Rollup merge of #40559 - nagisa:manually-drop, r=alexcrichton

Implement Manually Drop

As the RFC has been from approx a week in FCP without any major comments, I’m taking the opportunity to submit the PR early.

12 files changed, 147 insertions, 64 deletions

diff --git a/src/doc/unstable-book/src/SUMMARY.md b/src/doc/unstable-book/src/SUMMARY.md
index 81c75e1326e..54e602a81db 100644
--- a/src/doc/unstable-book/src/SUMMARY.md
+++ b/src/doc/unstable-book/src/SUMMARY.md
@@ -114,6 +114,7 @@
 - [loop_break_value](loop-break-value.md)
 - [macro_reexport](macro-reexport.md)
 - [main](main.md)
+- [manually_drop](manually-drop.md)
 - [map_entry_recover_keys](map-entry-recover-keys.md)
 - [mpsc_select](mpsc-select.md)
 - [n16](n16.md)
diff --git a/src/doc/unstable-book/src/manually-drop.md b/src/doc/unstable-book/src/manually-drop.md
new file mode 100644
index 00000000000..e69de29bb2d
--- /dev/null
+++ b/src/doc/unstable-book/src/manually-drop.md
diff --git a/src/libcollections/lib.rs b/src/libcollections/lib.rs
index 5360804f9e1..99afd08e811 100644
--- a/src/libcollections/lib.rs
+++ b/src/libcollections/lib.rs
@@ -44,6 +44,7 @@
 #![feature(heap_api)]
 #![feature(inclusive_range)]
 #![feature(lang_items)]
+#![feature(manually_drop)]
 #![feature(nonzero)]
 #![feature(pattern)]
 #![feature(placement_in)]
diff --git a/src/libcollections/slice.rs b/src/libcollections/slice.rs
index 6cff315a6cc..3069adb12e9 100644
--- a/src/libcollections/slice.rs
+++ b/src/libcollections/slice.rs
@@ -1558,7 +1558,7 @@ fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
             //    performance than with the 2nd method.
             //
             // All methods were benchmarked, and the 3rd showed best results. So we chose that one.
-            let mut tmp = NoDrop { value: ptr::read(&v[0]) };
+            let mut tmp = mem::ManuallyDrop::new(ptr::read(&v[0]));
 
             // Intermediate state of the insertion process is always tracked by `hole`, which
             // serves two purposes:
@@ -1571,13 +1571,13 @@ fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
             // fill the hole in `v` with `tmp`, thus ensuring that `v` still holds every object it
             // initially held exactly once.
             let mut hole = InsertionHole {
-                src: &mut tmp.value,
+                src: &mut *tmp,
                 dest: &mut v[1],
             };
             ptr::copy_nonoverlapping(&v[1], &mut v[0], 1);
 
             for i in 2..v.len() {
-                if !is_less(&v[i], &tmp.value) {
+                if !is_less(&v[i], &*tmp) {
                     break;
                 }
                 ptr::copy_nonoverlapping(&v[i], &mut v[i - 1], 1);
@@ -1587,12 +1587,6 @@ fn insert_head<T, F>(v: &mut [T], is_less: &mut F)
         }
     }
 
-    // Holds a value, but never drops it.
-    #[allow(unions_with_drop_fields)]
-    union NoDrop<T> {
-        value: T
-    }
-
     // When dropped, copies from `src` into `dest`.
     struct InsertionHole<T> {
         src: *mut T,
diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index e0a4707ff66..b0287631585 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -691,9 +691,6 @@ extern "rust-intrinsic" {
     /// initialize memory previous set to the result of `uninit`.
     pub fn uninit<T>() -> T;
 
-    /// Moves a value out of scope without running drop glue.
-    pub fn forget<T>(_: T) -> ();
-
     /// Reinterprets the bits of a value of one type as another type.
     ///
     /// Both types must have the same size. Neither the original, nor the result,
diff --git a/src/libcore/mem.rs b/src/libcore/mem.rs
index f5cf3724d07..7be927b28ed 100644
--- a/src/libcore/mem.rs
+++ b/src/libcore/mem.rs
@@ -171,7 +171,7 @@ pub use intrinsics::transmute;
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn forget<T>(t: T) {
-    unsafe { intrinsics::forget(t) }
+    ManuallyDrop::new(t);
 }
 
