pin-project does not interoperate well with #[cfg()]

[jonhoo]

Take this struct for instance:

#[pin_project]
#[derive(Debug)]
pub struct Socket {
    #[cfg_attr(unix, pin)]
    #[cfg(unix)]
    inner: tokio::net::unix::UnixStream,
    #[cfg_attr(windows, pin)]
    #[cfg(windows)]
    inner: tokio_named_pipes::NamedPipe,
}

I get the error

error[E0124]: field `inner` is already declared
  --> src/io/socket.rs:29:5
   |
26 |     inner: tokio::net::unix::UnixStream,
   |     ----------------------------------- `inner` first declared here
...
29 |     inner: tokio_named_pipes::NamedPipe,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ field already declared

Even though, as far as I can tell, this should work. My guess here is that pin-project is run before the #[cfg()]s are evaluated, and ends up not propagating them, but I'm not sure?

[jonhoo]

For now, I'm working around this by placing the #[cfg()] on the whole struct, but that won't easily work for structs with more fields that are shared.

[taiki-e]

@jonhoo Thanks for the reporting!

I will investigate.

[taiki-e]

This is because I missed that the timing when the cfg attributes are applied differs between proc-macro-derive and proc-macro-attribute (rust-lang/rust#44528), but, I'm not sure if this issue can be fixed immediately.

1. pin-project collects the type of field annotated with #[pin] attributes from the structure field and generates an Unpin implementation.
2. Since cfg is not applied at that time, pin-project collects all #[pin] fields and adds them to where-clause of the Unpin implementation.
   For example, in the example you wrote, the generated Unpin implementation would be:

impl Unpin for Socket 
where 
    tokio::net::unix::UnixStream: Unpin,
    tokio_named_pipes::NamedPipe: Unpin,
{}

To fix this, we probably need to delegate the Unpin implementation to proc-macro-derive.
(Other issues are just extracting cfg from the field and applying it, so it shouldn't be too difficult to fix: abf3700)

So, for now, it is preferable to create a type alias / new type or create structs for each cfg.

#[cfg(unix)]
type Inner = tokio::net::unix::UnixStream;
// or `struct Inner(tokio::net::unix::UnixStream);`

#[cfg(windows)]
type Inner = tokio_named_pipes::NamedPipe;
// or `struct Inner(tokio_named_pipes::NamedPipe);`

#[pin_project]
#[derive(Debug)]
pub struct Socket {
    #[pin]
    inner: Inner,
}

[taiki-e]

However, I would like to fix this by the release of 0.4 (the other remaining blocker at this time is #26).

[taiki-e]

Filed #77

[taiki-e]

Published 0.4.0-alpha.10 which fixes this issue.

By the way, if you use cfg on fields, it works for other attributes used on that field, so you can use #[pin] directly instead of #[cfg_attr(..., pin)].

#[pin_project]
#[derive(Debug)]
pub struct Socket {
    #[pin]
    #[cfg(unix)]
    inner: tokio::net::unix::UnixStream,
    #[pin]
    #[cfg(windows)]
    inner: tokio_named_pipes::NamedPipe,
}