Can I write an Iterator that mutates itself and then yields a reference into itself?

by

This is not possible. If it were allowed one could call next again and thus modify data that is also visible via & or even invalidate the reference entirely. This is because there is no connection between the self object itself and the returned reference: there is no explicit lifetime linking them.

For the compiler to reason about this and allow returning a reference into self next needs a signature like

fn next(&'a mut self) -> Option<&'a [i8]>

However, this differs from the signature of the trait which is not allowed as generic code that just takes an T: Iterator<...> cannot tell that there are different requirements on the use of the return value for some T; all have to be handled identically.

The Iterator trait is designed for return values that are independent of the iterator object, which is necessary for iterator adaptors like .collect to be correct and safe. This is more restrictive than necessary for many uses (e.g. a transient use inside a for loop) but it is just how it is at the moment. I don’t think we have the tools for generalising this trait/the for loop properly now (specifically, I think we need associated types with higher rank lifetimes), but maybe in the future.

More Related Contents:

  1. Why is a trait not implemented for a type that clearly has it implemented?
  2. How can I include a module from another file from the same project?
  3. What does “Sized is not implemented” mean?
  4. How do I implement a trait I don’t own for a type I don’t own?
  5. How do I implement Copy and Clone for a type that contains a String (or any type that doesn’t implement Copy)?
  6. What makes something a “trait object”?
  7. Cannot borrow as immutable because it is also borrowed as mutable in function arguments
  8. What is the relation between auto-dereferencing and deref coercion?
  9. What is the return type of the indexing operation?
  10. How to do a binary search on a Vec of floats?
  11. Change enum variant while moving the field to the new variant
  12. I implemented a trait for another trait but cannot call methods from both traits
  13. When is it useful to define multiple lifetimes in a struct?
  14. How to convert ‘struct’ to ‘&[u8]’?
  15. How does the mechanism behind the creation of boxed traits work?
  16. What is the syntax: `instance.method::()`?
  17. Why does passing a closure to function which accepts a function pointer not work?
  18. “expected type parameter, found struct”
  19. Adding an append method to a singly linked list
  20. Is it possible to declare the type of the variable in Rust for loops?
  21. Lifetime of variables passed to a new thread
  22. How can I co-sort two Vecs based on the values in one of the Vecs?
  23. Is there a way to combine multiple traits in order to define a new trait? [duplicate]
  24. How can I store a pattern in a variable in Rust?
  25. How to switch between Rust toolchains?
  26. What does “missing lifetime specifier” mean when storing a &str in a structure?
  27. Is there a list of all cfg features?
  28. Comparing every element in a vector with the next one
  29. How can I conditionally provide a default reference without performing unnecessary computation when it isn’t used?
  30. How can I build multiple binaries with Cargo?

Leave a Comment