FEAT: Use stable MaybeUninit when we can (feature detected)

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));
+// To guard against changes in this currently unstable feature, use
-    if has_stable_union_with_md {
+// a detection tests instead of a Rustc version and/or date test.
-        println!("cargo:rustc-cfg=has_manually_drop_in_union");
+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
+    }
+}