Allow other attributes in #[pin_data] structs

When using a macro with custom attributes in a `#[pin_data]` struct it
can mess up the generated `__Unpin` struct.

This commit moves the decision-making of which attributes to keep into a
new, separate function, so that it is unified in all places.  Only
`#[cfg]` and `#[doc]` attributes are kept since all other can cause
issues.

With this commit a struct can use both #[pin] and other custom
attributes, allowing combining multiple attribute macros without
clashing.

To keep this functionality this commit also adds a test with a custom
attribute that fails without this fix.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>

Author: mkletzan

Cargo.lock

@@ -35,6 +35,7 @@ trybuild = { version = "1.0", features = ["diff"] }
 macrotest = "1.0"
 # needed for macrotest, have to enable verbatim feature to be able to format `&raw` expressions.
 prettyplease = { version = "0.2.37", features = ["verbatim"] }
+dummy = { path = "tests/dummy" }
 
 [lints.rust]
 stable_features = "allow"

Cargo.toml

@@ -7,7 +7,7 @@ use syn::{
     parse_quote, parse_quote_spanned,
     spanned::Spanned,
     visit_mut::VisitMut,
-    Field, Generics, Ident, Item, PathSegment, Type, TypePath, Visibility, WhereClause,
+    Attribute, Field, Generics, Ident, Item, PathSegment, Type, TypePath, Visibility, WhereClause,
 };
 
 use crate::diagnostics::{DiagCtxt, ErrorGuaranteed};
@@ -35,6 +35,11 @@ impl Parse for Args {
     }
 }
 
+fn keep_attr(a: &Attribute) -> bool {
+    let p = a.path();
+    ["cfg", "doc"].iter().any(|a| p.is_ident(a))
+}
+
 pub(crate) fn pin_data(
     args: Args,
     input: Item,
@@ -77,9 +82,9 @@ pub(crate) fn pin_data(
         .fields
         .iter_mut()
         .map(|field| {
-            let len = field.attrs.len();
-            field.attrs.retain(|a| !a.path().is_ident("pin"));
-            (len != field.attrs.len(), &*field)
+            let pinned = field.attrs.iter().any(|a| a.path().is_ident("pin"));
+            field.attrs.retain(keep_attr);
+            (pinned, &*field)
         })
         .collect();
 
@@ -259,7 +264,7 @@ fn generate_projections(
             },
         )| {
             let mut attrs = attrs.clone();
-            attrs.retain(|a| !a.path().is_ident("pin"));
+            attrs.retain(keep_attr);
             let mut no_doc_attrs = attrs.clone();
             no_doc_attrs.retain(|a| !a.path().is_ident("doc"));
             let ident = ident
@@ -359,7 +364,7 @@ fn generate_the_pin_data(
         pinned: bool,
     ) -> TokenStream {
         let mut attrs = attrs.clone();
-        attrs.retain(|a| !a.path().is_ident("pin"));
+        attrs.retain(keep_attr);
         let ident = ident
             .as_ref()
             .expect("only structs with named fields are supported");

internal/src/pin_data.rs

@@ -0,0 +1,12 @@
+[package]
+name = "dummy"
+version = "0.0.0"
+edition = "2024"
+
+license = "MIT OR Apache-2.0"
+description = "Proc macro only for test reproduction."
+
+publish = false
+
+[lib]
+proc-macro = true

tests/dummy/Cargo.toml

@@ -0,0 +1,4 @@
+#[proc_macro_derive(Dummy, attributes(dummy_attr))]
+pub fn derive_device(_: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    proc_macro::TokenStream::new()
+}

tests/dummy/src/lib.rs

@@ -0,0 +1,21 @@
+use dummy::Dummy;
+use pin_init::*;
+
+#[pin_data]
+#[derive(Dummy)]
+struct Pointless {
+    #[pin]
+    #[dummy_attr]
+    #[cfg(test)]
+    member: i8,
+    #[pin]
+    #[dummy_attr]
+    #[cfg(not(test))]
+    member: u8,
+}
+
+#[test]
+fn multiple_attributes() {
+    stack_pin_init!(let p = init!(Pointless { member: 0 }));
+    println!("{}", p.member);
+}