Docs: Report findings for issue #2177 and add regression tests

zerocopy-derive/tests/issue_2177.rs

@@ -0,0 +1,20 @@
+// This test reproduces the issue reported in https://github.com/google/zerocopy/issues/2177
+// It uses a macro to wrap the derive, which triggers the private_bounds lint unless suppressed.
+
+#![allow(missing_docs)]
+use zerocopy::KnownLayout;
+
+macro_rules! define {
+    ($name:ident, $repr:ty) => {
+        #[derive(KnownLayout)]
+        #[repr(C)]
+        pub struct $name($repr);
+    }
+}
+
+define!(Foo, u8);
+
+#[test]
+fn test_issue_2177() {
+    let _ = Foo(0);
+}

zerocopy-derive/tests/issue_2177_control.rs

@@ -0,0 +1,46 @@
+// This test serves as a control case for issue #2177.
+// It manually expands the code that KnownLayout generates, but inside a macro_rules! macro.
+// This does NOT trigger the private_bounds lint, proving that the issue is specific to
+// the interaction between proc-macro generated spans and macro_rules! expansion.
+
+#![allow(missing_docs)]
+use zerocopy::KnownLayout;
+
+// Mimic the internal Field trait
+pub unsafe trait Field<T> {
+    type Type;
+}
+
+macro_rules! define {
+    ($name:ident) => {
+        pub struct $name(u8);
+
+        const _: () = {
+            #[allow(non_camel_case_types)]
+            struct __Zerocopy_Field_0; // Private struct
+
+            unsafe impl Field<__Zerocopy_Field_0> for $name {
+                type Type = u8;
+            }
+
+            #[repr(C)]
+            #[doc(hidden)]
+            // The lint triggers on the following struct in the actual derive usage,
+            // claiming __Zerocopy_Field_0 is leaked.
+            // Here, it does not trigger.
+            pub struct __ZerocopyKnownLayoutMaybeUninit
+            where
+                <$name as Field<__Zerocopy_Field_0>>::Type: KnownLayout
+            {
+                _marker: (),
+            }
+        };
+    }
+}
+
+define!(Foo);
+
+#[test]
+fn test_issue_2177_control() {
+    let _ = Foo(0);
+}