Add trybuild test to ensure deprecated types are derivable.

zerocopy-derive/src/lib.rs

@@ -1205,8 +1205,6 @@ fn impl_block<D: DataExt>(
     });
 
     quote! {
-        // TODO(#553): Add a test that generates a warning when
-        // `#[allow(deprecated)]` isn't present.
         #[allow(deprecated)]
         unsafe impl < #(#params),* > #trait_path for #type_ident < #(#param_idents),* >
         where

zerocopy-derive/tests/ui-nightly/absence_of_deprecated_warning.rs

@@ -0,0 +1,24 @@
+//! See: https://github.com/google/zerocopy/issues/553
+//! zerocopy must still allow derives of deprecated types.
+//! This test has a hand-written impl of a deprecated type, and should result in a compilation
+//! error. If zerocopy does not tack an allow(deprecated) annotation onto its impls, then this
+//! test will fail because more than one compile error will be generated.
+#![deny(deprecated)]
+
+extern crate zerocopy;
+
+use zerocopy::IntoBytes;
+
+#[deprecated = "Do not use"]
+#[derive(IntoBytes)]
+#[repr(C)]
+struct OldHeader {
+    field_a: usize,
+    collection: [u8; 8],
+}
+
+trait T {}
+
+impl T for OldHeader {}
+
+fn main() {}

zerocopy-derive/tests/ui-nightly/absence_of_deprecated_warning.stderr

@@ -0,0 +1,11 @@
+error: use of deprecated struct `OldHeader`: Do not use
+  --> tests/ui-nightly/absence_of_deprecated_warning.rs:22:12
+   |
+22 | impl T for OldHeader {}
+   |            ^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> tests/ui-nightly/absence_of_deprecated_warning.rs:6:9
+   |
+6  | #![deny(deprecated)]
+   |         ^^^^^^^^^^