Fix improper handling of overflow in len_usize (fix #128)

This will lead to implausibly large lengths, nearly endless iterators
and possibly unsound behaviour through the Drop implementation

Switches the `truncate` method to refer to the common defintion of
simply chopping of the test bits. (Implemented by `as`, which is defined
to do this).

Adds a new `saturate` method to replace the previous `truncate` method

src/sealed.rs

@@ -21,6 +21,9 @@ unsafe impl XCore for MultiCore {
 }
 
 pub unsafe trait Uxx: Into<usize> + Send {
+    #[doc(hidden)]
+    fn saturate(x: usize) -> Self;
+
     #[doc(hidden)]
     fn truncate(x: usize) -> Self;
 
@@ -39,15 +42,19 @@ pub unsafe trait Uxx: Into<usize> + Send {
 }
 
 unsafe impl Uxx for u8 {
-    fn truncate(x: usize) -> Self {
-        let max = ::core::u8::MAX;
+    fn saturate(x: usize) -> Self {
+        let max = Self::max_value() as usize;
         if x >= usize::from(max) {
-            max
+            max as Self
         } else {
-            x as u8
+            x as Self
         }
     }
 
+    fn truncate(x: usize) -> Self {
+        x as Self
+    }
+
     unsafe fn load_acquire<C>(x: *const Self) -> Self
     where
         C: XCore,
@@ -79,15 +86,19 @@ unsafe impl Uxx for u8 {
 }
 
 unsafe impl Uxx for u16 {
-    fn truncate(x: usize) -> Self {
-        let max = ::core::u16::MAX;
+    fn saturate(x: usize) -> Self {
+        let max = Self::max_value() as usize;
         if x >= usize::from(max) {
-            max
+            max as Self
         } else {
-            x as u16
+            x as Self
         }
     }
 
+    fn truncate(x: usize) -> Self {
+        x as Self
+    }
+
     unsafe fn load_acquire<C>(x: *const Self) -> Self
     where
         C: XCore,
@@ -119,6 +130,10 @@ unsafe impl Uxx for u16 {
 }
 
 unsafe impl Uxx for usize {
+    fn saturate(x: usize) -> Self {
+        x
+    }
+
     fn truncate(x: usize) -> Self {
         x
     }

src/spsc/mod.rs

@@ -174,7 +174,7 @@ where
 {
     /// Returns the maximum number of elements the queue can hold
     pub fn capacity(&self) -> U {
-        U::truncate(N::to_usize())
+        U::saturate(N::to_usize())
     }
 
     /// Returns `true` if the queue has a length of 0
@@ -205,7 +205,7 @@ where
         let head = self.0.head.load_relaxed().into();
         let tail = self.0.tail.load_relaxed().into();
 
-        tail.wrapping_sub(head)
+        U::truncate(tail.wrapping_sub(head)).into()
     }
 }
 
@@ -693,6 +693,20 @@ mod tests {
         assert_eq!(items.next(), None);
     }
 
+    #[test]
+    fn iter_overflow() {
+        let mut rb: Queue<i32, U4, u8> = Queue::u8();
+
+        rb.enqueue(0).unwrap();
+        for _ in 0..300 {
+            let mut items = rb.iter_mut();
+            assert_eq!(items.next(), Some(&mut 0));
+            assert_eq!(items.next(), None);
+            rb.dequeue().unwrap();
+            rb.enqueue(0).unwrap();
+        }
+    }
+
     #[test]
     fn iter_mut() {
         let mut rb: Queue<i32, U4> = Queue::new();