Work around new dead_code warning

    warning: field `0` is never read
      --> src/marker.rs:15:39
       |
    15 | pub(crate) struct ProcMacroAutoTraits(Rc<()>);
       |                   ------------------- ^^^^^^
       |                   |
       |                   field in this struct
       |
       = note: `#[warn(dead_code)]` on by default
    help: consider changing the field to be of unit type to suppress this warning while preserving the field numbering, or remove the field
       |
    15 | pub(crate) struct ProcMacroAutoTraits(());
       |                                       ~~

src/marker.rs

@@ -12,7 +12,7 @@ mod value {
     pub(crate) use core::marker::PhantomData as Marker;
 }
 
-pub(crate) struct ProcMacroAutoTraits(Rc<()>);
+pub(crate) struct ProcMacroAutoTraits(#[allow(dead_code)] Rc<()>);
 
 impl UnwindSafe for ProcMacroAutoTraits {}
 impl RefUnwindSafe for ProcMacroAutoTraits {}

tests/ui/test-not-send.stderr

@@ -31,7 +31,7 @@ error[E0277]: `Rc<()>` cannot be sent between threads safely
 note: required because it appears within the type `proc_macro2::marker::ProcMacroAutoTraits`
  --> $WORKSPACE/src/marker.rs
   |
-  | pub(crate) struct ProcMacroAutoTraits(Rc<()>);
+  | pub(crate) struct ProcMacroAutoTraits(#[allow(dead_code)] Rc<()>);
   |                   ^^^^^^^^^^^^^^^^^^^
 note: required because it appears within the type `PhantomData<proc_macro2::marker::ProcMacroAutoTraits>`
  --> $RUST/core/src/marker.rs