rfcs/compiler_changes.md

RFC policy - the compiler

We have not previously had an RFC system for compiler changes, so policy here is likely to change as we get the hang of things. We don't want to slow down most compiler development, but on the other hand we do want to do more design work ahead of time on large additions and refactorings.

Compiler RFCs will be managed by the compiler sub-team, and tagged T-compiler. The compiler sub-team will do an initial triage of new PRs within a week of submission. The result of triage will either be that the PR is assigned to a member of the sub-team for shepherding, the PR is closed because the sub-team believe it should be done without an RFC, or closed because the sub-team feel it should clearly not be done and further discussion is not necessary. We'll follow the standard procedure for shepherding, final comment period, etc.

Where there is significant design work for the implementation of a language feature, the preferred workflow is to submit two RFCs - one for the language design and one for the implementation design. The implementation RFC may be submitted later if there is scope for large changes to the language RFC.

Changes which need an RFC

• New lints (these fall under the lang team)
• Large refactorings or redesigns of the compiler
• Changing the API presented to syntax extensions or other compiler plugins in non-trivial ways
• Adding, removing, or changing a stable compiler flag
• The implementation of new language features where there is significant change or addition to the compiler. There is obviously some room for interpretation about what consitutes a "significant" change and how much detail the implementation RFC needs. For guidance, associated items and UFCS would clearly need an implementation RFC, type ascription and lifetime elision would not.
• Any other change which causes backwards incompatible changes to stable behaviour of the compiler, language, or libraries

Changes which don't need an RFC

• Bug fixes, improved error messages, etc.
• Minor refactoring/tidying up
• Implmenting language features which have an accepted RFC, where the implementation does not significantly change the compiler or require significant new design work
• Adding unstable API for tools (note that all compiler API is currently unstable)
• Adding, removing, or changing an unstable compiler flag (if the compiler flag is widely used there should be at least some discussion on discuss, or an RFC in some cases)

If in doubt it is probably best to just announce the change you want to make to the compiler subteam on discuss or IRC, and see if anyone feels it needs an RFC.