──────────────────────────────────────────────────
Warning: this tree is indicative only. Some tags may have been hidden.
0..   4..   8
| Act | Act |    └─┬──<TAG=root of the allocation>
| Frz |?Cel |      └────<TAG=a>
──────────────────────────────────────────────────
error: Undefined Behavior: write access through <TAG> (a) at ALLOC[0x0] is forbidden
  --> tests/fail/tree_borrows/cell-inside-struct.rs:LL:CC
   |
LL |         (*a).field1 = 88;
   |         ^^^^^^^^^^^^^^^^ 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> (a) has state Frozen which forbids this child write access
help: the accessed tag <TAG> was created here, in the initial state Cell
  --> tests/fail/tree_borrows/cell-inside-struct.rs:LL:CC
   |
LL |         let a = &root;
   |                 ^^^^^
   = note: BACKTRACE (of the first span):
   = note: inside `main` at tests/fail/tree_borrows/cell-inside-struct.rs:LL:CC

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

error: aborting due to 1 previous error