[vollbrecht]

Hmm i just tested out the claim that the following rust code would be rejected ( Example 4 in the paper).

And it seams to not be the case on the stable compiler version?

  fn write(x: &mut i32) {*x = 10}
  
  fn main() {
      let x = &mut 0;
      let y = x as *mut i32;
      //write(x); // this should use the mention implicit twophase borrow
      *x = 10; // this should not and therefore be rejected by the compiler
      unsafe {*y = 15 };
  }

[Arnavion]

Stacked borrows is miri's runtime model. Run it under miri and you will see the error reported for the `*x = 10;` version but not the `write(x);` version - "Undefined Behavior: attempting a write access using [...] but that tag does not exist in the borrow stack for this location".

rustc itself has no reason to reject either version, because y is a *mut and thus has no borrow/lifetime relation to the &mut that x is, from a compile-time/typesystem perspective.

[vollbrecht]

Ah that make sense. Thanks for clarifying.

[haberman]

The paper mentions that the authors implemented tree borrows in Miri. Is this change likely to be adopted by Miri as the default model going forward?

[ethan_smith]

The paper is describing the behavior under the proposed Tree Borrows model, not the current borrow checker implementation which uses a more limited analysis that doesn't detect this particular conflict between the raw pointer and mutable reference.