Rollup merge of rust-lang#61082 - RalfJung:vec, r=alexcrichton

fix dangling reference in Vec::append

Turns out I forgot to enable Miri again for the Vec tests. And there was a dangling reference hiding in there!  `get_unchecked_mut` is UB to call on an empty vector (there is no memory to get a reference to), and yet this code did it.

src/liballoc/tests/vec.rs

@@ -1,5 +1,3 @@
-#![cfg(not(miri))]
-
 use std::borrow::Cow;
 use std::mem::size_of;
 use std::{usize, isize};
@@ -763,6 +761,7 @@ fn from_into_inner() {
     it.next().unwrap();
     let vec = it.collect::<Vec<_>>();
     assert_eq!(vec, [2, 3]);
+    #[cfg(not(miri))] // Miri does not support comparing dangling pointers
     assert!(ptr != vec.as_ptr());
 }
 
@@ -971,6 +970,7 @@ fn test_reserve_exact() {
 }
 
 #[test]
+#[cfg(not(miri))] // Miri does not support signalling OOM
 fn test_try_reserve() {
 
     // These are the interesting cases:
@@ -1073,6 +1073,7 @@ fn test_try_reserve() {
 }
 
 #[test]
+#[cfg(not(miri))] // Miri does not support signalling OOM
 fn test_try_reserve_exact() {
 
     // This is exactly the same as test_try_reserve with the method changed.

src/liballoc/vec.rs

@@ -1094,7 +1094,7 @@ impl<T> Vec<T> {
         let count = (*other).len();
         self.reserve(count);
         let len = self.len();
-        ptr::copy_nonoverlapping(other as *const T, self.get_unchecked_mut(len), count);
+        ptr::copy_nonoverlapping(other as *const T, self.as_mut_ptr().add(len), count);
         self.len += count;
     }