fix for Miri and add Miri to CI

.travis.yml

@@ -1,9 +1,17 @@
 sudo: false
 language: rust
-rust:
-  - nightly
-  - beta
-  - stable
+
+matrix:
+    include:
+    - rust: stable
+    - rust: beta
+    - rust: nightly
+
+    - rust: nightly
+      os: linux
+      env: DESCRIPTION="Miri"
+      script:
+        - sh ci/miri.sh
 
 script:
   - cargo test

ci/miri.sh

@@ -0,0 +1,10 @@
+set -ex
+
+MIRI_NIGHTLY=nightly-$(curl -s https://rust-lang.github.io/rustup-components-history/x86_64-unknown-linux-gnu/miri)
+echo "Installing latest nightly with Miri: $MIRI_NIGHTLY"
+rustup default "$MIRI_NIGHTLY"
+
+rustup component add miri
+cargo miri setup
+
+cargo miri test

src/lib.rs

@@ -165,9 +165,13 @@ impl<T> Arena<T> {
     #[inline]
     fn alloc_fast_path(&self, value: T) -> Result<&mut T, T> {
         let mut chunks = self.chunks.borrow_mut();
-        if chunks.current.len() < chunks.current.capacity() {
+        let len = chunks.current.len();
+        if len < chunks.current.capacity() {
             chunks.current.push(value);
-            Ok(unsafe { mem::transmute(chunks.current.last_mut().unwrap()) })
+            // Avoid going through `Vec::deref_mut`, which overlaps
+            // other references we have already handed out!
+            debug_assert!(len < chunks.current.len()); // bounds check
+            Ok(unsafe { &mut *chunks.current.as_mut_ptr().add(len) })
         } else {
             Err(value)
         }