macro: ensure that cfg attrs are set for type alias verify generation

[ahayzen-kdab]

mod ffi {
  unsafe extern "C++" {
    #[cfg(enabled)]
    type A = crate::A;
  }
}

When using the bridge above we need to ensure that all generated code also has the cfg attribute set. Before this change the following error would occur cannot find type A in this scope. This appears to be due to the generated verify block below that was not copying the attributes from the original type.

const _: fn() = ::cxx::private::verify_external_type::<A, ...>;

After this change the attributes are copied and therefore when the type is not compiled the verify block is also not compiled.

[dtolnay]

Can you confirm that you have tested that this works? I would not expect cfg attributes to be present in attrs so this PR as currently written should have no effect on what you are describing.

[ahayzen-kdab]

I could see the cfg attribute appearing via cargo expand, but i would personally prefer this to be in a test to be sure as well.

@dtolnay do you have any recommendations as to where i could write a test for this ? As i had a quick look but it wasn't obvious if i should put this in tests/ or in an inline #[test] etc.

If you could point me to an example of a test taking a Rust bridge as input and asserting the generated Rust output that would be useful.

[dtolnay]

Here is where I got mixed up: I forgot that cfg attributes end up going into both cfg and attrs (unlike some other attributes, which are partitioned into only the Doc, or only the Vec<Derive>, or only the OtherAttrs). The PR makes perfect sense now. Thank you!

cxx/syntax/mod.rs

Lines 167 to 174 in 82765ef

	pub(crate) struct TypeAlias {
	#[allow(dead_code)] // only used by cxx-build, not cxxbridge-macro
	pub cfg: CfgExpr,
	#[allow(dead_code)] // only used by cxxbridge-macro, not cxx-build
	pub doc: Doc,
	pub derives: Vec<Derive>,
	#[allow(dead_code)] // only used by cxxbridge-macro, not cxx-build
	pub attrs: OtherAttrs,

cxx/syntax/attrs.rs

Lines 132 to 139 in 82765ef

	} else if attr_path.is_ident("cfg") {
	match cfg::parse_attribute(&attr) {
	Ok(cfg_expr) => {
	if let Some(cfg) = &mut parser.cfg {
	cfg.merge(cfg_expr);
	passthrough_attrs.push(attr);
	continue;
	}

[dtolnay]

Published in 1.0.144.