Change ManuallyDrop from an union to a struct and make it a lang item.

src/libcore/manually_drop_stage0.rs

@@ -0,0 +1,195 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+/// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
+///
+/// This wrapper is 0-cost.
+///
+/// # Examples
+///
+/// This wrapper helps with explicitly documenting the drop order dependencies between fields of
+/// the type:
+///
+/// ```rust
+/// use std::mem::ManuallyDrop;
+/// struct Peach;
+/// struct Banana;
+/// struct Melon;
+/// struct FruitBox {
+///     // Immediately clear there’s something non-trivial going on with these fields.
+///     peach: ManuallyDrop<Peach>,
+///     melon: Melon, // Field that’s independent of the other two.
+///     banana: ManuallyDrop<Banana>,
+/// }
+///
+/// impl Drop for FruitBox {
+///     fn drop(&mut self) {
+///         unsafe {
+///             // Explicit ordering in which field destructors are run specified in the intuitive
+///             // location – the destructor of the structure containing the fields.
+///             // Moreover, one can now reorder fields within the struct however much they want.
+///             ManuallyDrop::drop(&mut self.peach);
+///             ManuallyDrop::drop(&mut self.banana);
+///         }
+///         // After destructor for `FruitBox` runs (this function), the destructor for Melon gets
+///         // invoked in the usual manner, as it is not wrapped in `ManuallyDrop`.
+///     }
+/// }
+/// ```
+#[stable(feature = "manually_drop", since = "1.20.0")]
+#[allow(unions_with_drop_fields)]
+#[derive(Copy)]
+pub union ManuallyDrop<T>{ value: T }
+
+impl<T> ManuallyDrop<T> {
+    /// Wrap a value to be manually dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::mem::ManuallyDrop;
+    /// ManuallyDrop::new(Box::new(()));
+    /// ```
+    #[stable(feature = "manually_drop", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_manually_drop_new")]
+    #[inline]
+    pub const fn new(value: T) -> ManuallyDrop<T> {
+        ManuallyDrop { value: value }
+    }
+
+    /// Extract the value from the ManuallyDrop container.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// use std::mem::ManuallyDrop;
+    /// let x = ManuallyDrop::new(Box::new(()));
+    /// let _: Box<()> = ManuallyDrop::into_inner(x);
+    /// ```
+    #[stable(feature = "manually_drop", since = "1.20.0")]
+    #[inline]
+    pub fn into_inner(slot: ManuallyDrop<T>) -> T {
+        unsafe {
+            slot.value
+        }
+    }
+
+    /// Manually drops the contained value.
+    ///
+    /// # Safety
+    ///
+    /// This function runs the destructor of the contained value and thus the wrapped value
+    /// now represents uninitialized data. It is up to the user of this method to ensure the
+    /// uninitialized data is not actually used.
+    #[stable(feature = "manually_drop", since = "1.20.0")]
+    #[inline]
+    pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
+        ptr::drop_in_place(&mut slot.value)
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T> Deref for ManuallyDrop<T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        unsafe {
+            &self.value
+        }
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T> DerefMut for ManuallyDrop<T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        unsafe {
+            &mut self.value
+        }
+    }
+}
+
+#[stable(feature = "manually_drop", since = "1.20.0")]
+impl<T: ::fmt::Debug> ::fmt::Debug for ManuallyDrop<T> {
+    fn fmt(&self, fmt: &mut ::fmt::Formatter) -> ::fmt::Result {
+        unsafe {
+            fmt.debug_tuple("ManuallyDrop").field(&self.value).finish()
+        }
+    }
+}
+
+#[stable(feature = "manually_drop_impls", since = "1.22.0")]
+impl<T: Clone> Clone for ManuallyDrop<T> {
+    fn clone(&self) -> Self {
+        ManuallyDrop::new(self.deref().clone())
+    }
+
+    fn clone_from(&mut self, source: &Self) {
+        self.deref_mut().clone_from(source);
+    }
+}
+
+#[stable(feature = "manually_drop_impls", since = "1.22.0")]
+impl<T: Default> Default for ManuallyDrop<T> {
+    fn default() -> Self {
+        ManuallyDrop::new(Default::default())
+    }
+}
+
+#[stable(feature = "manually_drop_impls", since = "1.22.0")]
+impl<T: PartialEq> PartialEq for ManuallyDrop<T> {
+    fn eq(&self, other: &Self) -> bool {
+        self.deref().eq(other)
+    }
+
+    fn ne(&self, other: &Self) -> bool {
+        self.deref().ne(other)
+    }
+}
+
+#[stable(feature = "manually_drop_impls", since = "1.22.0")]
+impl<T: Eq> Eq for ManuallyDrop<T> {}
+
+#[stable(feature = "manually_drop_impls", since = "1.22.0")]
+impl<T: PartialOrd> PartialOrd for ManuallyDrop<T> {
+    fn partial_cmp(&self, other: &Self) -> Option<::cmp::Ordering> {
+        self.deref().partial_cmp(other)
+    }
+
+    fn lt(&self, other: &Self) -> bool {
+        self.deref().lt(other)
+    }
+
+    fn le(&self, other: &Self) -> bool {
+        self.deref().le(other)
+    }
+
+    fn gt(&self, other: &Self) -> bool {
+        self.deref().gt(other)
+    }
+
+    fn ge(&self, other: &Self) -> bool {
+        self.deref().ge(other)
+    }
+}
+
+#[stable(feature = "manually_drop_impls", since = "1.22.0")]
+impl<T: Ord> Ord for ManuallyDrop<T> {
+    fn cmp(&self, other: &Self) -> ::cmp::Ordering {
+        self.deref().cmp(other)
+    }
+}
+
+#[stable(feature = "manually_drop_impls", since = "1.22.0")]
+impl<T: ::hash::Hash> ::hash::Hash for ManuallyDrop<T> {
+    fn hash<H: ::hash::Hasher>(&self, state: &mut H) {
+        self.deref().hash(state);
+    }
+}

