Rustc-dev-guide: Document new mir-opt test format.

[vext01]

Hi,

It seems that the format of the mir-opt tests have recently changed. It'd be nice if this page could document the new format:
https://rustc-dev-guide.rust-lang.org/tests/adding.html

In the meantime, could someone confirm that mir-opt tests can no longer use elisions (...)?

I maintain a fork of rustc and I have a load of tests which rely heavily on elision. It wouldn't be possible to do exact comparisons as mir-opt now (seems to?) require, so I need to figure out what to do with them :

Thanks

[mark-i-m]

cc @rust-lang/wg-mir-opt They would know best.

[oli-obk]

I wrote docs in https://github.com/rust-lang/rust/blob/master/src/test/mir-opt/README.md Please let me know if you need more details

I maintain a fork of rustc and I have a load of tests which rely heavily on elision. It wouldn't be possible to do exact comparisons as mir-opt now (seems to?) require, so I need to figure out what to do with them :

The thing is that with the new tests, you just pass --bless and the comparison files are automatically updated. Or is there another reason but it being very hard to adjust such files by hand that you want to have elisions?

[oli-obk]

If you need other kind of diffs, I'd be happy to implement them, as we need a few others ourselves. E.g. what we want is to diff from the start of one optimization to the end of another, instead of just diffs of a single optimization.

[mark-i-m]

@oli-obk Thanks! Would it make sense to add some of those docs to the guide and point to that readme?

[spastorino]

Maybe even better if we can add self contained docs in the guide :).

[oli-obk]

I think we should only have these logs in one place. I'd be ok with moving them to the guide and making the src/tests/mir-opt/README.md just contain a link to the (rendered) guide.

[spastorino]

I'm not sure what's best but I remember we had a bunch of READMEs in the repo and this was more or less how rustc-dev-guide started, by extracting those into the guide. So maybe as @oli-obk has said, linking to the guide and in general migrating everything from READMEs to the guide and providing links from the READMEs to guide in general would be the best choice. Thoughts?.