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References: <20230628171108.1150742-1-gary@garyguo.net>
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From:   Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Date:   Thu, 10 Aug 2023 01:22:15 +0200
Message-ID: <CANiq72nm3ZHqzAWr-8q894_TbOayEx0xmDeeMPpqQz9b-jGU6Q@mail.gmail.com>
Subject: Re: [PATCH] rust: macros: add `paste!` proc macro
To:     Gary Guo <gary@garyguo.net>
Cc:     Miguel Ojeda <ojeda@kernel.org>,
        Alex Gaynor <alex.gaynor@gmail.com>,
        Wedson Almeida Filho <wedsonaf@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        =?UTF-8?Q?Bj=C3=B6rn_Roy_Baron?= <bjorn3_gh@protonmail.com>,
        Benno Lossin <benno.lossin@proton.me>,
        Andreas Hindborg <a.hindborg@samsung.com>,
        Alice Ryhl <aliceryhl@google.com>,
        linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org
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On Wed, Jun 28, 2023 at 7:12=E2=80=AFPM Gary Guo <gary@garyguo.net> wrote:
>
> This macro provides a flexible way to concatenated identifiers together
> and it allows the resulting identifier to be used to declare new items,
> which `concat_idents!` does not allow. It also allows identifiers to be
> transformed before concatenated.
>
> The `concat_idents!` example
>
>     let x_1 =3D 42;
>     let x_2 =3D concat_idents!(x, _1);
>     assert!(x_1 =3D=3D x_2);
>
> can be written with `paste!` macro like this:
>
>     let x_1 =3D 42;
>     let x_2 =3D paste!([<x _1>]);
>     assert!(x_1 =3D=3D x_2);
>
> However `paste!` macro is more flexible because it can be used to create
> a new variable:
>
>     let x_1 =3D 42;
>     paste!(let [<x _2>] =3D [<x _1>];);
>     assert!(x_1 =3D=3D x_2);
>
> While this is not possible with `concat_idents!`.
>
> This macro is similar to the `paste!` crate [1], but this is a fresh
> implementation to avoid vendoring large amount of code directly. Also, I
> have augmented it to provide a way to specify span of the resulting
> token, allowing precise control.
>
> For example, this code is broken because the variable is declared inside
> the macro, so Rust macro hygiene rules prevents access from the outside:
>
>     macro_rules! m {
>         ($id: ident) =3D> {
>             // The resulting token has hygiene of the macro.
>             paste!(let [<$id>] =3D 1;)
>         }
>     }
>
>     m!(a);
>     let _ =3D a;
>
> In this versionn of `paste!` macro I added a `span` modifier to allow
> this:
>
>     macro_rules! m {
>         ($id: ident) =3D> {
>             // The resulting token has hygiene of `$id`.
>             paste!(let [<$id:span>] =3D 1;)
>         }
>     }
>
>     m!(a);
>     let _ =3D a;
>
> Link: http://docs.rs/paste/ [1]
> Signed-off-by: Gary Guo <gary@garyguo.net>

Applied to `rust-next` with the SPDX license identifier added as
discussed and the typo fixed -- thanks everyone!

Cheers,
Miguel