Auto merge of #52711 - eddyb:unsized-manuallydrop, r=nikomatsakis

Change ManuallyDrop<T> to a lang item.

This PR implements the approach @RalfJung proposes in https://internals.rust-lang.org/t/pre-rfc-unions-drop-types-and-manuallydrop/8025 (lang item `struct` instead of `union`).

A followup PR can easily solve #47034 as well, by just adding a few `?Sized` to `libcore/mem.rs`.

r? @nikomatsakis

4 files changed, 236 insertions, 92 deletions

diff --git a/src/libcore/manually_drop_stage0.rs b/src/libcore/manually_drop_stage0.rs
new file mode 100644
index 00000000000..8643219cb61
--- /dev/null
+++ b/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);
+    }
+}
diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index a0fe6e98806..1a54f03bb00 100644
--- a/src/libcore/mem.rs
+++ b/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
     }
 }
 
diff --git a/src/libcore/tests/lib.rs b/src/libcore/tests/lib.rs
index ca7db6e4639..6fcfaae4535 100644
--- a/src/libcore/tests/lib.rs
+++ b/src/libcore/tests/lib.rs
@@ -62,6 +62,7 @@ mod fmt;
 mod hash;
 mod intrinsics;
 mod iter;
+mod manually_drop;
 mod mem;
 mod nonzero;
 mod num;
diff --git a/src/libcore/tests/manually_drop.rs b/src/libcore/tests/manually_drop.rs
new file mode 100644
index 00000000000..96bc9247da6
--- /dev/null
+++ b/src/libcore/tests/manually_drop.rs
@@ -0,0 +1,24 @@
+// 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.
+
+use core::mem::ManuallyDrop;
+
+#[test]
+fn smoke() {
+    struct TypeWithDrop;
+    impl Drop for TypeWithDrop {
+        fn drop(&mut self) {
+            unreachable!("Should not get dropped");
+        }
+    }
+
+    let x = ManuallyDrop::new(TypeWithDrop);
+    drop(x);
+}