Tracking Issue for RFC 3107: derive_default_enum

[pnkfelix]

This is a tracking issue for the RFC 3107: "#[derive(Default)] on enums with a #[default] attribute" (rust-lang/rfcs#3107).
The feature gate for the issue is #![feature(derive_default_enum)].

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.

Steps

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring
  instructions?)
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (done by Stabilize derive_default_enum #94457) (see instructions on rustc-dev-guide)

Unresolved Questions

None so far

Implementation history

[ibraheemdev]

Implementation PR was already created: #86735

[jhpratt]

Implementation has landed.

[jhpratt]

Actually, this is a duplicate of #86985. As the tracking issue in code links to this, I'll close that even though it was opened first.

Unresolved question for the record:

• Should multiple #[default] attributes on the same variant be denied?

[janriemer]

Unresolved question for the record:

* Should multiple `#[default]` attributes on the same variant be denied?

@jhpratt @pnkfelix I've seen this in the Unresolved Questions section of the duplicate #86985, but it doesn't show up in this issue (only after three comments when @jhpratt points it out). I think, we should put it here in the Unresolved Questions section as well, so people see it at first glance and don't have to scroll three comments to see it (and therefore don't miss it that easily).
Thank you for considering this.

[jhpratt]

With #88681 being merged, I'm inclined to resolve the question of multiple #[default] attributes in favor of prohibiting them. This is the current behavior. There's not really a reason to have it warn instead of outright error in my opinion, and it could conflict if #[default = val] is permitted in the future.

[fbstj]

https://github.com/rust-lang/rust/blob/master/src/test/ui/feature-gates/feature-gate-derive_default_enum.stderr still points at #86735. this file was the most "useful" result when I searched the feature gate on the repository/