src/libcore/mem.rs

@@ -918,7 +918,6 @@ pub fn discriminant<T>(v: &T) -> Discriminant<T> {
     }
 }
 
-
 /// A wrapper to inhibit compiler from automatically calling `T`’s destructor.
 ///
 /// This wrapper is 0-cost.
@@ -954,11 +953,18 @@ pub fn discriminant<T>(v: &T) -> Discriminant<T> {
 ///     }
 /// }
 /// ```
+#[cfg(not(stage0))]
 #[stable(feature = "manually_drop", since = "1.20.0")]
-#[allow(unions_with_drop_fields)]
-#[derive(Copy)]
-pub union ManuallyDrop<T>{ value: T }
+#[lang = "manually_drop"]
+#[derive(Copy, Clone, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct ManuallyDrop<T> {
+    value: T,
+}
+
+#[cfg(stage0)]
+include!("manually_drop_stage0.rs");
 
+#[cfg(not(stage0))]
 impl<T> ManuallyDrop<T> {
     /// Wrap a value to be manually dropped.
     ///
@@ -972,7 +978,7 @@ impl<T> ManuallyDrop<T> {
     #[rustc_const_unstable(feature = "const_manually_drop_new")]
     #[inline]
     pub const fn new(value: T) -> ManuallyDrop<T> {
-        ManuallyDrop { value: value }
+        ManuallyDrop { value }
     }
 
     /// Extract the value from the ManuallyDrop container.
@@ -987,9 +993,7 @@ impl<T> ManuallyDrop<T> {
     #[stable(feature = "manually_drop", since = "1.20.0")]
     #[inline]
     pub fn into_inner(slot: ManuallyDrop<T>) -> T {
-        unsafe {
-            slot.value
-        }
+        slot.value
     }
 
     /// Manually drops the contained value.
@@ -1006,102 +1010,22 @@ impl<T> ManuallyDrop<T> {
     }
 }
 
+#[cfg(not(stage0))]
 #[stable(feature = "manually_drop", since = "1.20.0")]
 impl<T> Deref for ManuallyDrop<T> {
     type Target = T;
     #[inline]
     fn deref(&self) -> &Self::Target {
-        unsafe {
-            &self.value
-        }
+        &self.value
     }
 }
 
