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Message-Id: <DFK1RR7HGFKN.XM2V8C9D0PR1@garyguo.net>
Date: Fri, 09 Jan 2026 12:12:01 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Benno Lossin" <lossin@...nel.org>, "Gary Guo" <gary@...yguo.net>,
"Miguel Ojeda" <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>,
Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Andreas
Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
"Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
"Fiona Behrens" <me@...enk.dev>, "Tamir Duberstein" <tamird@...il.com>,
"Alban Kurti" <kurti@...icto.ai>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/12] rust: pin-init: rewrite the `#[pinned_drop]`
attribute macro using `syn`
On Thu Jan 8, 2026 at 1:50 PM GMT, Benno Lossin wrote:
> Rewrite the attribute macro for implementing `PinnedDrop` using `syn`.
> Otherwise no functional changes intended aside from improved error
> messages on syntactic and semantical errors. For example:
>
> When missing the `drop` function in the implementation, the old error
> was:
>
> error: no rules expected `)`
> --> tests/ui/compile-fail/pinned_drop/no_fn.rs:6:1
> |
> 6 | #[pinned_drop]
> | ^^^^^^^^^^^^^^ no rules expected this token in macro call
> |
> note: while trying to match keyword `fn`
> --> src/macros.rs
> |
> | fn drop($($sig:tt)*) {
> | ^^
> = note: this error originates in the attribute macro `pinned_drop` (in Nightly builds, run with -Z macro-backtrace for more info)
>
> And the new one is:
>
> error[E0046]: not all trait items implemented, missing: `drop`
> --> tests/ui/compile-fail/pinned_drop/no_fn.rs:7:1
> |
> 7 | impl PinnedDrop for Foo {}
> | ^^^^^^^^^^^^^^^^^^^^^^^ missing `drop` in implementation
> |
> = help: implement the missing item: `fn drop(self: Pin<&mut Self>, _: OnlyCallFromDrop) { todo!() }`
>
> Signed-off-by: Benno Lossin <lossin@...nel.org>
> ---
> rust/pin-init/internal/src/lib.rs | 6 +-
> rust/pin-init/internal/src/pinned_drop.rs | 87 ++++++++++++-----------
> rust/pin-init/src/macros.rs | 28 --------
> 3 files changed, 52 insertions(+), 69 deletions(-)
>
> diff --git a/rust/pin-init/internal/src/lib.rs b/rust/pin-init/internal/src/lib.rs
> index ec593362c5ac..d0156b82b5d3 100644
> --- a/rust/pin-init/internal/src/lib.rs
> +++ b/rust/pin-init/internal/src/lib.rs
> @@ -25,7 +25,11 @@ pub fn pin_data(inner: TokenStream, item: TokenStream) -> TokenStream {
>
> #[proc_macro_attribute]
> pub fn pinned_drop(args: TokenStream, input: TokenStream) -> TokenStream {
> - pinned_drop::pinned_drop(args.into(), input.into()).into()
> + pinned_drop::pinned_drop(
> + parse_macro_input!(args as _),
> + parse_macro_input!(input as _),
> + )
`as _` can be removed.
> + .into()
> }
>
> #[proc_macro_derive(Zeroable)]
> 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`?
> + if let Some(unsafety) = input.unsafety {
> + errors.push(quote_spanned! {unsafety.span=>
> + ::core::compile_error!("implementing `PinnedDrop` is safe");
> + });
> + }
> + input.unsafety = Some(Token);
> + 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`");
> + });
> }
> - TokenTree::Punct(p) if p.as_char() == '>' => {
> - nesting = nesting.checked_sub(1).unwrap();
> - continue;
> + for (seg, expected) in path
> + .segments
> + .iter()
> + .rev()
> + .zip(["PinnedDrop", "pin_init", ""])
> + {
> + if expected.is_empty() || seg.ident != expected {
> + errors.push(quote_spanned! {seg.span()=>
> + ::core::compile_error!("bad import path for `PinnedDrop`");
> + });
> + }
> + if !seg.arguments.is_none() {
> + errors.push(quote_spanned! {seg.arguments.span()=>
> + ::core::compile_error!("unexpected arguments for `PinnedDrop` path");
> + });
> + }
> }
> - _ => {}
> - }
> - if i >= 1 && nesting == 0 {
> - // Found the end of the generics, this should be `PinnedDrop`.
> - assert!(
> - matches!(tt, TokenTree::Ident(i) if i == "PinnedDrop"),
> - "expected 'PinnedDrop', found: '{tt:?}'"
> - );
> - pinned_drop_idx = Some(i);
> - break;
> + *path = parse_quote!(::pin_init::PinnedDrop);
> }
> + None => errors.push(quote_spanned! {input.impl_token.span=>
nit: this can be `input.impl_token.span.join(input.self_ty.span())`.
Best,
Gary
> + ::core::compile_error!("expected `impl ... PinnedDrop for ...`, got inherent impl");
> + }),
> }
> - let idx = pinned_drop_idx
> - .unwrap_or_else(|| panic!("Expected an `impl` block implementing `PinnedDrop`."));
> - // Fully qualify the `PinnedDrop`, as to avoid any tampering.
> - toks.splice(idx..idx, quote!(::pin_init::));
> - // Take the `{}` body and call the declarative macro.
> - if let Some(TokenTree::Group(last)) = toks.pop() {
> - let last = last.stream();
> - quote!(::pin_init::__pinned_drop! {
> - @impl_sig(#(#toks)*),
> - @impl_body(#last),
> - })
> - } else {
> - TokenStream::from_iter(toks)
> + for item in &mut input.items {
> + if let ImplItem::Fn(fn_item) = item {
> + if fn_item.sig.ident == "drop" {
> + fn_item
> + .sig
> + .inputs
> + .push(parse_quote!(_: ::pin_init::__internal::OnlyCallFromDrop));
> + }
> + }
> }
> + quote!(#(#errors)* #input)
> }
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