fix for ?Sized type parameters

pin-project-internal/src/pin_project/derive.rs

@@ -973,7 +973,7 @@ impl<'a> Context<'a> {
                     // this 'public' type by creating this type in the inside of `const`.
                     #vis struct #struct_ident #proj_generics #where_clause {
                         __pin_project_use_generics: ::pin_project::__private::AlwaysUnpin<
-                            #lifetime, (#(#type_params),*)
+                            #lifetime, (#(::pin_project::__private::PhantomData<#type_params>),*)
                         >,
 
                         #(#fields,)*

src/lib.rs

@@ -222,7 +222,7 @@ pub mod __private {
     #[doc(hidden)]
     pub struct AlwaysUnpin<'a, T: ?Sized>(PhantomData<&'a ()>, PhantomData<T>);
 
-    impl<T: ?Sized> Unpin for AlwaysUnpin<'_, T> {}
+    impl<T> Unpin for AlwaysUnpin<'_, T> {}
 
     // This is an internal helper used to ensure a value is dropped.
     #[doc(hidden)]

tests/sized.rs

@@ -0,0 +1,13 @@
+#![warn(rust_2018_idioms, single_use_lifetimes)]
+#![allow(dead_code)]
+
+use pin_project::pin_project;
+
+#[pin_project]
+struct Foo<'a, I: ?Sized, Item>
+where
+    I: Iterator,
+{
+    iter: &'a mut I,
+    item: Option<Item>,
+}