add ?Sized bounds to traits in #[pin_data] macro

`#[pin_data]` uses some auxiliary traits to ensure that a user does not
implement `Drop` for the annotated struct, as that is unsound and can
lead to UB. However, if the struct that is annotated is `!Sized`, the
current bounds do not work, because `Sized` is an implicit bound for
generics.

This is *not* a soundness hole of pin-init, as it currently is
impossible to construct an unsized value using pin-init.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Author: BennoLossin

internal/src/syn_pin_data.rs

@@ -337,7 +337,7 @@ fn drop_impl(
             // if it also implements `Drop`
             trait MustNotImplDrop {}
             #[expect(drop_bounds)]
-            impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
+            impl<T: ::core::ops::Drop + ?::core::marker::Sized> MustNotImplDrop for T {}
             impl #impl_generics MustNotImplDrop for #ident #ty_generics
                 #whr
             {}
@@ -346,8 +346,8 @@ fn drop_impl(
             // `PinnedDrop` as the parameter to `#[pin_data]`.
             #[expect(non_camel_case_types)]
             trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
-            impl<T: ::pin_init::PinnedDrop> UselessPinnedDropImpl_you_need_to_specify_PinnedDrop
-                for T {}
+            impl<T: ::pin_init::PinnedDrop + ?::core::marker::Sized>
+                UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}
             impl #impl_generics
                 UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for #ident #ty_generics
                 #whr

src/macros.rs

@@ -942,15 +942,15 @@ macro_rules! __pin_data {
         // if it also implements `Drop`
         trait MustNotImplDrop {}
         #[expect(drop_bounds)]
-        impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
+        impl<T: ::core::ops::Drop + ?::core::marker::Sized> MustNotImplDrop for T {}
         impl<$($impl_generics)*> MustNotImplDrop for $name<$($ty_generics)*>
         where $($whr)* {}
         // We also take care to prevent users from writing a useless `PinnedDrop` implementation.
         // They might implement `PinnedDrop` correctly for the struct, but forget to give
         // `PinnedDrop` as the parameter to `#[pin_data]`.
         #[expect(non_camel_case_types)]
         trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
-        impl<T: $crate::PinnedDrop>
+        impl<T: $crate::PinnedDrop + ?::core::marker::Sized>
             UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}
         impl<$($impl_generics)*>
             UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for $name<$($ty_generics)*>

tests/ui/expand/pin-data.expanded.rs

@@ -68,11 +68,12 @@ const _: () = {
     {}
     trait MustNotImplDrop {}
     #[expect(drop_bounds)]
-    impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
+    impl<T: ::core::ops::Drop + ?::core::marker::Sized> MustNotImplDrop for T {}
     impl MustNotImplDrop for Foo {}
     #[expect(non_camel_case_types)]
     trait UselessPinnedDropImpl_you_need_to_specify_PinnedDrop {}
-    impl<T: ::pin_init::PinnedDrop> UselessPinnedDropImpl_you_need_to_specify_PinnedDrop
-    for T {}
+    impl<
+        T: ::pin_init::PinnedDrop + ?::core::marker::Sized,
+    > UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for T {}
     impl UselessPinnedDropImpl_you_need_to_specify_PinnedDrop for Foo {}
 };