stable clippy.toml: Create RFC

[blyxyas]

This PR creates an RFC for how clippy.toml should be handled. I made some improvements to some parts of the system. These are better to get fixed today than to wait until someone has a problem with them and then have to make a breaking change.

Based on Philipp's RFC. This would be the first stage of 3 to completely stabilize clippy.toml

Being that Philipp is on vacation, I'm not sure who could review this, it's a team-wide thing anyways.

changelog:none

Rendered

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[blyxyas]

@ojeda Is this more or less what you need?

The RFC stabilizes the existence of the Clippy configuration file, the algorithm to find it, the existence of CLIPPY_CONF_DIR and its behaviour, the existence of a new CLIPPY_CONF_PATH (which will be the preferred way to pass configuration into Clippy, but CLIPPY_CONF_DIR will not be removed anytime soon), and the format of the configuration itself as a subset of TOML.

The only thing needed apart from this would be stabilizing the configuration options themselves. Which, because of your usecase, the first ones will be msrv, check-private-items, disallowed-macros and doc-valid-idents.

[ojeda]

Thanks for working on this!

The only thing needed apart from this would be stabilizing the configuration options themselves. Which, because of your usecase, the first ones will be msrv, check-private-items, disallowed-macros and doc-valid-idents.

That would be great.

It may be good to mention in the RFC what would be the "stable mode" -- IIUC it would fail on unknown keys (like Clippy today), but it would not allow to use unstable keys (unlike Clippy today). Is that correct?

If so, then does that mean that unless all those options (msrv etc.) are stabilized before the default behavior changes, we would need to have 2 files to be able to make it work with old and new Clippy?

There isn't a good way to specify configuration values from the terminal, for the time being.

If Clippy plans to support that eventually, then an alternative would be to not stabilize the file and go for flags directly -- for us it would make it easier to pass flags to Clippy depending e.g. on the version.

But, of course, that would open again the questions about how to pass all what the TOML allows for non-trivial keys. IIRC we discussed it in Zulip a while ago.

[lucab]

Suggested change

	You can enable unstable features via the `unsable-conf` configuration option. If this configuration is set to `true`,
	You can enable unstable features via the `unstable-conf` configuration option. If this configuration is set to `true`,

[lucab]

Suggested change

	can use `unstable-conf = true` sparingly in their development cycle to check for unknown Rust to check for typos or
	can use `unstable-conf = true` sparingly in their development cycle to check for typos or