Rollup merge of #107954 - RalfJung:tree-borrows-fix, r=m-ou-se

avoid mixing accesses of ptrs derived from a mutable ref and parent ptrs

``@Vanille-N`` is working on a successor for Stacked Borrows. It will mostly accept strictly more code than Stacked Borrows did, with one exception: the following pattern no longer works.
```rust
let mut root = 6u8;
let mref = &mut root;
let ptr = mref as *mut u8;
*ptr = 0; // Write
assert_eq!(root, 0); // Parent Read
*ptr = 0; // Attempted Write
```
This worked in Stacked Borrows kind of by accident: when doing the "parent read", under SB we Disable `mref`, but the raw ptrs derived from it remain usable. The fact that we can still use the "children" of a reference that is no longer usable is quite nasty and leads to some undesirable effects (in particular it is the major blocker for resolving https://github.com/rust-lang/unsafe-code-guidelines/issues/257). So in Tree Borrows we no longer do that; instead, reading from `root` makes `mref` and all its children read-only.

Due to other improvements in Tree Borrows, the entire Miri test suite still passes with this new behavior, and even the entire libcore and liballoc test suite, except for these 2 cases this PR fixes. Both of these involve code where the programmer wrote `&mut` but then used pointers derived from that reference in ways that alias with the parent pointer, which arguably is violating uniqueness. They are fixed by properly using raw pointers throughout.

1 files changed, 3 insertions, 3 deletions

diff --git a/library/core/tests/ptr.rs b/library/core/tests/ptr.rs
index 80d30f14c66..c02cd99cc44 100644
--- a/library/core/tests/ptr.rs
+++ b/library/core/tests/ptr.rs
@@ -25,7 +25,7 @@ fn test() {
             snd: isize,
         }
         let mut p = Pair { fst: 10, snd: 20 };
-        let pptr: *mut Pair = &mut p;
+        let pptr: *mut Pair = addr_of_mut!(p);
         let iptr: *mut isize = pptr as *mut isize;
         assert_eq!(*iptr, 10);
         *iptr = 30;
@@ -1070,8 +1070,8 @@ fn swap_copy_untyped() {
     let mut x = 5u8;
     let mut y = 6u8;
 
-    let ptr1 = &mut x as *mut u8 as *mut bool;
-    let ptr2 = &mut y as *mut u8 as *mut bool;
+    let ptr1 = addr_of_mut!(x).cast::<bool>();
+    let ptr2 = addr_of_mut!(y).cast::<bool>();
 
     unsafe {
         ptr::swap(ptr1, ptr2);