@inverse

[Daniel N via Digitalmars-d]

Just throwing an idea out there... How about using annotations to 
teach the compiler which functions are inverses of each-other, in 
order to facilitate optimizing away certain redundant operations 
even if they are located inside a library(i.e. no source).

A little pseudo-code for illustrational purposes, in case my 
above text is incomprehensible:

void inc() pure nothrow @inverse(dec)
void dec() pure nothrow @inverse(inc)

void swap(T)(ref T lhs, ref T rhs) pure nothrow @inverse(swap!T)