Tracking Issue for `#[diagnostic::opaque]`

There is no RFC as adding new diagnostic attributes doesn't require one as per RFC-3368
The feature gate for the issue is #![feature(diagnostic_opaque)].

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.

Steps

• 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)

Unresolved Questions

• list all the "unresolved questions" found in the RFC to ensure they are
  not forgotten.

Implementation history

Initial implementation: https://github.com/rust-lang/rust/pull/158608

Start with the initial implementation in rust-lang/rust#158608 and review the tracking checklist for diagnostic_opaque. The remaining work is represented by the documentation, nightly style, and stabilization items; completion means resolving those checklist entries and addressing any unresolved questions before stabilization.