 /// Returns the size of a type in bytes.
@@ -736,3 +736,121 @@ pub fn discriminant<T>(v: &T) -> Discriminant<T> {
     }
 }
 
+
+/// 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
+/// # #![feature(manually_drop)]
+/// 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`.
+///     }
+/// }
+/// ```
+#[unstable(feature = "manually_drop", issue = "40673")]
+#[allow(unions_with_drop_fields)]
+pub union ManuallyDrop<T>{ value: T }
+
+impl<T> ManuallyDrop<T> {
+    /// Wrap a value to be manually dropped.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # #![feature(manually_drop)]
+    /// use std::mem::ManuallyDrop;
+    /// ManuallyDrop::new(Box::new(()));
+    /// ```
+    #[unstable(feature = "manually_drop", issue = "40673")]
+    #[inline]
+    pub fn new(value: T) -> ManuallyDrop<T> {
+        ManuallyDrop { value: value }
+    }
+
+    /// Extract the value from the ManuallyDrop container.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # #![feature(manually_drop)]
+    /// use std::mem::ManuallyDrop;
+    /// let x = ManuallyDrop::new(Box::new(()));
+    /// let _: Box<()> = ManuallyDrop::into_inner(x);
+    /// ```
+    #[unstable(feature = "manually_drop", issue = "40673")]
+    #[inline]
+    pub fn into_inner(slot: ManuallyDrop<T>) -> T {
+        unsafe {
+            slot.value
+        }
+    }
+
+    /// Manually drops the contained value.
+    ///
+    /// # Unsafety
+    ///
+    /// 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.
+    #[unstable(feature = "manually_drop", issue = "40673")]
+    #[inline]
+    pub unsafe fn drop(slot: &mut ManuallyDrop<T>) {
+        ptr::drop_in_place(&mut slot.value)
+    }
+}
+
+#[unstable(feature = "manually_drop", issue = "40673")]
+impl<T> ::ops::Deref for ManuallyDrop<T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        unsafe {
+            &self.value
+        }
+    }
+}
+
+#[unstable(feature = "manually_drop", issue = "40673")]
+impl<T> ::ops::DerefMut for ManuallyDrop<T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        unsafe {
+            &mut self.value
+        }
+    }
+}
+
+#[unstable(feature = "manually_drop", issue = "40673")]
+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()
+        }
+    }
+}
diff --git a/src/libcore/slice/sort.rs b/src/libcore/slice/sort.rs
index 7065fdb79fc..6f9f2915dfe 100644
--- a/src/libcore/slice/sort.rs
+++ b/src/libcore/slice/sort.rs
@@ -20,12 +20,6 @@ use cmp;
 use mem;
 use ptr;
 
-/// Holds a value, but never drops it.
-#[allow(unions_with_drop_fields)]
-union NoDrop<T> {
-    value: T
-}
-
 /// When dropped, copies from `src` into `dest`.
 struct CopyOnDrop<T> {
     src: *mut T,
@@ -49,15 +43,15 @@ fn shift_head<T, F>(v: &mut [T], is_less: &mut F)
             // Read the first element into a stack-allocated variable. If a following comparison
             // operation panics, `hole` will get dropped and automatically write the element back
             // into the slice.
-            let mut tmp = NoDrop { value: ptr::read(v.get_unchecked(0)) };
+            let mut tmp = mem::ManuallyDrop::new(ptr::read(v.get_unchecked(0)));
             let mut hole = CopyOnDrop {
-                src: &mut tmp.value,
+                src: &mut *tmp,
                 dest: v.get_unchecked_mut(1),
             };
             ptr::copy_nonoverlapping(v.get_unchecked(1), v.get_unchecked_mut(0), 1);
 
             for i in 2..len {
-                if !is_less(v.get_unchecked(i), &tmp.value) {
+                if !is_less(v.get_unchecked(i), &*tmp) {
                     break;
                 }
 
