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Last active Jan 21, 2020
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DoublyLinkedList!Something global; // error,
//DoublyLinkedList!Something contains headconst fields and is not marked as such (globals can't be anyway)
//also its null (=void wouldn't work either since then it wouldn't be tied to a stack)
void func() @system {
DoublyLinkedList!Something ll = new DoublyLinkedList!Something(); // ok in @system code but won't allow escaping because of headconst fields.
headconst DoublyLinkedList!Something ll = new DoublyLinkedList!Something(); // ok
func2(ll); // ok
func3(ll); // error
global = ll;
void func2(headconst DoublyLinkedList!Something ll) {} // could be @safe if you wanted it to be
void func3(DoublyLinkedList!Something) @system {} // error, DoublyLinkedList!T contains headconst fields.
final class DoublyLinkedList(T) {
private {
Node nullNode; // can't be escaped because of headconst fields,
//but they are =void so they don't need to be set in constructor.
headconst Node first;
headconst Node last;
this() @system {
// the refernces inside of nullNode won't be checked because they are =void and this is @system code
// the reference checks for first and last will be checked however as an invariant of the class so we must set them to something.
first = nullNode; // if this wasn't here, it would error as first would be 'null'
last = nullNode;
void add(T value) @safe nothrow {
if (last is nullNode) {
first = new Node(nullNode, nullNode, value); // the value's lifetime is that of 'this' DIP25
last = first;
} else { = new Node(last, nullNode, value);
last =;
void contains(T value) @safe nothrow {
headconst Node current = first;
while (current !is nullNode) {
if (current.value == value) {
return true;
current =;
return false;
void remove(T value) @safe nothrow {
headconst Node current = first;
while (current !is nullNode) {
if (current.value == value) {
if (current is first)
first =;
else =; // ok, doesn't escape the lifetime of 'this' DIP25
if (current is last)
last = current.previous;
else = current.previous; // no need to do null checks, because its guaranteed to not be null!
// GC will handle auto deallocation
current =;
static struct Node {
headconst Node previous = void, // ok, can only be read or modified in @system/@trusted code.
next = void;
T value;
Foo global;
Foo func() {
auto foo = scoped!Foo(1, 2, 3);
global = foo; // error, uses alias this which returns via headconst
func2(foo); // ok
/+return+/ headconst Foo bar = func3(foo); // ok
return bar; // error, DIP25 bar outlives foo.
return foo; // error, Scoped!T.get() is treated as return ref, so it can't be escaped
void func2(/+return+/ headconst Foo foo) {
foo.method(); // ok, doesn't matter if it was passed in via new Foo(1, 2, 3), or via scoped
/+return+/ headconst Foo func3(/+return+/ headconst Foo foo) { // DIP25 pass through checks
return foo;
void func4() {
Foo foo = new Foo(1, 2, 3);
func2(foo); // ok, func2 doesn't escape foo
headconst Foo value = func3(foo); // ok, func3 doesn't escape foo and the return value can't escape us
// since value is tied to foo's lifetime of this function body
template scoped(T) if (is(T == class)) {
Scoped scoped(Args...)(Args args) {
return Scoped(new T(args)); // ugh lets not replicate everything completely
struct Scoped {
private T payload;
@disable this(this);
/+return+/ headconst T get() {
return payload;
alias get this;
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