add example of &T -> *mut T

Author: nikomatsakis

litmus_tests/shared_ref_to_star_mut.md

@@ -0,0 +1,34 @@
+### Example
+
+```
+pub unsafe fn a() -> u8 {
+    let mut x = 11;
+    b(&x as *const u8 as *mut u8);
+    x
+}
+
+unsafe fn b(x: *mut u8) {
+    *x = 22;
+}
+```
+
+### Explanation
+
+Here the user takes a shared reference `&x` which is then cast to a
+`*mut` reference and ater used to mutate the variable `x`. Subtle
+aspects here:
+
+- The lifetime of the `&x` reference will be inferred very short (just
+  the duration of the cast, basically), so from a certain naive POV
+  `x` is "not borrowed" and yet its value changes.
+- The result of `&x` cannot, in safe code, be used to mutate `x`, and
+  yet its value changes.
+  
+Based on either of the above points, one might think the function
+above could return `11`, because the compiler might constant propagate
+`11` from the `let mux x = 11` to the return value under the
+assumption that it cannot change.
+
+### Source
+
+https://github.com/rust-lang/rust/issues/30424#issue-122623932