Re: [PATCH v2 05/28] rust: macros: add `concat_idents!` proc macro

[Gary Guo]

On Fri,  2 Dec 2022 17:14:36 +0100
ojeda@xxxxxxxxxx wrote:

> From: Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>
> 
> This macro provides similar functionality to the unstable feature
> `concat_idents` without having to rely on it.
> 
> For instance:
> 
>     let x_1 = 42;
>     let x_2 = concat_idents!(x, _1);
>     assert!(x_1 == x_2);
> 
> It has different behavior with respect to macro hygiene. Unlike
> the unstable `concat_idents!` macro, it allows, for example,
> referring to local variables by taking the span of the second
> macro as span for the output identifier.
> 
> Signed-off-by: Björn Roy Baron <bjorn3_gh@xxxxxxxxxxxxxx>
> Reviewed-by: Finn Behrens <me@xxxxxxxxxx>
> [Reworded, adapted for upstream and applied latest changes]
> Signed-off-by: Miguel Ojeda <ojeda@xxxxxxxxxx>
> ---
>  rust/macros/concat_idents.rs | 23 +++++++++++++++++++
>  rust/macros/lib.rs           | 44 ++++++++++++++++++++++++++++++++++++
>  2 files changed, 67 insertions(+)
>  create mode 100644 rust/macros/concat_idents.rs
> 
> diff --git a/rust/macros/concat_idents.rs b/rust/macros/concat_idents.rs
> new file mode 100644
> index 000000000000..7e4b450f3a50
> --- /dev/null
> +++ b/rust/macros/concat_idents.rs
> @@ -0,0 +1,23 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +use proc_macro::{token_stream, Ident, TokenStream, TokenTree};
> +
> +use crate::helpers::expect_punct;
> +
> +fn expect_ident(it: &mut token_stream::IntoIter) -> Ident {
> +    if let Some(TokenTree::Ident(ident)) = it.next() {
> +        ident
> +    } else {
> +        panic!("Expected Ident")
> +    }
> +}
> +
> +pub(crate) fn concat_idents(ts: TokenStream) -> TokenStream {
> +    let mut it = ts.into_iter();
> +    let a = expect_ident(&mut it);
> +    assert_eq!(expect_punct(&mut it), ',');
> +    let b = expect_ident(&mut it);
> +    assert!(it.next().is_none(), "only two idents can be concatenated");
> +    let res = Ident::new(&format!("{a}{b}"), b.span());
> +    TokenStream::from_iter([TokenTree::Ident(res)])
> +}
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index 91764bfb1f89..15555e7ff487 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -2,6 +2,7 @@
>  
>  //! Crate for all kernel procedural macros.
>  
> +mod concat_idents;
>  mod helpers;
>  mod module;
>  
> @@ -70,3 +71,46 @@ use proc_macro::TokenStream;
>  pub fn module(ts: TokenStream) -> TokenStream {
>      module::module(ts)
>  }
> +
> +/// Concatenate two identifiers.
> +///
> +/// This is useful in macros that need to declare or reference items with names
> +/// starting with a fixed prefix and ending in a user specified name. The resulting
> +/// identifier has the span of the second argument.
> +///
> +/// # Examples
> +///
> +/// ```ignore
> +/// use kernel::macro::concat_idents;
> +///
> +/// macro_rules! pub_no_prefix {
> +///     ($prefix:ident, $($newname:ident),+) => {
> +///         $(pub(crate) const $newname: u32 = kernel::macros::concat_idents!($prefix, $newname);)+
> +///     };
> +/// }
> +///
> +/// pub_no_prefix!(
> +///     binder_driver_return_protocol_,
> +///     BR_OK,
> +///     BR_ERROR,
> +///     BR_TRANSACTION,
> +///     BR_REPLY,
> +///     BR_DEAD_REPLY,
> +///     BR_TRANSACTION_COMPLETE,
> +///     BR_INCREFS,
> +///     BR_ACQUIRE,
> +///     BR_RELEASE,
> +///     BR_DECREFS,
> +///     BR_NOOP,
> +///     BR_SPAWN_LOOPER,
> +///     BR_DEAD_BINDER,
> +///     BR_CLEAR_DEATH_NOTIFICATION_DONE,
> +///     BR_FAILED_REPLY
> +/// );
> +///
> +/// assert_eq!(BR_OK, binder_driver_return_protocol_BR_OK);
> +/// ```
> +#[proc_macro]
> +pub fn concat_idents(ts: TokenStream) -> TokenStream {
> +    concat_idents::concat_idents(ts)
> +}

Reviewed-by: Gary Guo <gary@xxxxxxxxxxx>