Add __alloc__ for bytearray, add note to tests.

Lib/test/test_bytes.py

@@ -1449,6 +1449,11 @@ class ByteArrayTest(BaseBytesTest, unittest.TestCase):
         self.assertEqual(b, b1)
         self.assertIs(b, b1)
 
+    # RUSTPYTHON:
+    #
+    # The second instance of self.assertGreater was replaced with
+    # self.assertGreaterEqual since, in RustPython, the underlying storage
+    # is a Vec which doesn't require trailing null byte.
     # TODO: RUSTPYTHON
     @unittest.expectedFailure
     def test_alloc(self):
@@ -1459,10 +1464,15 @@ class ByteArrayTest(BaseBytesTest, unittest.TestCase):
         for i in range(100):
             b += b"x"
             alloc = b.__alloc__()
-            self.assertGreater(alloc, len(b))  # including trailing null byte
+            self.assertGreaterEqual(alloc, len(b))
             if alloc not in seq:
                 seq.append(alloc)
 
+    # RUSTPYTHON:
+    #
+    # The usages of self.assertGreater were replaced with
+    # self.assertGreaterEqual since, in RustPython, the underlying storage
+    # is a Vec which doesn't require trailing null byte.
     # TODO: RUSTPYTHON
     @unittest.expectedFailure
     def test_init_alloc(self):
@@ -1475,12 +1485,12 @@ class ByteArrayTest(BaseBytesTest, unittest.TestCase):
                 self.assertEqual(len(b), len(a))
                 self.assertLessEqual(len(b), i)
                 alloc = b.__alloc__()
-                self.assertGreater(alloc, len(b))  # including trailing null byte
+                self.assertGreaterEqual(alloc, len(b))
         b.__init__(g())
         self.assertEqual(list(b), list(range(1, 100)))
         self.assertEqual(len(b), 99)
         alloc = b.__alloc__()
-        self.assertGreater(alloc, len(b))
+        self.assertGreaterEqual(alloc, len(b))
 
     def test_extend(self):
         orig = b'hello'

vm/src/builtins/bytearray.rs

@@ -124,6 +124,11 @@ impl PyByteArray {
         self.inner().repr("bytearray(", ")")
     }
 
+    #[pymethod(magic)]
+    fn alloc(&self) -> usize {
+        self.inner().capacity()
+    }
+
     #[pymethod(name = "__len__")]
     fn len(&self) -> usize {
         self.borrow_buf().len()

vm/src/bytesinner.rs

@@ -293,6 +293,11 @@ impl PyBytesInner {
         self.elements.len()
     }
 
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.elements.capacity()
+    }
+
     #[inline]
     pub fn is_empty(&self) -> bool {
         self.elements.is_empty()