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

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Message-ID: <20230703113708.1758734-1-aliceryhl@google.com>
Date:   Mon,  3 Jul 2023 11:37:07 +0000
From:   Alice Ryhl <aliceryhl@...gle.com>
To:     gary@...yguo.net
Cc:     a.hindborg@...sung.com, alex.gaynor@...il.com,
        aliceryhl@...gle.com, benno.lossin@...ton.me,
        bjorn3_gh@...tonmail.com, boqun.feng@...il.com,
        linux-kernel@...r.kernel.org, ojeda@...nel.org,
        rust-for-linux@...r.kernel.org, wedsonaf@...il.com
Subject: Re: [PATCH] rust: macros: add `paste!` proc macro

Gary Guo <gary@...yguo.net> writes:
> 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: Alice Ryhl <aliceryhl@...gle.com>