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Backwards-Incompatible Assert Desugaring Change in Rust

Core Problem

A recent change to Rust's assert desugaring has broken the bitvec crate, causing a compilation error on stable versions of the language. This issue highlights the importance of backward compatibility in language design and development.

Solution & Analysis

#[derive(Debug)]
struct F {
    data: bool
}

impl std::ops::Not for F {
  type Output = bool;
  fn not(self) -> Self::Output { !self.data }
}

fn main() {
  let f = F { data: true };

  assert!(f);
}

The problem lies in the fact that the assert! macro now desugares to a call to std::panic::catch_unwind, which expects a closure with at least one argument. In the provided code, the F struct implements the Not trait, but its not method returns the value of data, not a boolean indicating whether the value is true or false.

To fix this issue, we need to modify the assert! macro to correctly handle the desugaring of closures that return values. One possible solution is to use the std::ops::Not trait's not method as intended:

fn main() {
  let f = F { data: true };

  assert!(f.not());
}

This change ensures that the assert! macro correctly desugares the closure and checks if the value of data is false.

Conclusion

The backwards-incompatible assert desugaring change in Rust highlights the importance of careful consideration when designing language features. By understanding the implications of such changes, developers can create more robust and reliable code that adheres to the evolving standards of the Rust programming language.

Reference