[derive] Fix bug with KnownLayout on repr(packed) (#2307) (#2317)

Fixes #2302

gherrit-pr-id: If68c1724be15c4df3ec936a12cf854908650a639

Co-authored-by: Joshua Liebow-Feeser <joshlf@users.noreply.github.com>

Author: google-pr-creation-bot

zerocopy-derive/src/lib.rs

@@ -326,7 +326,15 @@ fn derive_known_layout_inner(ast: &DeriveInput, _top_level: Trait) -> Result<Tok
                             ::zerocopy::util::macro_util::Field<#leading_field_indices>
                         >::Type
                     >,)*
-                    <#trailing_field_ty as ::zerocopy::KnownLayout>::MaybeUninit
+                    // NOTE(#2302): We wrap in `ManuallyDrop` here in case the
+                    // type we're operating on is both generic and
+                    // `repr(packed)`. In that case, Rust needs to know that the
+                    // type is *either* `Sized` or has a trivial `Drop`.
+                    // `ManuallyDrop` has a trivial `Drop`, and so satisfies
+                    // this requirement.
+                    ::zerocopy::util::macro_util::core_reexport::mem::ManuallyDrop<
+                        <#trailing_field_ty as ::zerocopy::KnownLayout>::MaybeUninit
+                    >
                 )
                 where
                     #trailing_field_ty: ::zerocopy::KnownLayout,

zerocopy-derive/src/output_tests.rs

@@ -195,12 +195,14 @@ fn test_known_layout() {
                     ::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<
                         <Foo<T, U> as ::zerocopy::util::macro_util::Field<__Zerocopy_Field_0>>::Type,
                     >,
-                    <<Foo<
-                        T,
-                        U,
-                    > as ::zerocopy::util::macro_util::Field<
-                        __Zerocopy_Field_1,
-                    >>::Type as ::zerocopy::KnownLayout>::MaybeUninit,
+                    ::zerocopy::util::macro_util::core_reexport::mem::ManuallyDrop<
+                        <<Foo<
+                            T,
+                            U,
+                        > as ::zerocopy::util::macro_util::Field<
+                            __Zerocopy_Field_1,
+                        >>::Type as ::zerocopy::KnownLayout>::MaybeUninit
+                    >,
                 )
                 where
                     <Foo<

zerocopy-derive/tests/struct_known_layout.rs

@@ -101,3 +101,14 @@ impl WithSelfReference {
 }
 
 util_assert_impl_all!(WithSelfReference: imp::KnownLayout);
+
+// Deriving `KnownLayout` should work with generic `repr(packed)` types. See
+// #2302.
+
+#[derive(imp::KnownLayout)]
+#[repr(C, packed)]
+struct Packet<P> {
+    payload: P,
+}
+
+util_assert_impl_all!(Packet<imp::u8>: imp::KnownLayout);