Deriving a trait results in unexpected compiler error, but the manual implementation works

by

The answer is buried in the error message:

    = note: the method `clone` exists but the following trait bounds were not satisfied:
            `T: std::clone::Clone`
            which is required by `Foo<'_, T>: std::clone::Clone`

When you derive Clone (and many other automatically-derived types), it adds a Clone bound on all generic types. Using rustc -Z unstable-options --pretty=expanded, we can see what it becomes:

impl <'a, T: ::std::clone::Clone + 'a> ::std::clone::Clone for Foo<'a, T> {
    #[inline]
    fn clone(&self) -> Foo<'a, T> {
        match *self {
            Foo { t: ref __self_0_0 } =>
            Foo{t: ::std::clone::Clone::clone(&(*__self_0_0)),},
        }
    }
}

In this case, the bound is not needed because the generic type is behind a reference.

For now, you will need to implement Clone yourself. There’s a Rust issue for this, but it’s a comparatively rare case with a workaround.

More Related Contents:

  1. Why can’t I store a value and a reference to that value in the same struct?
  2. The compiler suggests I add a ‘static lifetime because the parameter type may not live long enough, but I don’t think that’s what I want
  3. Is it possible to control the size of an array using the type parameter of a generic?
  4. How do I stop iteration and return an error when Iterator::map returns a Result::Err?
  5. Initialize a large, fixed-size array with non-Copy types
  6. Sending trait objects between threads in Rust
  7. Why would I implement methods on a trait instead of as part of the trait?
  8. How to initialize struct fields which reference each other
  9. “Expected fn item, found a different fn item” when working with function pointers
  10. How do you unwrap a Result on Ok or return from the function on Err?
  11. How do I read the entire body of a Tokio-based Hyper request?
  12. How can I downcast from Box to a trait object type?
  13. How do I ‘pass down’ feature flags to subdependencies in Cargo?
  14. When returning the outcome of consuming a StdinLock, why was the borrow to stdin retained?
  15. How does Rust pattern matching determine if the bound variable will be a reference or a value?
  16. Using generic trait methods like .into() when type inference is impossible
  17. What is a crate attribute and where do I add it?
  18. Parameter type may not live long enough?
  19. How to model complex recursive data structures (graphs)?
  20. Why can’t I reuse a &mut reference after passing it to a function that accepts a generic type?
  21. print! macro not executed until pressing Enter
  22. How can I define an async method in a trait?
  23. How do I specify the linker path in Rust?
  24. Why am I getting “parameter is never used [E0392]”?
  25. How to quiet a warning for a single statement in Rust?
  26. Is there an elegant solution to modifying a structure while iterating?
  27. How do I cope with lazy iterators?
  28. Can a struct containing a raw pointer implement Send and be FFI safe?
  29. Lazy sequence generation in Rust
  30. Is there a way to get the field names of a struct in a macro?

Leave a Comment