Tracking Issue for RFC 3762, "Make trait methods callable in const contexts"

This is a tracking issue for the RFC "3762" (rust-lang/rfcs#3762).
The feature gate for the issue is #![feature(const_trait_impl)].

[!NOTE]
Questions about the usage of this feature should be directed to the t-compiler/project-const-traits Zulip channel. Use t-lang/effects and the RFC (until it gets accepted) to discuss the language feature and future extensions.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Components

• #[const_trait] attribute Removed for good
• impl const Trait
• T: [const] Trait
• append_const_msg on rustc_on_unimplemented removed in https://github.com/rust-lang/rust/pull/154653
• #[derive_const]
• trait Destruct

Steps

• Implement the RFC
• Add const trait Trait and migrate away from #[const_trait] #143879
• Split derive_const, -> impl [const] Trait into separate feature[s] to make initial stabilization minimal
• add a perf stress test for const trait impls and complex bounds that need solving
• Adjust documentation (see instructions on rustc-dev-guide)
• Style updates for any new syntax (nightly-style-procedure)
  • Style team decision on new formatting
  • Formatting for new syntax has been added to the Style Guide
  • (non-blocking) Formatting has been implemented in rustfmt
• Stabilization PR (see instructions on rustc-dev-guide)
  • Error against putting rustc_const_unstable attribute on const impls as they are now insta-stable.
  • Treat const trait default method bodies and const impl bodies as stable const fn bodies. We need to prevent accidentally stabilizing an implementation that uses unstable lang/lib const fn features.
  • Change Rustdoc to display [const] bounds or what syntax we decided it to be.

Unresolved Questions

• #[derive_const] for custom derives (proc macros) #118304

Implementation history

• See #67792 for previous tracking issue and its history
• Incomplete list of PRs implementing/refining the feature: https://github.com/rust-lang/rust/pulls?q=is%3Apr+label%3AF-const_trait_impl+is%3Aclosed+
• https://github.com/rust-lang/rust/pull/148641

Start with RFC 3762, the const_trait_impl feature gate, and the unchecked components in the tracking checklist. Review the implementation history and linked closed pull requests before choosing a separately scoped task. Done means completing one defined checklist item, including any required documentation or style updates, and recording the progress here.