#[derive(Debug)] is triggering the unused_results lint

[canndrew]

The following code fails to build on nightly

#![deny(unused_results)]

#[derive(Debug)]
enum Foo {
    Bar(u32),
}

It generates this error:

<anon>:3:10: 3:15 error: unused result
<anon>:3 #[derive(Debug)]
                  ^~~~~
<anon>:3:10: 3:15 note: in this expansion of #[derive_Debug] (defined in <anon>)
<anon>:1:9: 1:23 note: lint level defined here
<anon>:1 #![deny(unused_results)]

[apasel422]

The expansion is:

#[automatically_derived]
#[allow(unused_qualifications)]
impl ::std::fmt::Debug for Foo {
    fn fmt(&self, __arg_0: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
        match (&*self,) {
            (&Foo::Bar(ref __self_0),) => {
                let mut builder = __arg_0.debug_tuple("Bar");
                builder.field(&&(*__self_0));
                builder.finish()
            }
        }
    }
}

The call to builder.field(...) returns self for chaining purposes, but the expansion obviously ignores it.

[apasel422]

This was a result of #29565.