Include stack trace in failed test info

[danielbom]

I opened an issue on the Gleam repository about a potential inconsistency in how let assert failures are reported (issue 4703). The behavior differs between gleam run and gleam test.

Here’s an example of the output from gleam test:

test/phone_number_test.gleam
   test: cleans_the_number_test
  error: Pattern match failed
   site: phone_number:9
  value: ["2", "2", "3", "4", "5", "6", "7", "8", "9", "0"]

And from gleam run:

runtime error: let assert

Pattern match failed, no pattern matched the value.

unmatched value:
  ["0", "0", "0", "0", "0", "0", "0", "0", "0", "0"]

stacktrace:
  phone_number.-clean/1-fun-0- src\phone_number.gleam:9

As I noted in the issue:

“It seems like gleam run is much more explicit—it clearly signals that the developer made a wrong assertion, with a red-colored title and a full stack trace (e.g., src\phone_number.gleam:9). On the other hand, gleam test reports the failure more subtly, almost as if everything ran normally.”

Wouldn’t it be a good idea for gleam test to adopt a stricter and more visible approach to let assert failures—similar to gleam run—since these are developer errors that benefit from being highlighted clearly?

[lpil]

Adding the stack trace sounds good!

The behavior differs between gleam run and gleam test.

There's no expectation for these two distinct projects to have the same output.