Edition 2021 and beyond

[nikomatsakis]

This RFC proposes revised rules and plans for editions:

• Announce plans for a Rust 2021 Edition, and for a regular cadence of editions every 3 years thereafter.
  • We will roll out an edition regardless of whether there are breaking changes.
• Unlike Rust 2018, we will avoid using editions as a "deadline" to tie together high-priority projects.
  • Instead, we embrace the train model, but editions are effectively a "somewhat bigger release", giving us an opportunity to give an overview of all the work that has landed over the previous three years.
• We specify a cadence for Edition lints.
  • "Edition idiom" lints for Edition N will warn for editions before N, and become "deny by default" in Edition N.
  • Since it would be disruptive to introduce deny-by-default lints for Rust 2018 now, the Rust 2018 lints are repurposed into Rust 2021 Edition lints.
• We specify a policy on reserving keywords and other prospective changes.
  • In short, reserving keywords is allowed only as part of an active project group.

Co-authored with @steveklabnik.

Rendered view

Pending edits

• Clarify the lint policy (context).
• Clarify that changing edition is not considered a "major semver" change except insofar as it implies that a newer version of rustc is needed. (context)