I really like _with style functions that accept a FnOnce callback. The scoping rules work out really well when using these functions. I was working with the slab crate recently and used the Slab#insert_with function. This function takes a callback where an object is supposed to be allocated before being inserted into the slab. The function returns an Option type. I was trying to figure out to drop the newly created object if the function returned None (meaning the insertion failed). After a f...
