Add link to dead_code autotrait bug

rust-lang/rust#119645

Author: dtolnay

src/marker.rs

@@ -12,7 +12,10 @@ mod value {
     pub(crate) use core::marker::PhantomData as Marker;
 }
 
-pub(crate) struct ProcMacroAutoTraits(#[allow(dead_code)] Rc<()>);
+pub(crate) struct ProcMacroAutoTraits(
+    #[allow(dead_code)] // https://github.com/rust-lang/rust/issues/119645
+    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(#[allow(dead_code)] Rc<()>);
+  | pub(crate) struct ProcMacroAutoTraits(
   |                   ^^^^^^^^^^^^^^^^^^^
 note: required because it appears within the type `PhantomData<proc_macro2::marker::ProcMacroAutoTraits>`
  --> $RUST/core/src/marker.rs