@@ -81,15 +75,15 @@ fn shift_tail<T, F>(v: &mut [T], is_less: &mut F)
             // Read the last element into a stack-allocated variable. If a following comparison
             // operation panics, `hole` will get dropped and automatically write the element back
             // into the slice.
-            let mut tmp = NoDrop { value: ptr::read(v.get_unchecked(len - 1)) };
+            let mut tmp = mem::ManuallyDrop::new(ptr::read(v.get_unchecked(len - 1)));
             let mut hole = CopyOnDrop {
-                src: &mut tmp.value,
+                src: &mut *tmp,
                 dest: v.get_unchecked_mut(len - 2),
             };
             ptr::copy_nonoverlapping(v.get_unchecked(len - 2), v.get_unchecked_mut(len - 1), 1);
 
             for i in (0..len-2).rev() {
-                if !is_less(&tmp.value, v.get_unchecked(i)) {
+                if !is_less(&*tmp, v.get_unchecked(i)) {
                     break;
                 }
 
@@ -403,12 +397,12 @@ fn partition<T, F>(v: &mut [T], pivot: usize, is_less: &mut F) -> (usize, bool)
 
         // Read the pivot into a stack-allocated variable for efficiency. If a following comparison
         // operation panics, the pivot will be automatically written back into the slice.
-        let mut tmp = NoDrop { value: unsafe { ptr::read(pivot) } };
+        let mut tmp = mem::ManuallyDrop::new(unsafe { ptr::read(pivot) });
         let _pivot_guard = CopyOnDrop {
-            src: unsafe { &mut tmp.value },
+            src: &mut *tmp,
             dest: pivot,
         };
-        let pivot = unsafe { &tmp.value };
+        let pivot = &*tmp;
 
         // Find the first pair of out-of-order elements.
         let mut l = 0;
@@ -452,12 +446,12 @@ fn partition_equal<T, F>(v: &mut [T], pivot: usize, is_less: &mut F) -> usize
 
     // Read the pivot into a stack-allocated variable for efficiency. If a following comparison
     // operation panics, the pivot will be automatically written back into the slice.
-    let mut tmp = NoDrop { value: unsafe { ptr::read(pivot) } };
+    let mut tmp = mem::ManuallyDrop::new(unsafe { ptr::read(pivot) });
     let _pivot_guard = CopyOnDrop {
-        src: unsafe { &mut tmp.value },
+        src: &mut *tmp,
         dest: pivot,
     };
-    let pivot = unsafe { &tmp.value };
+    let pivot = &*tmp;
 
     // Now partition the slice.
     let mut l = 0;
diff --git a/src/librustc_data_structures/array_vec.rs b/src/librustc_data_structures/array_vec.rs
index 29fbcb70756..adb22197226 100644
--- a/src/librustc_data_structures/array_vec.rs
+++ b/src/librustc_data_structures/array_vec.rs
@@ -20,10 +20,11 @@ use std::fmt;
 use std::mem;
 use std::collections::range::RangeArgument;
 use std::collections::Bound::{Excluded, Included, Unbounded};
+use std::mem::ManuallyDrop;
 
 pub unsafe trait Array {
     type Element;
-    type PartialStorage: Default + Unsize<[ManuallyDrop<Self::Element>]>;
+    type PartialStorage: Unsize<[ManuallyDrop<Self::Element>]>;
     const LEN: usize;
 }
 
@@ -66,7 +67,7 @@ impl<A: Array> ArrayVec<A> {
     pub fn new() -> Self {
         ArrayVec {
             count: 0,
-            values: Default::default(),
+            values: unsafe { ::std::mem::uninitialized() },
         }
     }
 
@@ -81,7 +82,7 @@ impl<A: Array> ArrayVec<A> {
     /// Panics when the stack vector is full.
     pub fn push(&mut self, el: A::Element) {
         let arr = &mut self.values as &mut [ManuallyDrop<_>];
-        arr[self.count] = ManuallyDrop { value: el };
+        arr[self.count] = ManuallyDrop::new(el);
         self.count += 1;
     }
 
