Cannot interoperate with other field attributes

[taiki-e]

In the current implementation, an error will occur if the field has an attribute other than #[pin].

pin_project! {
   #[derive(serde::Deserialize)]
   struct Struct1<T, U> {
       #[serde(default)] //~ ERROR 
       pinned: Option<T>,
       unpinned: U,
    }
}

[taiki-e]

This is a bug, but I'm not sure if it can be fully fixed.

[taiki-e]

This also affects doc-comment on fields (as that is actually#[doc = "..."] attributes).

[taiki-e]

Another case: https://github.com/taiki-e/pin-project-lite/pull/28#issuecomment-702176944

• The #[project] (and #[project_ref]) attribute must precede the other attributes except for #[doc]:

  pin_project! {
    /// documents (`#[doc]`) can be placed before `#[project]`.
    #[derive(Clone)] // <--- Error
    #[project = EnumProj] 
    #[derive(Debug)] // <--- Ok
    enum Enum<T, U> {
        Struct {
            #[pin]
            pinned: T,
            unpinned: U,
        },
        Unit,
    }
  }

[taiki-e]

Workaround: If the attribute is #[doc] (doc comment), this can be avoided by changing it to normal comment.

  pin_project! {
     #[derive(serde::Deserialize)]
     struct Struct1<T, U> {
-        /// description
+        // description
         pinned: Option<T>,
         unpinned: U,
      }
  }

[taiki-e]

Note: This issue is about attributes that apply only to the original struct/enum.

Proper handling of cfg/cfg_attr on fields and variants requires a more complex approach than the fix for this (it needs to be aware of the kind of attribute and propagate some of it to projected type). Even pin-project could not find the correct solution other than "leave to the compiler". And it is impossible to use the same trick in pin-project-lite.

See https://github.com/taiki-e/pin-project/issues/68#issuecomment-528318870 and https://github.com/hyperium/hyper/issues/2388#issuecomment-758304097 for a known workaround of the cfg issue.

(The cfg issue is not a "limitation that may be fixed" but a "limitation that is considered impossible to fix" and is a similar category as "wontfix". Actually, pin-project-lite cannot handle it properly on both fields and variants.)

[tesaguri]

I have written a macro as a workaround for #[cfg] attributes on enum variants and fields.

https://gist.github.com/tesaguri/2a1c0790a48bbda3dd7f71c26d02a793

What this does is to rewrite something like this:

enum Foo {
    #[cfg(a)]
    A { /* ... */ }
    #[cfg(b)]
    B { /* ... */ }
}

to this:

#[cfg(all(a, b))]
enum Foo {
    A { /* ... */ }
    B { /* ... */ }
}

#[cfg(all(a, not(b)))]
enum Foo {
    A { /* ... */ }
}

#[cfg(all(not(a), b))]
enum Foo {
    B { /* ... */ }
}

#[cfg(all(not(a), not(b)))]
enum Foo {
}

and pass the token tree to pin_project!.

[tmccombs]

Workaround: If the attribute is #[doc] (doc comment), this can be avoided by changing it to normal comment.

This doesn't work if you actually want the comment in the generated documentation. And if you have #![deny(missing_docs)] enabled on your crate, you are really in a pickle, because if the field is public, it will give you an error if it isn't documented, and you can't use #[allow(missing_docs)] or #[doc(hidden)] either.