Add regression test for suppressing private_bounds lint in KnownLayout derive (#2791)

* Suppress `private_bounds` lint in `KnownLayout` derive

This commit suppresses the `private_bounds` lint on the `__ZerocopyKnownLayoutMaybeUninit` struct generated by `#[derive(KnownLayout)]`. This lint can be falsely triggered when the derive macro is used on a public struct within a `macro_rules!` macro, where the generated code refers to private types (field indices) that are technically "more private" than the public struct, even though they are in the same module scope.

Fixes #2177.

* Suppress `private_bounds` lint in `KnownLayout` derive

This commit suppresses the `private_bounds` lint on the `__ZerocopyKnownLayoutMaybeUninit` struct generated by `#[derive(KnownLayout)]`. This lint can be falsely triggered when the derive macro is used on a public struct within a `macro_rules!` macro, where the generated code refers to private types (field indices) that are technically "more private" than the public struct, even though they are in the same module scope.

Fixes #2177.

---------

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

zerocopy-derive/tests/hygiene.rs

@@ -36,3 +36,23 @@ util_assert_impl_all!(
         _zerocopy::FromBytes,
         _zerocopy::Unaligned
 );
+
+// Regression test for #2177.
+//
+// This test ensures that `#[derive(KnownLayout)]` does not trigger the
+// `private_bounds` lint when used on a public struct in a macro.
+mod issue_2177 {
+    #![deny(private_bounds)]
+    // We need to access `_zerocopy` from the parent module.
+    use super::_zerocopy;
+
+    macro_rules! define {
+        ($name:ident, $repr:ty) => {
+            #[derive(_zerocopy::KnownLayout)]
+            #[repr(C)]
+            pub struct $name($repr);
+        };
+    }
+
+    define!(Foo, u8);
+}