RFCs have to describe migrations

Author: nikomatsakis

text/0000-edition-2021.md

@@ -102,6 +102,14 @@ Migrations typically come with one or more **migration lints**. Each lint warns
 
 The default edition for new projects will be 2021.
 
+## RFCs that propose edition changes
+
+RFCs that propose migrations should include details about how the migration between editions will work. This migration should be described in sufficient detail that the relevant teams can assess the feasibility of the migration. It will often make sense to consult the compiler team on this question specificially. 
+
+* To perform this evaluation, an RFC proposing a migration should enumerate the situations that will no longer compile with the change. 
+* For each such situation, it should describe how the code will be modified such that it works on both the old and new edition.
+    * Alternatively, if that scenario is deemed unlikely, the RFC can state that this sort of code will not be automatically ported between editions. It should then describe if it is possible to at least issue a warning that the code will no longer work or will change meaning in the new edition.
+
 ## Tooling workflow
 
 The expected workflow for upgrading to a new edition is as follows: