Add `crystal: previous` option

[Sija]

I'd like to suggest adding previous (or previous-minor) as the version option (following latest), allowing automatically selecting previous MINOR version for testing. That would help to catch any new additions in terms of BC.

[oprypin]

What's "BC"?

[Sija]

Backward Compatibility

[straight-shoota]

For backwards compatibility, wouldn't it be better to specify explicit versions that you want to ensure compatibility with? previous would just be a moving target, so it's not exactly clear which versions are considered to be backward compatible.

[Sija]

@straight-shoota That's exactly my point, the case at-hand is testing features that are working with Crystal latest onwards and checking whether the build still succeeds with the previous one, like in this PR: crystal-ameba/ameba#536

[straight-shoota]

But how is a floating target latest better than pinning a specific version?

[Sija]

It's not about better, but a specific use-case.

[oprypin]

I don't understand the use case. Why test with the 2nd newest version? What if you want to develop something for the 2nd newest version of Crystal but the 3rd newest doesn't support it? Instead test with your oldest supported version and that will cover everything.