Auto merge of #2077 - RalfJung:more-test, r=RalfJung

add another test for #2068

Author: bors

tests/run-pass/issue-miri-2068-2.rs

@@ -0,0 +1,14 @@
+// compile-flags: -Zmiri-disable-validation
+
+use std::mem::MaybeUninit;
+
+fn main() { unsafe {
+    let mut x = MaybeUninit::<i64>::uninit();
+    // Put in a ptr.
+    x.as_mut_ptr().cast::<&i32>().write_unaligned(&0);
+    // Overwrite parts of that pointer with 'uninit' through a Scalar.
+    let ptr = x.as_mut_ptr().cast::<i32>();
+    *ptr = MaybeUninit::uninit().assume_init();
+    // Reading this back should hence work fine.
+    let _c = *ptr;
+} }

tests/run-pass/stacked-borrows/stacked-borrows.rs

@@ -16,6 +16,7 @@ fn main() {
     disjoint_mutable_subborrows();
     raw_ref_to_part();
     array_casts();
+    mut_below_shr();
 }
 
 // Make sure that reading from an `&mut` does, like reborrowing to `&`,
@@ -186,3 +187,12 @@ fn array_casts() {
     let p = &x as *const usize;
     assert_eq!(unsafe { *p.add(1) }, 1);
 }
+
+/// Transmuting &&i32 to &&mut i32 is fine.
+fn mut_below_shr() {
+    let x = 0;
+    let y = &x;
+    let p = unsafe { core::mem::transmute::<&&i32,&&mut i32>(&y) };
+    let r = &**p;
+    let _val = *r;
+}