linux-kernel - Re: [PATCH] rust: macros: add `paste!` proc macro

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Date:   Thu, 10 Aug 2023 01:22:15 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Gary Guo <gary@...yguo.net>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Andreas Hindborg <a.hindborg@...sung.com>,
        Alice Ryhl <aliceryhl@...gle.com>,
        linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] rust: macros: add `paste!` proc macro

On Wed, Jun 28, 2023 at 7:12 PM Gary Guo <gary@...yguo.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 = 42;
>     let x_2 = concat_idents!(x, _1);
>     assert!(x_1 == x_2);
>
> can be written with `paste!` macro like this:
>
>     let x_1 = 42;
>     let x_2 = paste!([<x _1>]);
>     assert!(x_1 == x_2);
>
> However `paste!` macro is more flexible because it can be used to create
> a new variable:
>
>     let x_1 = 42;
>     paste!(let [<x _2>] = [<x _1>];);
>     assert!(x_1 == 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 {
>         ($id: ident) => {
>             // The resulting token has hygiene of the macro.
>             paste!(let [<$id>] = 1;)
>         }
>     }
>
>     m!(a);
>     let _ = a;
>
> In this versionn of `paste!` macro I added a `span` modifier to allow
> this:
>
>     macro_rules! m {
>         ($id: ident) => {
>             // The resulting token has hygiene of `$id`.
>             paste!(let [<$id:span>] = 1;)
>         }
>     }
>
>     m!(a);
>     let _ = a;
>
> Link: http://docs.rs/paste/ [1]
> Signed-off-by: Gary Guo <gary@...yguo.net>

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

Cheers,
Miguel