Rollup merge of rust-lang#34951 - tomgarcia:covariant-vec, r=brson

Make vec::Drain and binary_heap::Drain covariant

I removed all mutable pointers/references, and added covariance tests similar to the ones in rust-lang#32635. It builds and passes the tests, but I noticed that there weren't any tests of Drain's behaviour (at least not in libcollectionstest), so I'm not sure if my changes accidently broke Drain's behaviour. Should I add some tests for that (and if so, what should the tests include)?

src/libcollections/vec.rs

@@ -73,6 +73,7 @@ use core::mem;
 use core::ops::{Index, IndexMut};
 use core::ops;
 use core::ptr;
+use core::ptr::Shared;
 use core::slice;
 
 use super::SpecExtend;
@@ -899,8 +900,8 @@ impl<T> Vec<T> {
             Drain {
                 tail_start: end,
                 tail_len: len - end,
-                iter: range_slice.iter_mut(),
-                vec: self as *mut _,
+                iter: range_slice.iter(),
+                vec: Shared::new(self as *mut _),
             }
         }
     }
@@ -1806,8 +1807,8 @@ pub struct Drain<'a, T: 'a> {
     /// Length of tail
     tail_len: usize,
     /// Current remaining range to remove
-    iter: slice::IterMut<'a, T>,
-    vec: *mut Vec<T>,
+    iter: slice::Iter<'a, T>,
+    vec: Shared<Vec<T>>,
 }
 
 #[stable(feature = "drain", since = "1.6.0")]
@@ -1845,7 +1846,7 @@ impl<'a, T> Drop for Drain<'a, T> {
 
         if self.tail_len > 0 {
             unsafe {
-                let source_vec = &mut *self.vec;
+                let source_vec = &mut **self.vec;
                 // memmove back untouched tail, update to new length
                 let start = source_vec.len();
                 let tail = self.tail_start;

src/libcollectionstest/binary_heap.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 use std::collections::BinaryHeap;
+use std::collections::binary_heap::Drain;
 
 #[test]
 fn test_iterator() {
@@ -292,3 +293,8 @@ fn test_extend_specialization() {
 
     assert_eq!(a.into_sorted_vec(), [-20, -10, 1, 2, 3, 3, 5, 43]);
 }
+
+#[allow(dead_code)]
+fn assert_covariance() {
+    fn drain<'new>(d: Drain<'static, &'static str>) -> Drain<'new, &'new str> { d }
+}

src/libcollectionstest/vec.rs

@@ -11,6 +11,7 @@
 use std::borrow::Cow;
 use std::iter::{FromIterator, repeat};
 use std::mem::size_of;
+use std::vec::Drain;
 
 use test::Bencher;
 
@@ -510,6 +511,11 @@ fn test_cow_from() {
     }
 }
 
+#[allow(dead_code)]
+fn assert_covariance() {
+    fn drain<'new>(d: Drain<'static, &'static str>) -> Drain<'new, &'new str> { d }
+}
+
 #[bench]
 fn bench_new(b: &mut Bencher) {
     b.iter(|| {