Remove lifetime from wrapper types and impl generics

[taiki-e]

Context: #357 (comment)
I'm not worried about the compiler making changes that break the current code in the future since the compiler depends on pin-project-lite, but I do like reducing the generated code by this.

-impl<'pin, T, U> _pin_project::__private::Unpin for Struct<T, U>
+impl<T, U> _pin_project::__private::Unpin for Struct<T, U>
     where
-        ::pin_project::__private::PinnedFieldsOf<
-            __Struct<'pin, T, U>,
+        for<'pin> ::pin_project::__private::PinnedFieldsOf<
+           __Struct<T, U>,
         >: _pin_project::__private::Unpin,
     {}

FYI @danielhenrymantilla

[taiki-e]

Hmm, it seems lifetime in wrapper type in needed in old rustc.
https://github.com/taiki-e/pin-project-lite/actions/runs/11503119119/job/32019681641
https://github.com/taiki-e/pin-project-lite/actions/runs/11503253973/job/32020114753

error[E0277]: the trait bound `std::marker::PhantomPinned: std::marker::Unpin` is not satisfied
   --> tests/lint/lib.rs:93:5
    |
93  | /     pin_project! {
94  | |         /// Testing enum.
95  | |         #[project = EnumProj]
96  | |         #[project_ref = EnumProjRef]
...   |
112 | |         }
113 | |     }
    | |_____^ the trait `std::marker::Unpin` is not implemented for `std::marker::PhantomPinned`
    |
    = help: the following implementations were found:
              <std::marker::PhantomPinned as std::marker::Unpin>
    = help: see issue #48214
    = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

[taiki-e]

Nah, for<'a> approach doesn't seem to work well with the old rustc in the first place.
https://github.com/taiki-e/pin-project-lite/actions/runs/11503435474/job/32020719422?pr=85
taiki-e/pin-project-lite#85

error[E0277]: the trait bound `for<'__pin> <pin_project_lite::__private::PinnedFieldsOfHelperStruct<default::_::__Origin<'__pin, (), ()>> as pin_project_lite::__private::PinnedFieldsOfHelperTrait>::Actual: std::marker::Unpin` is not satisfied
  --> tests/proper_unpin.rs:28:5
   |
28 |     assert_unpin!(Struct<(), ()>);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |     |
   |     the trait `for<'__pin> std::marker::Unpin` is not implemented for `<pin_project_lite::__private::PinnedFieldsOfHelperStruct<default::_::__Origin<'__pin, (), ()>> as pin_project_lite::__private::PinnedFieldsOfHelperTrait>::Actual`
   |     in this macro invocation
   |
   = help: the following implementations were found:
             <&'a T as std::marker::Unpin>
             <&'a mut T as std::marker::Unpin>
   = note: required because of the requirements on the impl of `std::marker::Unpin` for `default::Struct<(), ()>`
note: required by `default::_::{{closure}}#0::assert_impl_all`
  --> tests/proper_unpin.rs:28:5
   |
28 |     assert_unpin!(Struct<(), ()>);
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ in this macro invocation
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

[danielhenrymantilla]

Interesting, TIL. Thanks for trying it out anyways!