+#[cfg(not(stage0))]
 #[stable(feature = "manually_drop", since = "1.20.0")]
 impl<T> DerefMut for ManuallyDrop<T> {
     #[inline]
     fn deref_mut(&mut self) -> &mut Self::Target {
-        unsafe {
-            &mut self.value
-        }
-    }
-}
-
-#[stable(feature = "manually_drop", since = "1.20.0")]
-impl<T: ::fmt::Debug> ::fmt::Debug for ManuallyDrop<T> {
-    fn fmt(&self, fmt: &mut ::fmt::Formatter) -> ::fmt::Result {
-        unsafe {
-            fmt.debug_tuple("ManuallyDrop").field(&self.value).finish()
-        }
-    }
-}
-
-#[stable(feature = "manually_drop_impls", since = "1.22.0")]
-impl<T: Clone> Clone for ManuallyDrop<T> {
-    fn clone(&self) -> Self {
-        ManuallyDrop::new(self.deref().clone())
-    }
-
-    fn clone_from(&mut self, source: &Self) {
-        self.deref_mut().clone_from(source);
-    }
-}
-
-#[stable(feature = "manually_drop_impls", since = "1.22.0")]
-impl<T: Default> Default for ManuallyDrop<T> {
-    fn default() -> Self {
-        ManuallyDrop::new(Default::default())
-    }
-}
-
-#[stable(feature = "manually_drop_impls", since = "1.22.0")]
-impl<T: PartialEq> PartialEq for ManuallyDrop<T> {
-    fn eq(&self, other: &Self) -> bool {
-        self.deref().eq(other)
-    }
-
-    fn ne(&self, other: &Self) -> bool {
-        self.deref().ne(other)
-    }
-}
-
-#[stable(feature = "manually_drop_impls", since = "1.22.0")]
-impl<T: Eq> Eq for ManuallyDrop<T> {}
-
-#[stable(feature = "manually_drop_impls", since = "1.22.0")]
-impl<T: PartialOrd> PartialOrd for ManuallyDrop<T> {
-    fn partial_cmp(&self, other: &Self) -> Option<::cmp::Ordering> {
-        self.deref().partial_cmp(other)
-    }
-
-    fn lt(&self, other: &Self) -> bool {
-        self.deref().lt(other)
-    }
-
-    fn le(&self, other: &Self) -> bool {
-        self.deref().le(other)
-    }
-
-    fn gt(&self, other: &Self) -> bool {
-        self.deref().gt(other)
-    }
-
-    fn ge(&self, other: &Self) -> bool {
-        self.deref().ge(other)
-    }
-}
-
-#[stable(feature = "manually_drop_impls", since = "1.22.0")]
-impl<T: Ord> Ord for ManuallyDrop<T> {
-    fn cmp(&self, other: &Self) -> ::cmp::Ordering {
-        self.deref().cmp(other)
-    }
-}
-
-#[stable(feature = "manually_drop_impls", since = "1.22.0")]
-impl<T: ::hash::Hash> ::hash::Hash for ManuallyDrop<T> {
-    fn hash<H: ::hash::Hasher>(&self, state: &mut H) {
-        self.deref().hash(state);
+        &mut self.value
     }
 }
 

src/librustc/middle/lang_items.rs

@@ -324,6 +324,8 @@ language_item_table! {
 
     NonZeroItem,                     "non_zero",                non_zero;
 
+    ManuallyDropItem,                "manually_drop",           manually_drop;
+
     DebugTraitLangItem,              "debug_trait",             debug_trait;
 
     // A lang item for each of the 128-bit operators we can optionally lower.

src/librustc/traits/query/dropck_outlives.rs

@@ -243,7 +243,10 @@ pub fn trivial_dropck_outlives<'tcx>(tcx: TyCtxt<'_, '_, 'tcx>, ty: Ty<'tcx>) ->
 
         ty::TyAdt(def, _) => {
             if def.is_union() {
-                // Unions never run have a dtor.
+                // Unions never have a dtor.
+                true
+            } else if Some(def.did) == tcx.lang_items().manually_drop() {
+                // `ManuallyDrop` never has a dtor.
                 true
             } else {
                 // Other types might. Moreover, PhantomData doesn't

src/librustc/ty/util.rs

@@ -932,6 +932,9 @@ fn needs_drop_raw<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         // Foreign types can never have destructors
         ty::TyForeign(..) => false,
 
+        // `ManuallyDrop` doesn't have a destructor regardless of field types.
+        ty::TyAdt(def, _) if Some(def.did) == tcx.lang_items().manually_drop() => false,
+
         // Issue #22536: We first query type_moves_by_default.  It sees a
         // normalized version of the type, and therefore will definitely
         // know whether the type implements Copy (and thus needs no
@@ -967,7 +970,8 @@ fn needs_drop_raw<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 
         ty::TyTuple(ref tys) => tys.iter().cloned().any(needs_drop),
 
-        // unions don't have destructors regardless of the child types
+        // unions don't have destructors because of the child types,
+        // only if they manually implement `Drop` (handled above).
         ty::TyAdt(def, _) if def.is_union() => false,
 
         ty::TyAdt(def, substs) =>