[PATCH v2 06/15] rust: pin-init: rewrite the `#[pinned_drop]` attribute macro using `syn`

[Benno Lossin <lossin@kernel.org>]

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@kernel.org>
---
Changes in v2:
* improved error handling
* improved error span
---
 rust/pin-init/internal/src/lib.rs         |  4 +-
 rust/pin-init/internal/src/pinned_drop.rs | 89 +++++++++++++----------
 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 d3878cbccd68..04794c864c78 100644
--- a/rust/pin-init/internal/src/lib.rs
+++ b/rust/pin-init/internal/src/lib.rs
@@ -25,7 +25,7 @@ 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), parse_macro_input!(input)).into()
 }
 
 #[proc_macro_derive(Zeroable)]
@@ -55,12 +55,10 @@ fn from(value: syn::Error) -> Self {
 }
 
 impl Error {
-    #[expect(dead_code)]
     pub(crate) fn none() -> Self {
         Self(None)
     }
 
-    #[expect(dead_code)]
     pub(crate) fn combine(&mut self, error: impl Into<Self>) {
         let error = error.into();
         if let Some(this) = self.0.as_mut() {
diff --git a/rust/pin-init/internal/src/pinned_drop.rs b/rust/pin-init/internal/src/pinned_drop.rs
index cf8cd1c42984..e1df034844bb 100644
--- a/rust/pin-init/internal/src/pinned_drop.rs
+++ b/rust/pin-init/internal/src/pinned_drop.rs
@@ -1,49 +1,62 @@
 // SPDX-License-Identifier: Apache-2.0 OR MIT
 
-use proc_macro2::{TokenStream, TokenTree};
+use proc_macro2::TokenStream;
 use quote::quote;
+use syn::{parse::Nothing, parse_quote, spanned::Spanned, Error, 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 error = crate::Error::none();
+    if let Some(unsafety) = input.unsafety {
+        error.combine(Error::new_spanned(
+            unsafety,
+            "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 {
+                error.combine(Error::new_spanned(not, "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 {
+                    error.combine(Error::new_spanned(seg, "bad import path for `PinnedDrop`"));
+                }
+                if !seg.arguments.is_none() {
+                    error.combine(Error::new_spanned(
+                        &seg.arguments,
+                        "unexpected arguments for `PinnedDrop` path",
+                    ));
+                }
             }
-            _ => {}
+            *path = parse_quote!(::pin_init::PinnedDrop);
         }
-        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;
+        None => {
+            let span = input
+                .impl_token
+                .span
+                .join(input.self_ty.span())
+                .unwrap_or(input.impl_token.span);
+            error.combine(Error::new(
+                span,
+                "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!(#error #input)
 }
diff --git a/rust/pin-init/src/macros.rs b/rust/pin-init/src/macros.rs
index 53ed5ce860fc..b80c95612fd6 100644
--- a/rust/pin-init/src/macros.rs
+++ b/rust/pin-init/src/macros.rs
@@ -503,34 +503,6 @@
 #[cfg(not(kernel))]
 pub use ::paste::paste;
 
-/// Creates a `unsafe impl<...> PinnedDrop for $type` block.
-///
-/// See [`PinnedDrop`] for more information.
-///
-/// [`PinnedDrop`]: crate::PinnedDrop
-#[doc(hidden)]
-#[macro_export]
-macro_rules! __pinned_drop {
-    (
-        @impl_sig($($impl_sig:tt)*),
-        @impl_body(
-            $(#[$($attr:tt)*])*
-            fn drop($($sig:tt)*) {
-                $($inner:tt)*
-            }
-        ),
-    ) => {
-        // SAFETY: TODO.
-        unsafe $($impl_sig)* {
-            // Inherit all attributes and the type/ident tokens for the signature.
-            $(#[$($attr)*])*
-            fn drop($($sig)*, _: $crate::__internal::OnlyCallFromDrop) {
-                $($inner)*
-            }
-        }
-    }
-}
-
 /// This macro first parses the struct definition such that it separates pinned and not pinned
 /// fields. Afterwards it declares the struct and implement the `PinData` trait safely.
 #[doc(hidden)]
-- 
2.52.0