add a test for shrink_in_place

tests/shrink_in_place.rs

@@ -0,0 +1,35 @@
+#![cfg_attr(feature = "alloc_trait", feature(allocator_api))]
+
+extern crate jemallocator;
+
+use jemallocator::Jemalloc;
+
+#[global_allocator]
+static A: Jemalloc = Jemalloc;
+
+#[test]
+#[cfg(feature = "alloc_trait")]
+fn shrink_in_place() {
+    unsafe {
+        use std::alloc::{Alloc, Layout};
+
+        // allocate a "large" block of memory:
+        let orig_sz = 10 * 4096;
+        let orig_l = Layout::from_size_align(orig_sz, 1).unwrap();
+        let ptr = Jemalloc.alloc(orig_l).unwrap();
+
+        // try to shrink it in place to 1 byte - if this succeeds,
+        // the size-class of the new allocation should be different
+        // than that of the original allocation:
+        let new_sz = 1;
+        if let Ok(()) = Jemalloc.shrink_in_place(ptr, orig_l, new_sz) {
+            // test that deallocating with the new layout succeeds:
+            let new_l = Layout::from_size_align(new_sz, 1).unwrap();
+            Jemalloc.dealloc(ptr, new_l);
+        } else {
+            assert!(false);
+            // if shrink in place failed - deallocate with the old layout
+            Jemalloc.dealloc(ptr, orig_l);
+        }
+    }
+}