error: Undefined Behavior: reborrow through <TAG> at ALLOC[0x0] is forbidden
  --> tests/fail/tree_borrows/write-during-2phase.rs:LL:CC
   |
LL |     fn add(&mut self, n: u64) -> u64 {
   |            ^^^^^^^^^ Undefined Behavior occurred here
   |
   = 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: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/tree-borrows.md for further information
   = help: the accessed tag <TAG> has state Disabled which forbids this reborrow (acting as a child read access)
help: the accessed tag <TAG> was created here, in the initial state Reserved
  --> tests/fail/tree_borrows/write-during-2phase.rs:LL:CC
   |
LL |     let res = f.add(unsafe {
   |               ^
help: the accessed tag <TAG> later transitioned to Disabled due to a foreign write access at offsets [0x0..0x8]
  --> tests/fail/tree_borrows/write-during-2phase.rs:LL:CC
   |
LL |         *alias = 42;
   |         ^^^^^^^^^^^
   = help: this transition corresponds to a loss of read and write permissions
   = note: BACKTRACE (of the first span):
   = note: inside `Foo::add` at tests/fail/tree_borrows/write-during-2phase.rs:LL:CC
note: inside `main`
  --> tests/fail/tree_borrows/write-during-2phase.rs:LL:CC
   |
LL |       let res = f.add(unsafe {
   |  _______________^
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 | |         *alias = 42;
LL | |         0
LL | |     });
   | |______^

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

error: aborting due to 1 previous error