Vendor MaybeUninit from libcore for old Rust versions

core/build.rs

@@ -3,4 +3,7 @@ fn main() {
     if cfg.probe_rustc_version(1, 34) {
         println!("cargo:rustc-cfg=has_sized_atomics");
     }
+    if cfg.probe_rustc_version(1, 36) {
+        println!("cargo:rustc-cfg=has_maybe_uninit");
+    }
 }

core/src/lib.rs

@@ -56,6 +56,7 @@
     feature(thread_local, checked_duration_since)
 )]
 
+mod maybe_uninit;
 mod parking_lot;
 mod spinwait;
 mod thread_parker;

core/src/maybe_uninit.rs

@@ -0,0 +1,41 @@
+//! This module contains a re-export or vendored version of `core::mem::MaybeUninit` depending
+//! on which Rust version it's compiled for.
+//!
+//! Remove this module and use `core::mem::MaybeUninit` directly when dropping support for <1.36
+#![allow(dead_code)]
+
+#[cfg(has_maybe_uninit)]
+pub use core::mem::MaybeUninit;
+
+#[cfg(not(has_maybe_uninit))]
+use core::mem::ManuallyDrop;
+
+/// Copied from `core::mem::MaybeUninit` to support Rust older than 1.36
+#[cfg(not(has_maybe_uninit))]
+pub union MaybeUninit<T: Copy> {
+    uninit: (),
+    value: ManuallyDrop<T>,
+}
+
+#[cfg(not(has_maybe_uninit))]
+impl<T: Copy> MaybeUninit<T> {
+    #[inline(always)]
+    pub fn uninit() -> MaybeUninit<T> {
+        MaybeUninit { uninit: () }
+    }
+
+    #[inline(always)]
+    pub fn as_ptr(&self) -> *const T {
+        unsafe { &*self.value as *const T }
+    }
+
+    #[inline(always)]
+    pub fn as_mut_ptr(&mut self) -> *mut T {
+        unsafe { &mut *self.value as *mut T }
+    }
+
+    #[inline(always)]
+    pub unsafe fn assume_init(self) -> T {
+        ManuallyDrop::into_inner(self.value)
+    }
+}