/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/panic.rs:100:13
   4: cucumber::cucumber::Cucumber<W,P,I,R,Wr,Cli>::filter_run_and_exit::{{closure}}
             at /Users/<user>/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cucumber-0.21.1/src/cucumber.rs:1287:13
   5: cucumber::cucumber::Cucumber<W,P,I,R,Wr,Cli>::run_and_exit::{{closure}}
             at /Users/<user>/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cucumber-0.21.1/src/cucumber.rs:1195:57
   6: futures_executor::local_pool::block_on::{{closure}}
             at /Users/<user>/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-executor-0.3.31/src/local_pool.rs:316:23
   7: futures_executor::local_pool::run_executor::{{closure}}
             at /Users/<user>/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-executor-0.3.31/src/local_pool.rs:90:37
   8: std::thread::local::LocalKey<T>::try_with
             at /Users/<user>/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/thread/local.rs:283:12
   9: std::thread::local::LocalKey<T>::with
             at /Users/<user>/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/std/src/thread/local.rs:260:9
  10: futures_executor::local_pool::run_executor
             at /Users/<user>/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-executor-0.3.31/src/local_pool.rs:86:5
  11: futures_executor::local_pool::block_on
             at /Users/<user>/.cargo/registry/src/index.crates.io-6f17d22bba15001f/futures-executor-0.3.31/src/local_pool.rs:316:5
  12: example::main
             at ./tests/example.rs:23:5
  13: core::ops::function::FnOnce::call_once
             at /Users/<user>/.rustup/toolchains/stable-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5

[Dr-Emann]

As a workaround, I've figured out I can currently add a step that re-enables the default panic hook:

Feature File:

Background:
    Given panics have backtraces

Rust File:

#[given("panics have backtraces")]
fn enable_backtraces(_world: &mut PanicWorld) {
    // Re-Enable real backtraces for panics by reinstalling the default panic hook
    _ = std::panic::take_hook();
}

[tyranron]

@Dr-Emann

As a workaround, I've figured out I can currently add a step that re-enables the default panic hook

You also should be able to do this in the before_hook. This also demonstrated in the "Scenario hooks" chapter of the Book. This way you won't mess your .feature files with technical details.

---

Regarding the original issue, I'll try to look into it after the 0.22.0 release, whether it could be improved.

Thank you for reporting this and the suggestion!