Avoid lossy ptr-int transmutes by using AtomicPtr

src/native/arc_list.rs

@@ -3,18 +3,18 @@
 use std::marker;
 use std::ops::Deref;
 use std::sync::atomic::Ordering::SeqCst;
-use std::sync::atomic::{AtomicBool, AtomicUsize};
+use std::sync::atomic::{AtomicBool, AtomicPtr};
 use std::sync::Arc;
 
 pub struct ArcList<T> {
-    list: AtomicUsize,
+    list: AtomicPtr<Node<T>>,
     _marker: marker::PhantomData<T>,
 }
 
 impl<T> ArcList<T> {
     pub fn new() -> ArcList<T> {
         ArcList {
-            list: AtomicUsize::new(0),
+            list: AtomicPtr::new(Node::EMPTY),
             _marker: marker::PhantomData,
         }
     }
@@ -31,10 +31,10 @@ impl<T> ArcList<T> {
             return Ok(());
         }
         let mut head = self.list.load(SeqCst);
-        let node = Arc::into_raw(data.clone()) as usize;
+        let node = Arc::into_raw(data.clone()) as *mut Node<T>;
         loop {
             // If we've been sealed off, abort and return an error
-            if head == 1 {
+            if head == Node::SEALED {
                 unsafe {
                     drop(Arc::from_raw(node as *mut Node<T>));
                 }
@@ -55,16 +55,19 @@ impl<T> ArcList<T> {
     pub fn take(&self) -> ArcList<T> {
         let mut list = self.list.load(SeqCst);
         loop {
-            if list == 1 {
+            if list == Node::SEALED {
                 break;
             }
-            match self.list.compare_exchange(list, 0, SeqCst, SeqCst) {
+            match self
+                .list
+                .compare_exchange(list, Node::EMPTY, SeqCst, SeqCst)
+            {
                 Ok(_) => break,
                 Err(l) => list = l,
             }
         }
         ArcList {
-            list: AtomicUsize::new(list),
+            list: AtomicPtr::new(list),
             _marker: marker::PhantomData,
         }
     }
@@ -73,7 +76,7 @@ impl<T> ArcList<T> {
     /// `push`.
     pub fn take_and_seal(&self) -> ArcList<T> {
         ArcList {
-            list: AtomicUsize::new(self.list.swap(1, SeqCst)),
+            list: AtomicPtr::new(self.list.swap(Node::SEALED, SeqCst)),
             _marker: marker::PhantomData,
         }
     }
@@ -82,7 +85,7 @@ impl<T> ArcList<T> {
     /// empty list.
     pub fn pop(&mut self) -> Option<Arc<Node<T>>> {
         let head = *self.list.get_mut();
-        if head == 0 || head == 1 {
+        if head == Node::EMPTY || head == Node::SEALED {
             return None;
         }
         let head = unsafe { Arc::from_raw(head as *const Node<T>) };
@@ -103,15 +106,19 @@ impl<T> Drop for ArcList<T> {
 }
 
 pub struct Node<T> {
-    next: AtomicUsize,
+    next: AtomicPtr<Node<T>>,
     enqueued: AtomicBool,
     data: T,
 }
 
 impl<T> Node<T> {
+    const EMPTY: *mut Node<T> = std::ptr::null_mut();
+
+    const SEALED: *mut Node<T> = std::ptr::null_mut::<Node<T>>().wrapping_add(1);
+
     pub fn new(data: T) -> Node<T> {
         Node {
-            next: AtomicUsize::new(0),
+            next: AtomicPtr::new(Node::EMPTY),
             enqueued: AtomicBool::new(false),
             data,
         }

src/native/timer.rs

@@ -1,8 +1,7 @@
 use std::fmt;
-use std::mem;
 use std::pin::Pin;
-use std::sync::atomic::AtomicUsize;
 use std::sync::atomic::Ordering::SeqCst;
+use std::sync::atomic::{AtomicPtr, AtomicUsize};
 use std::sync::{Arc, Mutex, Weak};
 use std::task::{Context, Poll};
 use std::time::Instant;
@@ -216,7 +215,8 @@ impl Default for Timer {
     }
 }
 
-static HANDLE_FALLBACK: AtomicUsize = AtomicUsize::new(0);
+static HANDLE_FALLBACK: AtomicPtr<Inner> = AtomicPtr::new(EMPTY_HANDLE);
+const EMPTY_HANDLE: *mut Inner = std::ptr::null_mut();
 
 /// Error returned from `TimerHandle::set_fallback`.
 #[derive(Clone, Debug)]
@@ -247,23 +247,23 @@ impl TimerHandle {
     /// successful then no future calls may succeed.
     fn set_as_global_fallback(self) -> Result<(), SetDefaultError> {
         unsafe {
-            let val = self.into_usize();
-            match HANDLE_FALLBACK.compare_exchange(0, val, SeqCst, SeqCst) {
+            let val = self.into_raw();
+            match HANDLE_FALLBACK.compare_exchange(EMPTY_HANDLE, val, SeqCst, SeqCst) {
                 Ok(_) => Ok(()),
                 Err(_) => {
-                    drop(TimerHandle::from_usize(val));
+                    drop(TimerHandle::from_raw(val));
                     Err(SetDefaultError(()))
                 }
             }
         }
     }
 
-    fn into_usize(self) -> usize {
-        unsafe { mem::transmute::<Weak<Inner>, usize>(self.inner) }
+    fn into_raw(self) -> *mut Inner {
+        self.inner.into_raw() as *mut Inner
     }
 
-    unsafe fn from_usize(val: usize) -> TimerHandle {
-        let inner = mem::transmute::<usize, Weak<Inner>>(val);
+    unsafe fn from_raw(val: *mut Inner) -> TimerHandle {
+        let inner = Weak::from_raw(val);
         TimerHandle { inner }
     }
 }
@@ -277,7 +277,7 @@ impl Default for TimerHandle {
         // actually create a helper thread then we'll just return a "defunkt"
         // handle which will return errors when timer objects are attempted to
         // be associated.
-        if fallback == 0 {
+        if fallback == EMPTY_HANDLE {
             let helper = match global::HelperThread::new() {
                 Ok(helper) => helper,
                 Err(_) => return TimerHandle { inner: Weak::new() },
@@ -301,11 +301,11 @@ impl Default for TimerHandle {
         // At this point our fallback handle global was configured so we use
         // its value to reify a handle, clone it, and then forget our reified
         // handle as we don't actually have an owning reference to it.
-        assert!(fallback != 0);
+        assert!(fallback != EMPTY_HANDLE);
         unsafe {
-            let handle = TimerHandle::from_usize(fallback);
+            let handle = TimerHandle::from_raw(fallback);
             let ret = handle.clone();
-            let _ = handle.into_usize();
+            let _ = handle.into_raw();
             ret
         }
     }