missing extra bytes

[ikravets]

I see this project is not very active, unfortunately. Anyway, we use it in some internal project and I've discovered and fixed a bug more than a year ago. Noticed I haven't upstreamed it, so I'm going to submit a PR in a moment. Here is a context.

The problem arises when using extra > 0 in Pool::with_capacity<F>(count: usize, mut extra: usize, init: F). We see that in PoolInner::entry(&self, idx: usize) pointer is calculated based on size of Entry<T> only, not on size_of::<Entry<T>>() + extra. Essentially, extra argument is ignored.

My simple fix is the following:

@@ -293,7 +293,7 @@ impl<T> PoolInner<T> {
     fn entry(&self, idx: usize) -> &Entry<T> {
         unsafe {
             debug_assert!(idx < self.count, "invalid index");
-            let ptr = self.ptr.offset(idx as isize);
+            let ptr = (self.ptr as *mut u8).offset((idx * self.entry_size) as isize) as *mut Entry<T>;
             mem::transmute(ptr)
         }
     }

Note that self.ptr is of *mut Entry<T> and self.entry_size is defined at https://github.com/carllerche/pool/blob/77eee7c6de50ccb7a6c3770b136e3f9220d1f059/src/lib.rs#L205