Rollup merge of rust-lang#78476 - RalfJung:btree-alias, r=Mark-Simulacrum

fix some incorrect aliasing in the BTree

This line is wrong:
```
ptr::copy(slice.as_ptr().add(idx), slice.as_mut_ptr().add(idx + 1), slice.len() - idx);
```
When `slice.as_mut_ptr()` is called, that creates a mutable reference to the entire slice, which invalidates the raw pointer previously returned by `slice.as_ptr()`. (Miri currently misses this because raw pointers are not tracked properly.)

Cc ````````@ssomers````````

Author: Dylan-DPC

library/alloc/src/collections/btree/node.rs

@@ -1608,15 +1608,19 @@ pub mod marker {
 
 unsafe fn slice_insert<T>(slice: &mut [T], idx: usize, val: T) {
     unsafe {
-        ptr::copy(slice.as_ptr().add(idx), slice.as_mut_ptr().add(idx + 1), slice.len() - idx);
-        ptr::write(slice.get_unchecked_mut(idx), val);
+        let len = slice.len();
+        let slice_ptr = slice.as_mut_ptr();
+        ptr::copy(slice_ptr.add(idx), slice_ptr.add(idx + 1), len - idx);
+        ptr::write(slice_ptr.add(idx), val);
     }
 }
 
 unsafe fn slice_remove<T>(slice: &mut [T], idx: usize) -> T {
     unsafe {
-        let ret = ptr::read(slice.get_unchecked(idx));
-        ptr::copy(slice.as_ptr().add(idx + 1), slice.as_mut_ptr().add(idx), slice.len() - idx - 1);
+        let len = slice.len();
+        let slice_ptr = slice.as_mut_ptr();
+        let ret = ptr::read(slice_ptr.add(idx));
+        ptr::copy(slice_ptr.add(idx + 1), slice_ptr.add(idx), len - idx - 1);
         ret
     }
 }