rustdoc generates repeated documentation for doc comment outside module

[alxgnon]

rustdoc unusually repeats a doc comment twice when it is outside an external module. Check out alxgnon/docrepeat for an example of the bug in action.

I previously fixed this issue for Rust's documentation with #13812, by simply moving the problematic doc comments.

However, #13812 does not fix the issue with rustdoc itself. I am therefore opening this as a separate issue.

[alxgnon]

I did some more testing.

rustdoc --no-defaults lib.rs                          # The bug does not occur
rustdoc --no-defaults --passes collapse-docs lib.rs   # The bug occurs

So the problematic behaviour is probably somewhere within rustdoc::passes::collapse_docs.

[lifthrasiir]

It is not a fault of rustdoc: you can verify that by rustc -Z ast-json lib.rs. It duplicates AttrInner variants twice.

Edit: Ugh, unlike other Parser::parse_item_* methods, Parser::parse_item_mod returns a list of extra attrs plus original attrs, something unexpected for parse_item_or_view_item.

[alxgnon]

Yeah, it seems the reason disabling the collapse-docs pass fixes the bug is it doesn't concatenate the duplicate element. Thank you for noticing that :-)