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

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Date:   Wed, 28 Jun 2023 21:51:28 -0300
From:   Martin Rodriguez Reboredo <yakoyoku@...il.com>
To:     Gary Guo <gary@...yguo.net>, 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>
Cc:     linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] rust: macros: add `paste!` proc macro

On 6/28/23 14:11, Gary Guo 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>
> ---
> [...]

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>