Design meeting: Redesigning `super let`

[dianne]

Summary

The super let feature (rust-lang/rust#139076) is currently in design limbo. There's discussion of how to move forward with it on the tracking issue and on Zulip, but there seemingly isn't a clear consensus on how exactly it should look. As such, I think it could be helpful to have a design meeting about it as a next step, to try and pin down what we want from the feature and what the most promising direction is to get there. To aid in this, I've put together a design exploration document; this further develops the ideas I've seen and evaluates them against design principles based on the need to express macros like pin! and format_args!.

cc @m-ou-se @nikomatsakis @tmandry @yoshuawuyts (one of the designs I investigate is adjacent to placing functions)

Background reading

• The design exploration document would be the most important. It links to other sources, as well.
• The discussions on the tracking issue and on Zulip are the most up-to-date discussions I could find on super let's design.

About this issue

This issue corresponds to a lang-team design meeting proposal. It corresponds to a possible topic of discussion that may be scheduled for deeper discussion during one of our design meetings.