[Discussion] Super borrow, for super let, placement new, and hybrid borrow

[cg909]

Your first example looks similar to my proposal "Downgradable mutable borrows" from December 2023:

Pre-RFC: Downgradable mutable borrows

Some months ago I've read the following comment by steffahn and I think a generalized &mut T could be viable. So here's my very first Pre-RFC based on that idea: Summary Generalize &mut with a second, optional lifetime to allow opt-in downgradeable borrows. Motivation Currently coercing or reborrowing a &mut as an immutable reference always extends the unique borrow to the lifetime of the resulting reference. When interior mutability is involved, this behavior is neccessary for correctness …