Re: [PATCH 05/12] rust: pin-init: rewrite the `#[pinned_drop]` attribute macro using `syn`

["Benno Lossin" <lossin@kernel.org>]

On Fri Jan 9, 2026 at 1:12 PM CET, Gary Guo wrote:
> On Thu Jan 8, 2026 at 1:50 PM GMT, Benno Lossin wrote:
>> diff --git a/rust/pin-init/internal/src/pinned_drop.rs b/rust/pin-init/internal/src/pinned_drop.rs
>> index cf8cd1c42984..4df2cb9959fb 100644
>> --- a/rust/pin-init/internal/src/pinned_drop.rs
>> +++ b/rust/pin-init/internal/src/pinned_drop.rs
>> @@ -1,49 +1,56 @@
>>  // SPDX-License-Identifier: Apache-2.0 OR MIT
>>  
>> -use proc_macro2::{TokenStream, TokenTree};
>> -use quote::quote;
>> +use proc_macro2::TokenStream;
>> +use quote::{quote, quote_spanned};
>> +use syn::{parse::Nothing, parse_quote, spanned::Spanned, ImplItem, ItemImpl, Token};
>>  
>> -pub(crate) fn pinned_drop(_args: TokenStream, input: TokenStream) -> TokenStream {
>> -    let mut toks = input.into_iter().collect::<Vec<_>>();
>> -    assert!(!toks.is_empty());
>> -    // Ensure that we have an `impl` item.
>> -    assert!(matches!(&toks[0], TokenTree::Ident(i) if i == "impl"));
>> -    // Ensure that we are implementing `PinnedDrop`.
>> -    let mut nesting: usize = 0;
>> -    let mut pinned_drop_idx = None;
>> -    for (i, tt) in toks.iter().enumerate() {
>> -        match tt {
>> -            TokenTree::Punct(p) if p.as_char() == '<' => {
>> -                nesting += 1;
>> +pub(crate) fn pinned_drop(_args: Nothing, mut input: ItemImpl) -> TokenStream {
>> +    let mut errors = vec![];
>
> Any reason to not make this `Vec<Error>` and use `syn::Error::combine`?

Is there a way to have an "empty" error? I dislike using `Option<Error>`
here... If not I'll just create my own helper type instead.

Cheers,
Benno

>> +    if let Some(unsafety) = input.unsafety {
>> +        errors.push(quote_spanned! {unsafety.span=>
>> +            ::core::compile_error!("implementing `PinnedDrop` is safe");
>> +        });
>> +    }
>> +    input.unsafety = Some(Token![unsafe](input.impl_token.span));
>> +    match &mut input.trait_ {
>> +        Some((not, path, _for)) => {
>> +            if let Some(not) = not {
>> +                errors.push(quote_spanned! {not.span=>
>> +                    ::core::compile_error!("cannot implement `!PinnedDrop`");
>> +                });