Cannot transmute type parameters

[tbu-]

It would be nice if there would be some way to std::mem::transmute in functions, even with type parameters.

Maybe the prototype of the transmute function could be changed to

fn transmute<T,U:Transmutable<T>>(x: T) -> U;

where Transmutable is a compiler-implemented type, like Send, Copy or Share.

[alexcrichton]

See #14859 and #14903 for rationales around this. Otherwise closing as a duplicate of rust-lang/rfcs#91 which was postponed for now.