Rust Edition 2021

This document will be turned into tracking issues and an RFC for more structure at some point. For now, we'll try to collect everything here. Please also add ideas that are not very concrete yet, just so we don't miss anything.

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Discussion: #edition on Zulip

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Milestone planning

December 2020:

• Start this document.
• Ask teams to check this document and add things.
• Preliminary roadmap with rough dates.
• Add unstable rustc --edition=2021: #79576

January 2021:

• Make sure all relevant teams have brought attention to the 2021 edition in one of their meetings.
  • compiler
  • lang
  • libs
  • rustdoc
  • cargo
• Add initial support for edition 2021 to all tools.
  (Just to make them aware of the new edition. Any edition-specific changes will be added later as part of that edition change itself.)
  • Add edition = "2021" to cargo: cargo#8922
  • 🧹 Add edition 2021 to rustfmt: rustfmt#4618
  • Add edition 2021 to rls: rls#1709
  • Add edition 2021 to racer: racer#1152
  • Add edition 2021 to clippy: rust-clippy#6530
  • Add edition 2021 to rust-analyzer: rust-analyzer#7123
  • Add edition2021 to rustdoc: No change required.
• Make sure the relevant teams discuss each of the proposed changes.
  • compiler
  • lang
  • libs
  • rustdoc
  • cargo

February 2021:

• Final list of changes. All proposed changes have:
  • An approved RFC
  • Migration+idiom lint plan
  • Partial implementation or plan for implementation

March 2021:

April 2021:

May 2021:

• All proposed changes implemented and available in nightly.

June 2021:

• All lints implemented on nightly and tested on crater.

July 2021:

• Announce start of public testing.

August 2021:

• Finish marketing materials, docs, etc.
  • Blog post(s)
  • Edition guide

September 2021:

• Stabilize --edition=2021 on nightly (1.56).

October 2021:

• --edition=2021 available on stable in Rust 1.56.

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Edition Changes

Unify std::panic and core::panic

Team(s)	Status	RFC	Tracking issue	Who
libs, lang	RFC accepted	RFC 3007	#80162	Mara

This makes panic!() consistent with print!():
panic!("{}") becomes an error, and panic!("{x}") will format x once implicit formatting arguments are stabilized.

To do:

• Implement things. See tracking issue.

Promote 2018 warn lints to deny

Team(s)	Status	RFC	Tracking issue	Who
compiler	-	-	#80165	Ryan Levick

To do:

• Decide on final list of promotions
• Implementation

Forbid bare_trait_objects

Team(s)	Status	RFC	Tracking issue	Who
lang	Not yet decided	None (yet?)	lang-team#65	t-lang

• Context: this Zulip topic.

To do:

• Make decision
• Implementation

Change Cargo's feature resolver to resolver = "2" by default for edition 2021

Team(s)	Status	RFC	Tracking issue	Who
cargo	Accepted	-	cargo#9048	?

Edition specific or-patterns

Team(s)	Status	RFC	Tracking issue	Who
lang	Team consensus	RFC 2535	#54883	mark-i-m

• Consensus from t-lang: comment on #54883
• Implementation: #80100
• Stabilization FCP: #79278

Disjoint captures in closures

Team(s)	Status	RFC	Tracking issue	Who
lang	?	RFC 2229	#53488	?

• Migrations needed for changes in drop order and around auto traits
• Update: https://hackmd.io/wPs-tHJZR1Cj9j_vwsxwbw?view
• Implentation Status: Github Project Board

Fix cargo fix --edition for edition 2021

Team(s)	Status	RFC	Tracking issue	Who
cargo	Implemented	N/A	cargo#9047	?

Prelude changes

Team(s)	Status	RFC	Tracking issue	Who
libs, lang	Incomplete idea	RFC 3090	#65512	t-libs?

• Add TryFrom / TryInto
• Add other things?
• Needs a compatibility warning (See Zulip)
• Prioritization of glob import method resolution the same as item resolution (language change)
• Doesn't need to be edition-specific:
  • Add std::iter? (#80216)
• Might not need to be edition-specific:
  (but have potential breakage from trait method conflicts, so would probably need at least a crater run)
  • Add ops::Not? (thread on Internals)
  • Add Debug and Display (#80360 and Zulip discussion)
  • Add Hash (#80361 and Zulip discussion)

To do:

• Complete the list of changes (see tracking issue)
• Implement glob and prelude import changes in compiler? Compatibility warning? (RFC/MCP first?)
• Implement new prelude

Removing #[macro_export]

Team(s)	Status	RFC	Tracking issue	Who
lang	Not yet decided	-	None yet	petrochenkov?

To do:

• Support pub on macro_rules!: #78166
• Remove #[macro_export]
• Could also be just a deprecation warning.

IntoIterator for [T; N]

Team(s)	Status	RFC	Tracking issue	Who
libs	Incomplete idea	None (yet?)	None yet	Mara

• Context: this comment and a few more after that one.

Replace #[rustc_args_required_const(..)] with const generics

Team(s)	Status	RFC	Tracking issue	Who
libs, lang	Incomplete idea	None (yet?)	None yet	Amanieu

• Context: this comment, and also the rest of that issue.
• Could also be a warning on all editions. (So, not deny on 2021.)

Reserve new keywords

Team(s)	Status	RFC	Tracking issue	Who
lang	Incomplete idea	None	None	t-lang?

To do:

• List keywords to reserve
• Implement it

New/fixed range types (make a..b Copy)

Team(s)	Status	RFC	Tracking issue	Who
libs, lang	Incomplete idea	None	None	t-libs? sfackler?

To do:

• Investigate alternative solutions, see rfcs#2848
• Decide on the best solution
• Implement it

Change rustdoc lints to have a tool prefix and make changing the names deny-by-default

Team(s)	Status	RFC	Tracking issue	Who
rustdoc	waiting on FCP (see Zulip thread)	-	-	Joshua Nelson

To do:

• Add rustdoc:: lints and suggest changing builtin rustdoc lints to use rustdoc:: prefix (#80527)
• Add a deny-by-default lint to rename the old lints
• Cohesive lints suggests all tools should be namespacing lints, is this true?

Does this actually need to be deny by default? Warn-by-default seems good enough; it wouldn't allow simplifying code since the compiler has to support all editions.

Run doctests in a single binary?

Team(s)	Status	RFC	Tracking issue	Who
rustdoc	Incomplete idea	-	#75341	-

It's unclear whether we should do this. Preferably we could find a smaller hammer to throw at this that still gives performance gains.

Separate compile-time from run-time working directory for doctests?

Team(s)	Status	RFC	Tracking issue	Who
rustdoc	Incomplete idea	-	#8993	-

Similar to doctests in a single binary, I would prefer to find a smaller hammer that doesn't require breaking changes.

Change Rustdoc's URL structure?

Team(s)	Status	RFC	Tracking issue	Who
rustdoc	Incomplete idea	rfcs#2988	#25879, #80504, docs.rs#1055, #55160, #71912	Joshua Nelson

This change might be too big even for an edition. It also needs better planning of what the end result is - if we're changing URLs anyway, it probably makes sense to solve the case-insensitive files issue at the same time.

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Notes

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Links

• Project Board: Edition 2021
• RFC 2052: Editions
• RFC 2966: Edition 2021 and Beyond (Closed)