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

Question

This code (playground):

#[derive(Clone)]
struct Foo<'a, T: 'a> {
    t: &'a T,
}

fn bar<'a, T>(foo: Foo<'a, T>) {
    foo.clone();
}

... does not compile:

error[E0599]: no method named `clone` found for struct `Foo<'a, T>` in the current scope
   --> src/main.rs:16:9
    |
3   | struct Foo<'a, T: 'a> {
    | ---------------------
    | |
    | method `clone` not found for this
    | doesn't satisfy `Foo<'_, T>: std::clone::Clone`
...
16  |     foo.clone();
    |         ^^^^^ method not found in `Foo<'a, T>`
    |
    = 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`
help: consider restricting the type parameter to satisfy the trait bound
    |
3   | struct Foo<'a, T: 'a> where T: std::clone::Clone {
    |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^

Adding use std::clone::Clone; doesn't change anything, as it's already in the prelude anyway.

When I remove the #[derive(Clone)] and manually implement Clone for Foo, it compiles as expected!

impl<'a, T> Clone for Foo<'a, T> {
    fn clone(&self) -> Self {
        Foo {
            t: self.t,
        }
    }
}

What is going on here?

• Is there a difference between #[derive()]-impls and manual ones?
• Is this a compiler bug?
• Something else I didn't think of?

Answer 1

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.