@@ -90,8 +91,8 @@ impl<A: Array> ArrayVec<A> {
             let arr = &mut self.values as &mut [ManuallyDrop<_>];
             self.count -= 1;
             unsafe {
-                let value = ptr::read(&arr[self.count]);
-                Some(value.value)
+                let value = ptr::read(&*arr[self.count]);
+                Some(value)
             }
         } else {
             None
@@ -210,7 +211,7 @@ impl<A: Array> Iterator for Iter<A> {
     fn next(&mut self) -> Option<A::Element> {
         let arr = &self.store as &[ManuallyDrop<_>];
         unsafe {
-            self.indices.next().map(|i| ptr::read(&arr[i]).value)
+            self.indices.next().map(|i| ptr::read(&*arr[i]))
         }
     }
 
@@ -233,7 +234,7 @@ impl<'a, A: Array> Iterator for Drain<'a, A> {
 
     #[inline]
     fn next(&mut self) -> Option<A::Element> {
-        self.iter.next().map(|elt| unsafe { ptr::read(elt as *const ManuallyDrop<_>).value })
+        self.iter.next().map(|elt| unsafe { ptr::read(&**elt) })
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
@@ -295,25 +296,3 @@ impl<'a, A: Array> IntoIterator for &'a mut ArrayVec<A> {
         self.iter_mut()
     }
 }
-
-// FIXME: This should use repr(transparent) from rust-lang/rfcs#1758.
-#[allow(unions_with_drop_fields)]
-pub union ManuallyDrop<T> {
-    value: T,
-    #[allow(dead_code)]
-    empty: (),
-}
-
-impl<T> ManuallyDrop<T> {
-    fn new() -> ManuallyDrop<T> {
-        ManuallyDrop {
-            empty: ()
-        }
-    }
-}
-
-impl<T> Default for ManuallyDrop<T> {
-    fn default() -> Self {
-        ManuallyDrop::new()
-    }
-}
diff --git a/src/librustc_data_structures/lib.rs b/src/librustc_data_structures/lib.rs
index c1735b4a4ec..72c533a7461 100644
--- a/src/librustc_data_structures/lib.rs
+++ b/src/librustc_data_structures/lib.rs
@@ -39,6 +39,7 @@
 #![feature(conservative_impl_trait)]
 #![feature(discriminant_value)]
 #![feature(specialization)]
+#![feature(manually_drop)]
 
 #![cfg_attr(unix, feature(libc))]
 #![cfg_attr(test, feature(test))]
diff --git a/src/librustc_trans/intrinsic.rs b/src/librustc_trans/intrinsic.rs
index 7cfc28622c4..7077eade611 100644
--- a/src/librustc_trans/intrinsic.rs
+++ b/src/librustc_trans/intrinsic.rs
@@ -186,7 +186,7 @@ pub fn trans_intrinsic_call<'a, 'tcx>(bcx: &Builder<'a, 'tcx>,
             C_nil(ccx)
         }
         // Effectively no-ops
-        "uninit" | "forget" => {
+        "uninit" => {
             C_nil(ccx)
         }
         "needs_drop" => {
diff --git a/src/librustc_typeck/check/intrinsic.rs b/src/librustc_typeck/check/intrinsic.rs
index 2861fd28832..cd58fcd4806 100644
--- a/src/librustc_typeck/check/intrinsic.rs
+++ b/src/librustc_typeck/check/intrinsic.rs
@@ -124,7 +124,6 @@ pub fn check_intrinsic_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
             "rustc_peek" => (1, vec![param(0)], param(0)),
             "init" => (1, Vec::new(), param(0)),
             "uninit" => (1, Vec::new(), param(0)),
-            "forget" => (1, vec![ param(0) ], tcx.mk_nil()),
             "transmute" => (2, vec![ param(0) ], param(1)),
             "move_val_init" => {
                 (1,
diff --git a/src/test/compile-fail/forget-init-unsafe.rs b/src/test/compile-fail/forget-init-unsafe.rs
index 521f122f8af..48c9fda31e8 100644
--- a/src/test/compile-fail/forget-init-unsafe.rs
+++ b/src/test/compile-fail/forget-init-unsafe.rs
@@ -10,10 +10,9 @@
 
 #![feature(core_intrinsics)]
 
-use std::intrinsics::{init, forget};
+use std::intrinsics::{init};
 
 // Test that the `forget` and `init` intrinsics are really unsafe
 pub fn main() {
     let stuff = init::<isize>(); //~ ERROR call to unsafe function requires unsafe
-    forget(stuff);             //~ ERROR call to unsafe function requires unsafe
 }