DIP25

DIP25: Sealed references

Title:	Sealed references
DIP:	25
Version:	1
Status:	Draft
Created:	2013-02-05
Last Modified:	2014-12-27
Author:	Walter Bright and Andrei Alexandrescu
Links:

Abstract

D offers a number of features aimed at systems-level coding, such as unrestricted pointers, casting between integers and pointers, and the @system attribute. These means, combined with the other features of D, make it a complete and expressive language for systems-level tasks. On the other hand, economy of means should be exercised in defining such powerful but dangerous features. Most other features should offer good safety guarantees with little or no loss in efficiency or expressiveness. This proposal makes ref provide such a guarantee: with the proposed rules, it is impossible in safe code to have ref refer to a destroyed object. The restrictions introduced are not entirely backward compatible, but disallow code that is stylistically questionable and that can be easily replaced either with equivalent and clearer code.

In a nutshell

This DIP proposes that any ref parameter that a function received and also wants to return must be also annotated with inout. Example:

@safe:
ref int fun(ref int a} { return a; } // ERROR
ref int gun(ref inout int a} { return a; } // FINE, works for unqualified int only
ref inout int hun(ref inout int a} { return a; } // FINE, works for qualified int too

Description

Currently, D has some provisions for avoiding dangling references:

ref int fun(int x) {
  return x; // Error: escaping reference to local variable x 
}

ref int gun() {
  int x;
  return x; // Error: escaping reference to local variable x 
}

struct S {
    int x;
}

ref int hun() {
  S s;
  return s.x; // see https://issues.dlang.org/show_bug.cgi?id=13902
}

ref int iun() {
  int a[42];
  return a[5]; // see https://issues.dlang.org/show_bug.cgi?id=13902
}

However, this enforcement is shallow (even after fixing issue 13902). The following code compiles and allows reads and writes through defunct stack locations, bypassing scoping and lifetime rules:

ref int identity(ref int x) {
  return x; // pass-through function that does nothing 
}

ref int fun(int x) {
  return identity(x); // escape the address of a parameter 
}

ref int gun() {
  int x;
  return identity(x); // escape the address of a local
}

struct S {
    int x;
    ref int get() { return x; }
}

ref int hun(S x) {
  return x.get; // escape the address of a part of a parameter 
}

ref int iun() {
  S s;
  return s.get; // escape the address of part of a local
}

ref int jun() {
  return S().get; // worst contender: escape the address of a part of an rvalue
}

The escape patterns are obvious in these simple examples that make all code available and use no recursion, and may be found automatically. The problem is that generally the compiler cannot see the body of identity or S.get(). We need to devise a method that derives enough information for safety analysis only given the function signatures, not their bodies.

This DIP devises rules that allow passing objects by reference down into functions, and return references up from functions, whilst disallowing cases such as the above when a reference passed up ends up referring to a deallocated temporary.

Enhancing inout

The main issue is typechecking functions that return a ref T. Those that attempt to return locals or parts thereof are already addressed directly, contingent to Issue 13902. The one case remaining is allowing a function returning ref T to return a (part of a) parameter passed by ref.

The key is to distinguish legal from illegal cases. One simple but overly conservative option would be to simply disallow returning a ref parameter or part thereof. That makes identity impossible to implement, and as a consequence accessing elements of a container by reference becomes difficult or impossible to typecheck properly. Also, heap-allocated structures with deterministic destruction (e.g. reference counted) must insert member copies for all accesses.

Cases that should work include:

@safe ref int identity(ref int x) { 
    return x; // should work
}

@safe ref int fun(ref int input) {
    static int[42] data;
    return data[input]; // should work
}

@safe struct S {
    private int x;
    ref int get() { return x; } // should work 
}

This proposal promotes enhancing the charter of the inout qualifier to propagate the lifetime of a parameter to the return value of a function. With the proposed semantics, a function is disallowed to return a ref parameter of a part thereof UNLESS the parameter is also annotated with inout. Under the proposed semantics identity will be spelled as follows:

@safe ref int wrongIdentity(ref int x) { 
    return x; // ERROR! Cannot return a ref, please use "ref inout"
}
@safe ref int identity(ref inout int x) { 
    return x; // fine
}

Just by seeing the signature ref int identity(ref inout int x) the compiler assumes that the result of identity must have a shorter lifetime than x and typechecks callers accordingly. Example (given the previous definition of identity):

@safe ref int fun(ref inout int x) { 
    int a;
    return a; // ERROR per current language rules
    static int b;
    return b; // fine per current language rules
    return identity(a); // ERROR, this may escape the address of a local
    return x; // fine, propagate x's lifetime to output
    return identity(x); // fine, propagate x's lifetime through identity to the output
    return identity(identity(x)); // fine, propagate x's lifetime twice through identity to the output
}

Existing Semantics of inout

Currently (prior to this DIP) inout is used to propagate qualifiers such as const and immutable from a parameter to the result. That semantics remains, and is enhanced by the propagation of lifetime. This DIP assumes no major idioms are disabled by conflating qualifier propagation with lifetime propagation.

Types of Result vs. Parameters

Consider:

@safe ref int fun(ref inout float x);

This function arguably cannot return a value scoped within the lifetime of its argument for the simple reason it's impossible to find an int somewhere in a float (apart from unsafe address manipulation). However, this DIP ignores types; if a parameter is ref inout, it is always considered potentially escaped as a result. It is in fact possible that the author of fun wants to constrain its output's lifetime for unrelated reasons.

Future versions of this DIP may relax this rule.

Multiple Parameters

If multiple ref inout parameters are present, the result's lifetime is conservatively assumed to be enclosed in the lifetime of the shortest-lived of those arguments.

Member Functions

Member functions of structs must qualify this with inout if they want to return a result by ref that won't outlive this. Example:

@safe struct S {
    static int a;
    int b;
    ref int fun() { return a; } // fine, callers assume infinite lifetime
    ref int gun() { return b; } // ERROR! Cannot return a direct member
    ref int hun() inout { return b; } // fine, result is scoped within this
    ref inout int iun() inout { return b; } // even better, propagate qualifier to output
}

Copyright

This document has been placed in the Public Domain.