Use stable MaybeUninit when we can (feature detected)

.travis.yml

@@ -15,6 +15,7 @@ matrix:
     - rust: stable
       env:
       - FEATURES='array-sizes-33-128 array-sizes-129-255'
+      - ARRAYVECTEST_ENSURE_MAYBEUNINIT=1
     - rust: beta
     - rust: nightly
       env:
@@ -23,12 +24,12 @@ matrix:
     - rust: nightly
       env:
       - NODROP_FEATURES='use_needs_drop'
-      - ARRAYVECTEST_ENSURE_UNION=1
+      - ARRAYVECTEST_ENSURE_MAYBEUNINIT=1
     - rust: nightly
       env:
       - FEATURES='serde use_union'
       - NODROP_FEATURES='use_union'
-      - ARRAYVECTEST_ENSURE_UNION=1
+      - ARRAYVECTEST_ENSURE_MAYBEUNINIT=1
 branches:
   only:
     - master

build.rs

@@ -11,17 +11,28 @@ fn main() {
 }
 
 fn detect_maybe_uninit() {
+    let has_stable_maybe_uninit = probe(&stable_maybe_uninit());
+    if has_stable_maybe_uninit {
+        println!("cargo:rustc-cfg=has_stable_maybe_uninit");
+        return;
+    }
     let has_unstable_union_with_md = probe(&maybe_uninit_code(true));
     if has_unstable_union_with_md {
         println!("cargo:rustc-cfg=has_manually_drop_in_union");
         println!("cargo:rustc-cfg=has_union_feature");
-        return;
     }
+}
 
-    let has_stable_union_with_md = probe(&maybe_uninit_code(false));
-    if has_stable_union_with_md {
-        println!("cargo:rustc-cfg=has_manually_drop_in_union");
-    }
+// To guard against changes in this currently unstable feature, use
+// a detection tests instead of a Rustc version and/or date test.
+fn stable_maybe_uninit() -> String {
+    let code = "
+    #![allow(warnings)]
+    use std::mem::MaybeUninit;
+
+    fn main() { }
+    ";
+    code.to_string()
 }
 
 // To guard against changes in this currently unstable feature, use

src/lib.rs

@@ -50,9 +50,12 @@ use std::fmt;
 use std::io;
 
 
-#[cfg(has_manually_drop_in_union)]
+#[cfg(has_stable_maybe_uninit)]
+#[path="maybe_uninit_stable.rs"]
 mod maybe_uninit;
-#[cfg(not(has_manually_drop_in_union))]
+#[cfg(all(not(has_stable_maybe_uninit), has_manually_drop_in_union))]
+mod maybe_uninit;
+#[cfg(all(not(has_stable_maybe_uninit), not(has_manually_drop_in_union)))]
 #[path="maybe_uninit_nodrop.rs"]
 mod maybe_uninit;
 

src/maybe_uninit_stable.rs

@@ -0,0 +1,40 @@
+
+
+use array::Array;
+use std::mem::MaybeUninit as StdMaybeUninit;
+
+pub struct MaybeUninit<T> {
+    inner: StdMaybeUninit<T>,
+}
+
+impl<T> MaybeUninit<T> {
+    /// Create a new MaybeUninit with uninitialized interior
+    pub unsafe fn uninitialized() -> Self {
+        MaybeUninit { inner: StdMaybeUninit::uninit() }
+    }
+
+    /// Create a new MaybeUninit from the value `v`.
+    pub fn from(v: T) -> Self {
+        MaybeUninit { inner: StdMaybeUninit::new(v) }
+    }
+
+    // Raw pointer casts written so that we don't reference or access the
+    // uninitialized interior value
+
+    /// Return a raw pointer to the start of the interior array
+    pub fn ptr(&self) -> *const T::Item
+        where T: Array
+    {
+        // std MaybeUninit creates a &self.value reference here which is
+        // not guaranteed to be sound in our case - we will partially
+        // initialize the value, not always wholly.
+        self.inner.as_ptr() as *const T::Item
+    }
+
+    /// Return a mut raw pointer to the start of the interior array
+    pub fn ptr_mut(&mut self) -> *mut T::Item
+        where T: Array
+    {
+        self.inner.as_mut_ptr() as *mut T::Item
+    }
+}

tests/tests.rs

@@ -510,10 +510,8 @@ fn test_sizes_129_255() {
 
 
 #[test]
-fn test_nightly_uses_maybe_uninit() {
-    if option_env!("ARRAYVECTEST_ENSURE_UNION").map(|s| !s.is_empty()).unwrap_or(false) {
-        assert!(cfg!(has_manually_drop_in_union));
-        type ByteArray = ArrayVec<[u8; 4]>;
-        assert!(mem::size_of::<ByteArray>() == 5);
+fn test_newish_stable_uses_maybe_uninit() {
+    if option_env!("ARRAYVECTEST_ENSURE_MAYBEUNINIT").map(|s| !s.is_empty()).unwrap_or(false) {
+        assert!(cfg!(has_stable_maybe_uninit));
     }
 }