modifying immutable refenrences via unsafe no longer works, when building a nightly release about rust

[Nilstrieb]

closing as not a bug

[saethlin]

This code is trying to execute UB. It happened to work as you wanted before, by sheer luck. The problem is reported by running the code in Miri, which you can do via the playground or locally. The error is this:

error: Undefined Behavior: trying to retag from <2677> for Unique permission at alloc1188[0x0], but that tag only grants SharedReadOnly permission for this location
  --> src/main.rs:15:15
   |
15 |             f(&mut *self.0);
   |               ^^^^^^^^^^^^
   |               |
   |               trying to retag from <2677> for Unique permission at alloc1188[0x0], but that tag only grants SharedReadOnly permission for this location
   |               this error occurs as part of retag at alloc1188[0x0..0x1]
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <2677> was created by a SharedReadOnly retag at offsets [0x0..0x1]
  --> src/main.rs:7:14
   |
7  |         Self(t as *const T as *mut T)
   |              ^
   = note: BACKTRACE (of the first span):
   = note: inside `ResMut::<A>::patch_by::<{closure@src/main.rs:45:19: 45:22}>` at src/main.rs:15:15: 15:27
note: inside `foo`
  --> src/main.rs:44:5
   |
44 | /     ResMut::new(&v[0])
45 | |         .patch_by(|x| x.a = B::Bar).unwrap();
   | |___________________________________^
note: inside `main`
  --> src/main.rs:39:5
   |
39 |     foo(&v);
   |     ^^^^^^^

[Soveu]

Well, in more simple terms, you're just modifying immutable data - that is instant UB in Rust.
Maybe what you're looking for is UnsafeCell or some kind of its derivative (RefCell, Mutex, RwLock)