[swift-evolution] Proposal: Extend the &x -> UnsafePointer behavior to work with immutable values

[Kevin Ballard]

# Introduction

Swift allows you to pass a &x ref to a function taking Unsafe[Mutable]Pointer, as long as x is mutable.

# Problem

There's no way to pass an immutable UnsafePointer that points to immutable data. Any such immutable data has to be copied into a mutable variable before the &x ref works.

# Solution

Allow for using &x with immutable values if and only if the reference is passed as a parameter to a function expecting UnsafePointer

# Drawbacks

It's easy to cast an UnsafePointer to an UnsafeMutablePointer and then mutate it. This problem exists in C and C++ as well. But UnsafePointer/UnsafeMutablePointer can already be used to bypass many of the safety mechanisms of the language, so I don't think this is a big deal. Any such mutations will either crash (because it's mutating something that's not in a writeable page), or it will invoke undefined behavior, but that shouldn't surprise anyone.

# Alternatives

Do nothing and hope that at some point Swift introduces some form of first-class support for explicit immutable references. That's probably not happening any time soon.

-Kevin Ballard