error[E0119]: conflicting implementations of trait `Unpin` for type `Struct<_>`
  --> tests/ui/unstable-features/marker_trait_attr.rs:13:1
   |
13 | #[pin_project] //~ ERROR E0119
   | ^^^^^^^^^^^^^^ conflicting implementation for `Struct<_>`
...
20 | impl<T> Unpin for Struct<T> {}
   | --------------------------- first implementation here
   |
   = note: this error originates in the derive macro `::pin_project::__private::__PinProjectInternalDerive` (in Nightly builds, run with -Z macro-backtrace for more info)