constify const Fn*: Destruct
makes closures const destruct where their upvars are. i think this makes sense
and is how this should be implemented.
r? @oli-obk
The output of Array::map is intended to be an array of the same size, and does not modify the
original in place nor is it intended for side-effects. Thus, under normal circumstances it should be consumed.
See [discussion](https://internals.rust-lang.org/t/array-map-annotate-with-must-use/22813/26).
Attaching to tracking issue #75243
Jonathan Brouwer