[derive] IntoBytes on unions requires --cfg

Makes progress on #1792

tools/cargo-zerocopy/src/main.rs

@@ -164,10 +164,11 @@ fn install_toolchain_or_exit(versions: &Versions, name: &str) -> Result<(), Erro
 }
 
 fn get_rustflags(name: &str) -> &'static str {
+    // See #1792 for context on zerocopy_derive_union_into_bytes.
     if name == "nightly" {
-        "--cfg __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS "
+        "--cfg __ZEROCOPY_INTERNAL_USE_ONLY_NIGHTLY_FEATURES_IN_TESTS --cfg zerocopy_derive_union_into_bytes "
     } else {
-        ""
+        "--cfg zerocopy_derive_union_into_bytes "
     }
 }
 

zerocopy-derive/Cargo.toml

@@ -22,6 +22,10 @@ repository = "https://github.com/google/zerocopy"
 # [1] https://github.com/rust-lang/crater
 exclude = [".*", "tests/enum_from_bytes.rs", "tests/ui-nightly/enum_from_bytes_u16_too_few.rs.disabled"]
 
+[lints.rust]
+# See #1792 for more context.
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(zerocopy_derive_union_into_bytes)'] }
+
 [lib]
 proc-macro = true
 

zerocopy-derive/src/lib.rs

@@ -934,6 +934,15 @@ fn derive_into_bytes_enum(ast: &DeriveInput, enm: &DataEnum) -> Result<TokenStre
 /// - `repr(C)`, `repr(transparent)`, or `repr(packed)`
 /// - no padding (size of union equals size of each field type)
 fn derive_into_bytes_union(ast: &DeriveInput, unn: &DataUnion) -> Result<TokenStream, Error> {
+    if !cfg!(zerocopy_derive_union_into_bytes) {
+        // See #1792 for more context.
+        return Err(Error::new(
+            Span::call_site(),
+            "requires --cfg zerocopy_derive_union_into_bytes;
+please let us know you use this feature: https://github.com/google/zerocopy/discussions/1802",
+        ));
+    }
+
     // TODO(#10): Support type parameters.
     if !ast.generics.params.is_empty() {
         return Err(Error::new(Span::call_site(), "unsupported on types with type parameters"));