Actually keep PlaceMention if requested.

Author: cjgillot

src/lib.rs

@@ -130,6 +130,7 @@ pub const MIRI_DEFAULT_ARGS: &[&str] = &[
     "-Zalways-encode-mir",
     "-Zextra-const-ub-checks",
     "-Zmir-emit-retag",
+    "-Zmir-keep-place-mention",
     "-Zmir-opt-level=0",
     "-Zmir-enable-passes=-CheckAlignment",
 ];

tests/fail/dangling_pointers/dangling_pointer_deref_underscore.rs

@@ -6,6 +6,8 @@ fn main() {
         let b = Box::new(42);
         &*b as *const i32
     };
-    let _ = unsafe { *p }; //~ ERROR: dereferenced after this allocation got freed
+    unsafe {
+        let _ = *p; //~ ERROR: dereferenced after this allocation got freed
+    }
     panic!("this should never print");
 }

tests/fail/dangling_pointers/dangling_pointer_deref_underscore.stderr

@@ -1,8 +1,8 @@
 error: Undefined Behavior: pointer to ALLOC was dereferenced after this allocation got freed
   --> $DIR/dangling_pointer_deref_underscore.rs:LL:CC
    |
-LL |     let _ = unsafe { *p };
-   |                      ^^ pointer to ALLOC was dereferenced after this allocation got freed
+LL |         let _ = *p;
+   |                 ^^ pointer to ALLOC was dereferenced after this allocation got freed
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information