Create a gist now

Instantly share code, notes, and snippets.

@Kimundi /
Last active Aug 29, 2015

What would you like to do?
use std::collections::HashMap;
macro_rules! lazy_init {
($(static ref $N:ident : $T:ty = $e:expr;)*) => {
struct $N {__unit__: ()}
static $N: $N = $N {__unit__: ()};
impl Deref<$T> for $N {
fn deref<'a>(&'a self) -> &'a $T {
use std::sync::{Once, ONCE_INIT};
use std::mem::transmute;
fn require_share<T: Share>(_: &T) { }
unsafe {
static mut s: *$T = 0 as *$T;
static mut ONCE: Once = ONCE_INIT;
ONCE.doit(|| {
s = transmute::<Box<$T>, *$T>(box() ($e));
let static_ref = &*s;
lazy_init! {
static ref NUMBER: uint = times_two(3);
static ref VEC: [Box<uint>, ..3] = [box 1, box 2, box 3];
static ref OWNED_STRING: String = "hello".to_string();
static ref HASHMAP: HashMap<uint, &'static str> = {
let mut m = HashMap::new();
m.insert(0u, "abc");
m.insert(1, "def");
m.insert(2, "ghi");
fn times_two(n: uint) -> uint {
n * 2
fn test_basic() {
assert_eq!(OWNED_STRING.as_slice(), "hello");
assert_eq!(*NUMBER, 6);
assert_eq!(VEC.as_slice(), &[box 1, box 2, box 3]);
fn test_repeat() {
assert_eq!(*NUMBER, 6);
assert_eq!(*NUMBER, 6);
assert_eq!(*NUMBER, 6);

Why $N {__unit__: ()}; rather than a $N; empty struct?


Kimundi commented May 13, 2014

Answered on IRC already, but for future reference: struct $N {__unit__: ()}; only lives in the type namespace, allowing a static $N; to coexist.

struct $N; would also live in the value namespace, as its constructuor, and thus prevent a static of the same name

This is only memory-safe if $T satisfy the Share kind, right? Does/could/should this macro enforce Share? (I.e. fail at compile time when it would be unsafe.)


Kimundi commented May 16, 2014

You're right, I'm currently investigating how to make this safe.

I updated the current code to show how you can do a unsafe mutation of a static, which needs to be prevented.


Kimundi commented May 16, 2014

I think with the planned-to-be-re-added bounds on structs a phantom type parameter could work:

struct $N<T: Share> { unit: () }
static $N: $N<$T>= $N { unit: () }

EDIT: Adding a fn require_share<T: Share>() works just as well.

I removed the #[allow(dead_code)] line when integrating this in Servo. I found the dead code warning to be relevant, only showing up when a given static was actually unused.


Kimundi commented Jun 24, 2014

This now officially lives in

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment