Miri

Miri is a Rust interpreter for MIR — the compiler's mid-level IR. It runs your program's tests one MIR statement at a time, with full provenance tracking, and screams the moment you violate the abstract machine. It's the only tool that will catch the bug from the previous section.

1. 01
   Miri ships as a rustup component on the nightly toolchain. Install once, then cargo +nightly miri test runs your test suite under Miri. It is slow — expect 50–500x slowdown — but it is exhaustive.

   $ rustup toolchain install nightly
   $ rustup +nightly component add miri
   $ cargo +nightly miri test

2. 02
   Run it against the previous section's queue. Miri flags the first deref of self.tail after the box has been moved, with a stacked-borrows trace pointing at the offending tag.

   error: Undefined Behavior: trying to retag from <1234> for SharedReadWrite
     permission, but that tag does not exist in the borrow stack for this location
     --> src/lib.rs:23:13
      |
   23 |   unsafe { (*self.tail).next = Some(new_node); }
      |            ^^^^^^^^^^^^^^^^
      |            |
      |            trying to retag from <1234> for SharedReadWrite permission
      |            <1234> was created by a SharedReadWrite retag at offsets [0..24]
      = help: this indicates a potential bug in the program: it performed an
              invalid operation, but the rules it violated are still experimental

3. 03
   Miri does not just run on tests — it runs on any binary or example. Use MIRIFLAGS="-Zmiri-tree-borrows" to switch from stacked borrows to the newer tree-borrows model; some programs that fail under stacked borrows pass under tree borrows and vice versa. The lesson: write code that satisfies both.

   $ cargo +nightly miri test                            # stacked borrows (default)
   $ MIRIFLAGS="-Zmiri-tree-borrows" cargo +nightly miri test  # tree borrows