Give a better error when rustdoc tests fail
- Run the default rustdoc against the current rustdoc
- Diff output recursively
- Colorize diff output
Closes https://github.com/rust-lang/rust/issues/78750.
## Resolved questions
- Should this be opt-in instead of on by default?
+ No
- Should this call through to `delta`? That's not a very common program to have installed, but I'm not sure how to do diffs after the fact. Maybe `compiletest` can take a `--syntax-highlighter` parameter or something?
+ I decided to use `delta` if available and `diff --color` otherwise. It prints a warning if delta isn't installed so you know you can get nicer diffs
## Open questions.
- What version of rustdoc would this compare against? Ideally it would compare against `$(git merge-base HEAD origin/master)` - maybe that's feasible if we install those artifacts from CI?
- Does it always make sense to compare the tests? Especially for new tests, I'm not sure how useful it would be ... but then again, one of the questions I want to know most as a reviewer is 'did it break before?'.
r? `@GuillaumeGomez`
cc `@Mark-Simulacrum`
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