Update guide on compiler RFCs

Lots of this is very outdated, mentioning IRC (:D) and not mentioning
MCPs, which are very significant. I adjusted it to something that I
feel like is roughly true today. I don't know whether it's fully
accurate, but it's definitely more accurate than before.

compiler_changes.md

@@ -1,10 +1,5 @@
 # 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
@@ -13,41 +8,36 @@ 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.
-
+Most compiler decisions that go beyond the scope of a simple PR are done using [MCP]s,
+not RFCs. It is therefore likely that you should file an MCP instead of an RFC for your problem.
 
 ## 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 constitutes a "significant" change and how much detail the
-  implementation RFC needs. For guidance, [associated items](text/0195-associated-items.md)
-  and [UFCS](text/0132-ufcs.md) would clearly need an implementation RFC,
-  [type ascription](text/0803-type-ascription.md) and
-  [lifetime elision](text/0141-lifetime-elision.md) would not.
-* Any other change which causes backwards incompatible changes to stable
+* Significant user-facing changes to the compiler with a complex design space,
+  especially if they involve other teams as well (for example, [path sanitization]).
+* Any other change which causes significant 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
-* Implementing language features which have an accepted RFC, where the
-  implementation does not significantly change the compiler or require
-  significant new design work
+* Large refactorings or redesigns of the compiler (needs an [MCP])
+* Implementing language features which have an accepted RFC.
+* New lints (these fall under the lang team). Lints are best first tried out in clippy
+  and then uplifted later.
+* Changing the API presented to syntax extensions or other compiler plugins in
+  non-trivial ways
+* Adding, removing, or changing a stable compiler flag (needs an [MCP])
 * 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.
+the compiler subteam on [Zulip], and see if anyone feels it needs an RFC.
+
+[MCP]: https://github.com/rust-lang/compiler-team/issues
+[path sanitization]: https://github.com/rust-lang/rfcs/pull/3127
+[Zulip]: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler
+