Improve `assert_eq` failure message formatting to increase legibility (original) (raw)

Feature Name: prettier-assert
Start Date: 2017-01-22
RFC PR:
Rust Issue:

Summary

Improve assert_eq failure message formatting to increase legibility.

Motivation

Currently when assert_eq fails the default panic text has all the information on one long line, which is difficult to parse. This is more difficult when working with larger data structures. I'd like to alter the format of this text in order improve legibility, putting each piece of information on a different line.

Detailed design

Here is an failing test with the current format:

---- log_packet::tests::syntax_error_test stdout ----
        thread 'log_packet::tests::syntax_error_test' panicked at 'assertion failed: `(left == right)` (left: `"Syntax Error: a.rb:1: syntax error, unexpected end-of-input\n\n"`, right: `"Syntax error: a.rb:1: syntax error, unexpected end-of-input\n\n"`)', src/log_packet.rs:102
note: Run with `RUST_BACKTRACE=1` for a backtrace.

Here is a failing test with an alternate format:

---- log_packet::tests::syntax_error_test stdout ----
        thread 'log_packet::tests::syntax_error_test' panicked at 'assertion failed: `(left == right)`

left:  `"Syntax Error: a.rb:1: syntax error, unexpected end-of-input\n\n"`
right: `"Syntax error: a.rb:1: syntax error, unexpected end-of-input\n\n"`

', src/log_packet.rs:102
note: Run with `RUST_BACKTRACE=1` for a backtrace.

In addition to putting each expression on a separate line I've also padding the word "left" with an extra space. This makes the values line up and easier to visually diff.

This could be further improved with coloured diff'ing. i.e. If two strings are between a certain levenshtein distance colour additional chars green and missing ones red.

Here is a screenshot of the output of the Elixir lang ExUnit test assertion macro, which I think is extremely clear:

Drawbacks

Unresolved questions

This could be implemented as an alternative assert macro in a library. Is there more value in doing this? Is there value in keeping the stdlib assert_eq as it is?

Have I opened the RFC correctly? I'm unsure. ;P

Etc

Thank you.

+ I'm happy to work on implementing this feature, though I am a newbie Rustacean :)