Regression: cannot return value referencing function parameter self

[taiki-e]

Caused by #47.

Such code is used a lot in futures etc., so I would like to support it...
However, I'm not sure if this can be fixed by ways other than changing self argument to self: Pin<&mut Self>.

Code:

use std::pin::Pin;

#[pin_project::pin_project]
struct Struct<T, U> {
    #[pin]
    pinned: T,
    unpinned: U,
}

impl<T, U> Struct<T, U> {
    fn get_pin_mut<'a>(mut self: Pin<&'a mut Self>) -> Pin<&'a mut T> {
        self.project().pinned
    }
}

Error:

error[E0515]: cannot return value referencing function parameter `self`
  --> src/lib.rs:33:9
   |
33 |         self.project().pinned
   |         ----^^^^^^^^^^^^^^^^^
   |         |
   |         returns a value referencing data owned by the current function
   |         `self` is borrowed here

playground (current)

playground (previous)

Related: rust-lang/rust#54934

[taiki-e]

cc @Aaron1011

[taiki-e]

By the way, even if arbitrary_self_types is used, this seems to be an error.

[Aaron1011]

I've found a solution - I'll be opening a PR soon.