Silence private_bounds lint in KnownLayout derive (#2182)

For reasons not-yet-known, the `private_bounds` lint flags the
output of `derive(KnownLayout)` on `repr(C)` structs generated
by macros. This is likely a rustc bug, but to preserve a good
experience for our users, we nonetheless, `allow(private_bounds)`.

Fixes #2177

zerocopy-derive/src/lib.rs

@@ -320,6 +320,10 @@ fn derive_known_layout_inner(ast: &DeriveInput, _top_level: Trait) -> Result<Tok
                 // `#ty`, not `__ZerocopyKnownLayoutMaybeUninit` (see #2116).
                 #repr
                 #[doc(hidden)]
+                // Required on some rustc versions due to a lint that is only
+                // triggered when `derive(KnownLayout)` is applied to `repr(C)`
+                // structs that are generated by macros. See #2177 for details.
+                #[allow(private_bounds)]
                 #vis struct __ZerocopyKnownLayoutMaybeUninit<#params> (
                     #(::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<
                         <#ident #ty_generics as

zerocopy-derive/src/output_tests.rs

@@ -191,6 +191,7 @@ fn test_known_layout() {
                 #[repr(C)]
                 #[repr(align(2))]
                 #[doc(hidden)]
+                #[allow(private_bounds)]
                 struct __ZerocopyKnownLayoutMaybeUninit<T, U>(
                     ::zerocopy::util::macro_util::core_reexport::mem::MaybeUninit<
                         <Foo<T, U> as ::zerocopy::util::macro_util::Field<__Zerocopy_Field_0>>::Type,