Make using stable or nightly configurable

[danreeves]

Feature Request

Summary

Add a flag which toggles between generating stable or nightly config.

Motivation

Currently crossgen only supports generating config to test on nightly. If someone want's to test on stable instead they have to manually change all the places which say nightly.

Expected Behavior

crossgen: current default is to generate nightly config
crossgen --stable: create config for stable

There's currently only a handful of places where a switch would be needed. In .travis.yml and in https://github.com/yoshuawuyts/crossgen/blob/master/templates/lib/script.sh#L22

Drawbacks

Not sure how it might be implemented: some sort of templating or just bash conditionals?

Rationale and alternatives

The only other alternative is manual changes after running crossgen

Unresolved Questions

Should stable or nightly be the default. I'd consider changing this a breaking change.

[yoshuawuyts]

I'd argue we could probably default to stable, and perhaps add an extra check for beta? nightly on CI has traditionally seemed rather volatile, and was only a requirement because until recently Clippy wouldn't work on any other channel. But now that it does I reckon we can probably switch?

[danreeves]

Sounds good to me! If we include beta does it make sense to just use a --channel flag which defaults to stable?

crossgen --channel beta
crossgen --channel nightly
# vs
crossgen --beta
crossgen --nightly

[yoshuawuyts]

@danreeves yeah, I think so! It'd also be cool if we could write them as shorthands too:

$ crossgen -c stable

---

@danreeves it's probably also worth considering how much of channels we're testing. E.g. for a default configuration we'd probably want something like:

• test on {linux,windows,macOS,BSD} on stable
• test linux on beta

There's probably a balance to be found in how many tests we run.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[danreeves]

pls no @Stale bot 🙏