The feature gate for the issue is #![feature(relaxed_struct_unsize)].

This features changes the when a struct implements Unsize which
is required for unsize coercion.

A generic struct previously implemented Unsize if and only if:

• the tail field depends on at least one type or const parameter
• all type and const parameters used in the last field must only be used in the last field
• the target struct can be created from the source by replacing only the parameters found in the last struct field
• the tail field implements Unsize from source to target

With this feature gate the requirements are changed to:

• the tail field depends on at least one type or const parameter not used in any other field
• the target struct can be created from the source by replacing only the parameters only found in the last struct field
• the tail field implements Unsize from source to target

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

• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Implementation history

This feature has been implemented in #80726