Strict Value Semantics

Andrew_Trick · January 30, 2019, 1:34am

Well, to me it depends on the level of semantics you care about. If you're asking about whether a variable is substitutable or can observe "reentrant mutation", then inout behaves just like copy-in, copy-out as long as exclusivity access is 100% fully enforced, which we now claim.
See

github.com

apple/swift-evolution/blob/main/proposals/0176-enforce-exclusive-access-to-memory.md

# Enforce Exclusive Access to Memory

* Proposal: [SE-0176](0176-enforce-exclusive-access-to-memory.md)
* Author: [John McCall](https://github.com/rjmccall)
* Review Manager: [Ben Cohen](https://github.com/airspeedswift)
* Status: **Implemented (Swift 4)**
* Previous Revision: [1](https://github.com/apple/swift-evolution/blob/7e6816c22a29b0ba9bdf63ff92b380f9e963860a/proposals/0176-enforce-exclusive-access-to-memory.md)
* Previous Discussion: [Email Thread](https://forums.swift.org/t/review-se-0176-enforce-exclusive-access-to-memory/5836)

## Introduction

In Swift 3, it is possible to modify a variable while it's being used or
modified by another part of the program.  This can lead to unexpected and
confusing results.  It also forces a great deal of conservatism onto the
implementation of the compiler and the standard libraries, which must
generally ensure the basic soundness of the program (no crashes or
undefined behavior) even in unusual circumstances.

We propose that Swift should instead enforce a general rule that potential
modifications of variables must be exclusive with any other access to that

This file has been truncated. show original

I'm working on writing up the current implementation status, and I'll try to remember to post a link here when its ready.

If you're talking about semantics at the level of what I would call the zero-cost abstraction guarantee, rather than correctness, then inout is actually prohibited from making a copy. Otherwise, isKnownUniquelyReferenced would never work. Keep in mind though, that It's actually illegal for your programs correctness to depend on the result of isKnownUniquelyReferenced.