cargo doc error in using projects

[rbtcollins]

I've just filed rust-lang/rust#71908 about an error from cargo doc --no-deps which is ultimately reporting an issue coming from async-std. I'm not entirely sure what is triggering it yet :/.

That bug is about the quality of the error; this bug is about fixing the block, presumably in the macro generated output, or something - I haven't tracked down what I"m using that is triggering it as I mentioned, so I can't be certain of the context yet.

warning: doc comment contains an invalid Rust code block
   --> /home/robertc/.cargo/registry/src/github.com-1ecc6299db9ec823/async-std-1.5.0/src/utils.rs:243:42
    |
243 |         $($head)* extension_trait!(@ext (#[doc = $d]) $($tail)*);
    |                                          ^^^^^^^^^^^
    |
    = help: mark blocks that do not contain Rust code as text: ```text