[RFC PATCH 4/6] rust: pin-init: add `?Sized` bounds to traits in `#[pin_data]` macro

[Benno Lossin <benno.lossin@proton.me>]

`#[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>
---
 rust/pin-init/internal/src/syn_pin_data.rs | 6 +++---
 rust/pin-init/src/macros.rs                | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/rust/pin-init/internal/src/syn_pin_data.rs b/rust/pin-init/internal/src/syn_pin_data.rs
index d95176389e17..30cd4bc35fd0 100644
--- a/rust/pin-init/internal/src/syn_pin_data.rs
+++ b/rust/pin-init/internal/src/syn_pin_data.rs
@@ -293,7 +293,7 @@ fn drop(&mut self) {
             // 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
             {}
@@ -302,8 +302,8 @@ impl #impl_generics MustNotImplDrop for #ident #ty_generics
             // `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
diff --git a/rust/pin-init/src/macros.rs b/rust/pin-init/src/macros.rs
index 361623324d5c..03e2588423cc 100644
--- a/rust/pin-init/src/macros.rs
+++ b/rust/pin-init/src/macros.rs
@@ -931,7 +931,7 @@ impl<'__pin, $($impl_generics)*> ::core::marker::Unpin for $name<$($ty_generics)
         // if it also implements `Drop`
         trait MustNotImplDrop {}
         #[expect(drop_bounds)]
-        impl<T: ::core::ops::Drop> MustNotImplDrop for T {}
+        impl<T: ::core::ops::Drop + ?::core::marker::Sized> MustNotImplDrop for T {}
         impl<$($impl_generics)*> MustNotImplDrop for $name<$($ty_generics)*>
         where $($whr)* {}
         // We also take care to prevent users from writing a useless `PinnedDrop` implementation.
@@ -939,7 +939,7 @@ impl<$($impl_generics)*> MustNotImplDrop for $name<$($ty_generics)*>
         // `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)*>
-- 
2.48.1