Rollup merge of #108255 - fee1-dead-contrib:fix-old-fixme, r=cjgillot

Remove old FIXMEs referring to #19596

Having an inner function that accepts a mutable reference seems to be the only way this can be expressed. Taking a mutable reference would call the same function with a new type &mut F which then causes the infinite recursion error in #19596.

Author: matthiaskrgr