Tracking Issue for unsized const parameter types: `feature(unsized_const_params)`

This is a tracking issue for supporting unsized types in const generics.
The feature gate for the issue is #![feature(unsized_const_params)].

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

Most of the work here will be coordinated in the #project-const-generics stream on zulip.

• Experiment with supporting { *"hi" } as a const argument to const N: str
• Resolve #120961 somehow (e.g. by forbidding references if the previous step works out)
• Author an RFC for the feature
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide) merges the ConstParamTy and UnsizedConstParamTy traits

Unresolved Questions

• Can we forbid references and support !Sized types some other way

Implementation history

• Feature was split out of adt_const_params in #127722

cc @rust-lang/project-const-generics

Start with the checklist in issue #128028 and the unresolved question about references and !Sized types. Coordinate in the #project-const-generics Zulip stream, then review the linked rustc-dev-guide instructions for RFC, documentation, and stabilization work. Done means the relevant design and implementation milestones are completed and the stabilization PR merges the two traits.