[PATCH 4/8] rust: pin-init: internal: add utility for extract a single attribute

[Gary Guo <gary@garyguo.net>]

More attributes are to be added which has the requirement that they should
only be specified once. Add a utility code to extract attributes for a
specific path and report error when appeared multiple times.

Convert `#[pin]` parsing to use the added utility.

Signed-off-by: Gary Guo <gary@garyguo.net>
---
 rust/pin-init/internal/src/pin_data.rs |  9 ++-------
 rust/pin-init/internal/src/util.rs     | 25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/rust/pin-init/internal/src/pin_data.rs b/rust/pin-init/internal/src/pin_data.rs
index 4ffa35bbbccd..ac83f0f73e61 100644
--- a/rust/pin-init/internal/src/pin_data.rs
+++ b/rust/pin-init/internal/src/pin_data.rs
@@ -137,12 +137,7 @@ pub(crate) fn pin_data(
         .iter_mut()
         .enumerate()
         .map(|(index, field)| {
-            let len = field.attrs.len();
-            field.attrs.retain(|a| !a.path().is_ident("pin"));
-            let pinned_count = len - field.attrs.len();
-            if pinned_count > 1 {
-                dcx.error(&field, "#[pin] attribute specified more than once");
-            }
+            let pinned = field.attrs.extract_single_attr(dcx, "pin").is_some();
 
             assert!(
                 !field.attrs.iter().any(|a| a.path().is_ident("cfg")),
@@ -159,7 +154,7 @@ pub(crate) fn pin_data(
             FieldInfo {
                 field: &*field,
                 member,
-                pinned: pinned_count != 0,
+                pinned,
             }
         })
         .collect();
diff --git a/rust/pin-init/internal/src/util.rs b/rust/pin-init/internal/src/util.rs
index 2a57a25b70ae..3ce498cea754 100644
--- a/rust/pin-init/internal/src/util.rs
+++ b/rust/pin-init/internal/src/util.rs
@@ -4,8 +4,15 @@
 use quote::{format_ident, ToTokens};
 use syn::{Attribute, GenericParam, Generics, Index, Member, Token};
 
+use crate::DiagCtxt;
+
 pub(crate) trait AttrListExt {
     fn extract_cfg_attrs(&mut self) -> Vec<TokenStream>;
+
+    /// Extract attribute with identifier `path`.
+    ///
+    /// Report error if the attribute appears multiple times.
+    fn extract_single_attr(&mut self, dcx: &DiagCtxt, path: &str) -> Option<Attribute>;
 }
 
 impl AttrListExt for Vec<Attribute> {
@@ -25,6 +32,24 @@ fn extract_cfg_attrs(&mut self) -> Vec<TokenStream> {
 
         cfg
     }
+
+    fn extract_single_attr(&mut self, dcx: &DiagCtxt, path: &str) -> Option<Attribute> {
+        // FIXME: Replace with `extract_if` when MSRV >= 1.85.
+        let attr_pos = self.iter().position(|attr| attr.path().is_ident(path))?;
+        let attr = self.remove(attr_pos);
+        self.retain(|attr| {
+            if !attr.path().is_ident(path) {
+                return true;
+            }
+
+            dcx.error(
+                attr,
+                format!("`#[{path}]` attribute specified more than once"),
+            );
+            false
+        });
+        Some(attr)
+    }
 }
 
 pub(crate) trait MemberExt {

-- 
2.54.0