PhantomData should implement Clone

[Yoric]

I don't see any good reason to prevent it, and this would have saved me lots of manual implementations of Clone.

[apasel422]

PhantomData does implement Clone: https://doc.rust-lang.org/stable/std/marker/struct.PhantomData.html.

What version of Rust are you using? Can you post some source code that doesn't compile due to this issue?

[alexcrichton]

Agreed with @apasel422, so closing, but feel free to post an error message here so we can help debug!

[Yoric]

Here's a minimized misleading sample:

use std::marker::PhantomData;

trait Foo<T>: Clone {
}

#[derive(Clone)]
pub struct KeyedIgnoring<T> {
    witness: PhantomData<T>,
}

impl<T> Foo<T> for KeyedIgnoring<T> /* where Foo<T> : Clone */ {
    // error: the trait `core::clone::Clone` is not implemented for the type `T`
}

fn main() {
    println!("Hello, world!");
}

Now, if I uncomment where Foo<T>: Clone, this works. But I only tried this after you had closed the bug, because the error message led me to believe that the issue was in PhantomData.

[Yoric]

Also, adding where Foo<T>: Clone doesn't scale very well in my code, as I find myself adding numerous such clauses for each impl.

[eefriedman]

See #26925; you should be able to get around the issue by implementing Clone manually. (impl<T> Clone for KeyedIgnoring<T> { fn clone(&self) -> Self { KeyedIgnoring { witness: PhantomData } } }).

[Yoric]

Yes, hence

this would have saved me lots of manual implementations of Clone

in my original comment.

As it is, I had to revert my change in which I added where Foo<T>: Clone, as it breaks client code.

[alexcrichton]

Yes currently derive(Clone) will put a Clone requirement on all type parameters of the struct, so the dependency on Clone for T comes from KeyedIgnoring instead of the PhantomData itself.