error: Undefined Behavior: read access through <TAG> is forbidden
  --> $DIR/write-during-2phase.rs:LL:CC
   |
LL |     fn add(&mut self, n: u64) -> u64 {
   |            ^^^^^^^^^ read access through <TAG> is forbidden
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Tree Borrows rules it violated are still experimental
   = help: the accessed tag <TAG> has state Disabled which forbids child read accesses
help: the accessed tag <TAG> was created here, in the initial state Reserved
  --> $DIR/write-during-2phase.rs:LL:CC
   |
LL |       let _res = f.add(unsafe {
   |  ________________^
LL | |         let n = f.0;
LL | |         // This is the access at fault, but it's not immediately apparent because
LL | |         // the reference that got invalidated is not under a Protector.
LL | |         *inner = 42;
LL | |         n
LL | |     });
   | |______^
help: the accessed tag <TAG> then transitioned from Reserved to Disabled due to a foreign write access at offsets [0x0..0x8]
  --> $DIR/write-during-2phase.rs:LL:CC
   |
LL |         *inner = 42;
   |         ^^^^^^^^^^^
   = help: this corresponds to a loss of read and write permissions
   = note: BACKTRACE (of the first span):
   = note: inside `Foo::add` at $DIR/write-during-2phase.rs:LL:CC
note: inside `main`
  --> $DIR/write-during-2phase.rs:LL:CC
   |
LL |       let _res = f.add(unsafe {
   |  ________________^
LL | |         let n = f.0;
LL | |         // This is the access at fault, but it's not immediately apparent because
LL | |         // the reference that got invalidated is not under a Protector.
LL | |         *inner = 42;
LL | |         n
LL | |     });
   | |______^

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error