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

[Alice Ryhl <aliceryhl@google.com>]

Gary Guo <gary@garyguo.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@garyguo.net>

Reviewed-by: Alice Ryhl <aliceryhl@google.com>