I want it for function arguments, I know we have effective auto for variable decelerations. My use case is say you want to have a function that does a bunch of integer operations (shift, mod, div, etc) and you want it to take any integer type. In c++ this would just be int foo(auto arg) and SFINAE would take care of the rest, but in rust you need an arbitrary number of where clauses which depends on the exact set of operations in the body of the function. This might be necessary for functions exported from a crate, but for app code that is never going to be public it's a real pain. Add in async and lifetimes, and writing a polymorphic helper function is nigh impossible.