Fix condition in realloc() implementation of Vec

Poolable and Realloc trait method implementations for Vec
operate on the vector itself rather than its underlying buffer.

This is why Poolable::capacity() implementation calls Vec::len()
and Realloc::realloc() implementation uses Vec::resize() rather
than Vec::reserve() or Vec::reserve_exact().

However, the condition checking whether the vector should be resized
called Vec::len() rather than Vec::capacity().
Apparently it was an attempt to call Poolable::capacity(&self),
but by default ambiguous call is resolved to Vec::capacity().

The problem is resolved by calling .len() explicitly.

Regression test and assertions failing for byte-pool 0.2.2 are added.

Author: link2xt

src/poolable.rs

@@ -41,11 +41,13 @@ impl<T: Default + Clone> Realloc for Vec<T> {
         use std::cmp::Ordering::*;
 
         assert!(new_size > 0);
-        match new_size.cmp(&self.capacity()) {
+        match new_size.cmp(&self.len()) {
             Greater => self.resize(new_size, T::default()),
             Less => {
                 self.truncate(new_size);
+                debug_assert_eq!(self.len(), new_size);
                 self.shrink_to_fit();
+                debug_assert_eq!(self.capacity(), new_size);
             }
             Equal => {}
         }
@@ -74,3 +76,25 @@ where
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Regression test for `Realloc` trait implementation of `Vec`.
+    ///
+    /// Previously `Realloc::realloc()` called `self.capacity()` instead
+    /// of `self.len()` when checking whether the vector should be expanded.
+    /// As a result, it sometimes attempted to shrink the vector
+    /// when it should be expanded, and effectively did nothing.
+    #[test]
+    fn realloc_vec() {
+        let mut v: Vec<u8> = Poolable::alloc(100);
+
+        for i in 1..100 {
+            let new_size = Poolable::capacity(&v) + i;
+            v.realloc(new_size);
+            assert_eq!(Poolable::capacity(&v), new_size);
+        }
+    }
+}