Process for config option stabilisation

[SimonSapin]

The trailing_semicolon configuration for example is currently unstable. What needs to happen in order for it to be stabilized?

(I would like to use it in a project that is otherwise compatible with the Stable channel.)

[nrc]

I don't plan on stabilising any more options before 1.0 (for the edition). After that I'd like to slowly stabilise options as we go along and think about a process for that. The main motivation will be the trade-off between implementation complexity and trying to minimise the overwhelming-ness of a huge number of options vs the utility of the option. I would also like to think about an overall strategy - do we want many small options or a smaller number of sweeping ones, or high level styles only.

[SimonSapin]

To be clear, I don’t think there is much to discuss about this particular option. This is mostly about what is the process in general.

[SimonSapin]

Hello @rust-lang/dev-tools team. Now that the 2018 edition and rustfmt 1.0 have shipped, what are your thoughts on the desired process for configuration option stabilization?

[scampi]

Just writing down some ideas for stabilising an option, comments welcome!

Conditions

• the option is well tested, both in unit tests and, optimally, in real usage
• there is no open bug about the option that prevents its use

Steps for the stabilisation

Open a PR that closes the tracking issue:

• update the Config enum marking the option as stable
• update the the Configuration.md file marking the option as stable

Also do these actions:

• update the unstable options wiki page: https://github.com/rust-lang/rustfmt/wiki/Stability-of-Config-Options

After the stabilisation

Backward compatibility

The option should remain compatible with previous parameters of the option. For instance, if the option is an enum enum Foo { Alice, Bob } and the variant Foo::Bob is removed/renamed, existing use of the Foo::Bob variant should map to the new logic.
Maybe this is covered by the current version-gate scheme but I wanted to bring this aspect forward.