As always the Nim Manual is a treasure trove. I just discovered this [move
optimization](https://nim-lang.org/docs/manual.html#ast-based-overloading-move-optimization)
With the {call} constraint you can have 2 procs, one if there are multiple
references and one if the compiler knows the reference is unique !
proc `[]=`*(t: var Table, key: string, val: string) =
## puts a (key, value)-pair into `t`. The semantics of string require
## a copy here:
let idx = findInsertionPosition(key)
t[idx].key = key
t[idx].val = val
proc `[]=`*(t: var Table, key: string{call}, val: string{call}) =
## puts a (key, value)-pair into `t`. Optimized version that knows that
## the strings are unique and thus don't need to be copied:
let idx = findInsertionPosition(key)
shallowCopy t[idx].key, key
shallowCopy t[idx].val, val
var t: Table
# overloading resolution ensures that the optimized []= is called here:
t[f()] = g()
