Typed throws in withDependencies

[manmal]

Hi,

While integrating typed throws in our codebase, I've noticed that withDependencies isn't quite as ergonomic because it's using rethrows instead of typed throws. The reason being that rethrows erases the inner error type to any Error. My pitch is to convert all instances of rethrows to typed errors.

---

You're very likely aware, just reiterating the concept for completeness:

Instead of rethrows, we can now use typed throws:

-func myFunc(_ transform: () throws -> Void) rethrows {
+func myFunc<E: Error>(_ transform: () throws(E) -> Void) throws(E) {
  try transform()
}

The compiler knows to infer that if E == Never, there's no try required, which matches the behavior we get from rethrows:

  try myFunc {
    throw CancellationError() // E == CancellationError
  }
  
  myFunc {} // E == Never