Prepare const traits for stabilization

[nikomatsakis]

Metadata
Point of contact	@oli-obk
Team(s)	compiler, lang, types
Goal document	2025h1/const-trait

Summary

Prepare const Trait bounds for stabilization.

Tasks and status

• Lang-team experiment (lang )
• Experimental implementation (@fee1-dead, @compiler-errors)
• Author RFC (@oli-obk)
• Adjust implementation to match RFC ()
• Call for testing (@oli-obk)
• Standard reviews (compiler )
• Design meeting (lang )
• Lang-team champion (lang )
• RFC decision (lang )
• RFC secondary review (types )
• Author specification 1st draft (@oli-obk)

Formalize const-traits in a-mir-formality

• Discussion and moral support (types )
• Implementation (@tiif)
• Mentoring and kibbitzing (@nikomatsakis, @oli-obk)

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[fee1-dead]

Update 2024-07-27:

• Key developments: We recently landed my new proposed desugaring in #120639, which is a big step towards having a robust model of const traits in the type system.

[fee1-dead]

Update 2024-08-25:

• Key developments: A handful of cleanups (#126552 #126737 #127452 #128424 #129500), opened a project group repo for tracking design considerations. A proposed solution to making implied bounds with super traits work (project-const-traits#2) is open: #129499

[fee1-dead]

Update 2024-10-02:

• Key developments: With the PR for supporting implied super trait bounds landed (#129499), the current implementation is mostly complete in that it allows most code that should compile, and should reject all code that shouldn't. Further testing is required, with our next steps being improving diagnostics (#131152), and fixing more holes before we'd be comfortable with adding const traits back to core.

[fee1-dead]

Update 2024-11-20:

#131985 by @compiler-errors completely restructured the way const traits are desugared, which made the design easier to understand and more robust against our current unit tests. Since then, there had been significant development and cleanup for the feature, with #131968 #132118 #132168 #132169 #132227 #132275 #132344 #132366 #132368 #132479 #132823 among the ones that got merged, and #133216 #133218 that are still open.

With the recent work, we are very close to being able to dogfooding the feature on the standard library again, and the current implementation is the closest we have towards something that can be stabilized.

[fee1-dead]

Update 2024-12-30: (sorry for the delay!)

We began adding a small set of const traits and implementation in the compiler, while carefully looking for improvements in our implementation. We constified Add in rust-lang/rust#133237 and Deref/DerefMut in rust-lang/rust#133260.

A lot of the credits for implementing this feature goes to @compiler-errors, who authored these PRs (let me know if I have missed some): rust-lang/rust#133926 rust-lang/rust#134637 rust-lang/rust#134638

There was more progress on implementing stability for the const traits feature: rust-lang/rust#132823 (didn't include in the previous update, sorry), and rust-lang/rust#133999.

A couple of PRs constifying more traits are also open: rust-lang/rust#133995 rust-lang/rust#134628

[nikomatsakis]

This is a continuing project goal, and the updates below this comment will be for the new period 2025h1

[oli-obk]

I opened an RFC (rust-lang/rfcs#3762) and we had a lang team meeting about it. Some design exploration and bikeshedding later we have settled on using (const)instead of ~const along with some more annotations for explicitness and some fewer annotations in other places. The RFC has been updated accordingly. There is still ongoing discussions about reintroducing the "fewer annotations" for redundancy and easier processing by humans.