Fix ?Sized type parameters in Debug

[tyranron]

Synopsis

At the moment, derive_more::Debug fails to work with ?Sized generics:

#[derive(derive_more::Debug)]
struct UnnamedGenericStructUnsized<T: ?Sized>(T);

And generates the following error:

error[E0277]: the size for values of type `T` cannot be known at compilation time
   --> tests/debug.rs:845:14
    |
845 |     #[derive(Debug)]
    |              ^^^^^ doesn't have a size known at compile-time
846 |     struct UnnamedGenericStructUnsized<T: ?Sized>(T);
    |                                        - this type parameter needs to be `std::marker::Sized`
    |
    = note: required for the cast from `&T` to `&dyn Debug`
    = note: this error originates in the derive macro `Debug` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider borrowing the value, since `&&T` can be coerced into `&dyn Debug`
    |
845 |     #[derive(&Debug)]
    |              +
help: consider removing the `?Sized` bound to make the type parameter `Sized`
    |
846 -     struct UnnamedGenericStructUnsized<T: ?Sized>(T);
846 +     struct UnnamedGenericStructUnsized<T>(T);
    |

At the same moment, std::Debug works OK:

#[derive(std::Debug)]
struct UnnamedGenericStructUnsized<T: ?Sized>(T);

If we look at its expansion:

#[automatically_derived]
impl<T: ::core::fmt::Debug + ?Sized> ::core::fmt::Debug for
    UnnamedGenericStructUnsized<T> {
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        ::core::fmt::Formatter::debug_tuple_field1_finish(f,
            "UnnamedGenericStructUnsized", &&self.0)
    }
}

We can see that std::Debug always uses fields as &&self.0, while we in our derive_more::Debug expansion use &self.0 only.

Solution

Simply use double-ref in the expansion, as the error suggests, and std::Debug does.

Checklist

• Documentation is updated (not required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (not required)

[JelteF]

Does the same issue exist for other derives? Especially Display seems likely to have the same bug

[tyranron]

@JelteF seems no. The bug goes from the &dyn Debug usage in the debug building API. Display-like and other traits derives don't use such API atm.

I can add similar tests for other traits as a separate PR. Just to be sure.

[tyranron]

@JelteF ping.