Merge rayon-rs#1032

1032: Use phantomdata in CopyOnDrop r=cuviper a=Manishearth

There is a struct with a destructor borrowing from a local, it would be good to have some lifetimes protecting it from doing the wrong thing.

mergesort.rs also has this type but it's constructing it from raw pointers and I didn't want to infect everything with lifetimes. It's less of a problem there since it's *not* borrowing from locals.

Co-authored-by: Manish Goregaokar <manishsmail@gmail.com>

Author: bors[bot]

src/slice/quicksort.rs

@@ -5,16 +5,34 @@
 //! `rayon_core::join`.
 
 use std::cmp;
+use std::marker::PhantomData;
 use std::mem::{self, MaybeUninit};
 use std::ptr;
 
 /// When dropped, copies from `src` into `dest`.
-struct CopyOnDrop<T> {
+#[must_use]
+struct CopyOnDrop<'a, T> {
     src: *const T,
     dest: *mut T,
+    /// `src` is often a local pointer here, make sure we have appropriate
+    /// PhantomData so that dropck can protect us.
+    marker: PhantomData<&'a mut T>,
 }
 
-impl<T> Drop for CopyOnDrop<T> {
+impl<'a, T> CopyOnDrop<'a, T> {
+    /// Construct from a source pointer and a destination
+    /// Assumes dest lives longer than src, since there is no easy way to
+    /// copy down lifetime information from another pointer
+    unsafe fn new(src: &'a T, dest: *mut T) -> Self {
+        CopyOnDrop {
+            src,
+            dest,
+            marker: PhantomData,
+        }
+    }
+}
+
+impl<T> Drop for CopyOnDrop<'_, T> {
     fn drop(&mut self) {
         // SAFETY:  This is a helper class.
         //          Please refer to its usage for correctness.
@@ -54,10 +72,7 @@ where
             // into the slice.
             let tmp = mem::ManuallyDrop::new(ptr::read(v.get_unchecked(0)));
             let v = v.as_mut_ptr();
-            let mut hole = CopyOnDrop {
-                src: &*tmp,
-                dest: v.add(1),
-            };
+            let mut hole = CopyOnDrop::new(&*tmp, v.add(1));
             ptr::copy_nonoverlapping(v.add(1), v.add(0), 1);
 
             for i in 2..len {
@@ -103,10 +118,7 @@ where
             // into the slice.
             let tmp = mem::ManuallyDrop::new(ptr::read(v.get_unchecked(len - 1)));
             let v = v.as_mut_ptr();
-            let mut hole = CopyOnDrop {
-                src: &*tmp,
-                dest: v.add(len - 2),
-            };
+            let mut hole = CopyOnDrop::new(&*tmp, v.add(len - 2));
             ptr::copy_nonoverlapping(v.add(len - 2), v.add(len - 1), 1);
 
             for i in (0..len - 2).rev() {
@@ -510,10 +522,7 @@ where
 
         // SAFETY: `pivot` is a reference to the first element of `v`, so `ptr::read` is safe.
         let tmp = mem::ManuallyDrop::new(unsafe { ptr::read(pivot) });
-        let _pivot_guard = CopyOnDrop {
-            src: &*tmp,
-            dest: pivot,
-        };
+        let _pivot_guard = unsafe { CopyOnDrop::new(&*tmp, pivot) };
         let pivot = &*tmp;
 
         // Find the first pair of out-of-order elements.
@@ -569,10 +578,7 @@ where
     // operation panics, the pivot will be automatically written back into the slice.
     // SAFETY: The pointer here is valid because it is obtained from a reference to a slice.
     let tmp = mem::ManuallyDrop::new(unsafe { ptr::read(pivot) });
-    let _pivot_guard = CopyOnDrop {
-        src: &*tmp,
-        dest: pivot,
-    };
+    let _pivot_guard = unsafe { CopyOnDrop::new(&*tmp, pivot) };
     let pivot = &*tmp;
 
     // Now partition the slice.