Create an empty "Primitive" trait implemented by primitive (and only primitive) types to improve inferring "as" availability in generics

I've been trying to do something along the lines of:

fn foo<T> (buffer: &[u8]) {
  let foo: T = SomeTValue();
  &[0..foo as usize]
}

And discovered that as won't work there and there seems to be no way to add a 'where' bound to the T type to make sure the compiler accepts "as" here knowing that it'll work because it cannot infer that T is a primitive type.

Having a trait that allows <T: Primitive> type bounds would be handy here to let the compiler know that the code should work.

Start with the generic example in the issue and examine how Rust's as expressions and generic bounds currently handle primitive types. No source files, tests, or entry points are identified; done would require a settled design for a Primitive bound that is implemented only by primitive types and supports the intended generic cast.