#[project] on non-statement expression does not work without unstable features

[taiki-e]

playground

Code:

#[project]
fn foo() {
    #[pin_project]
    enum Enum<A> {
        Variant(#[pin] A),
    }

    let mut x = Enum::Variant(1);
    let mut x = Pin::new(&mut x).project();

    Some(
        #[project]
        match x {
            Enum::Variant(x) => {}
        },
    );
}

Error:

error[E0658]: attributes on expressions are experimental
   --> tests/project.rs:230:9
    |
230 |         #[project]
    |         ^^^^^^^^^^
    |
    = note: see issue #15701 <https://github.com/rust-lang/rust/issues/15701> for more information
    = help: add `#![feature(stmt_expr_attributes)]` to the crate attributes to enable

error[E0658]: custom attributes cannot be applied to expressions
   --> tests/project.rs:230:9
    |
230 |         #[project]
    |         ^^^^^^^^^^
    |
    = note: see issue #54727 <https://github.com/rust-lang/rust/issues/54727> for more information
    = help: add `#![feature(proc_macro_hygiene)]` to the crate attributes to enable

Same as taiki-e/auto_enums#90