Merge pull request #1667 from dtolnay/punctuatedfold

Optimize punctuated::fold

Author: dtolnay

codegen/src/fold.rs

@@ -42,7 +42,7 @@ fn visit(
             let Type::Syn(t) = t else { unimplemented!() };
             let method = method_name(t);
             Some(quote! {
-                FoldHelper::lift(#name, f, F::#method)
+                crate::punctuated::fold(#name, f, F::#method)
             })
         }
         Type::Option(t) => {

src/gen/fold.rs

@@ -1,7 +1,6 @@
 #[cfg(feature = "fold")]
 pub(crate) mod fold {
     use crate::fold::Fold;
-    use crate::punctuated::{Pair, Punctuated};
 
     pub(crate) trait FoldHelper {
         type Item;
@@ -21,20 +20,4 @@ pub(crate) mod fold {
             self.into_iter().map(|it| f(fold, it)).collect()
         }
     }
-
-    impl<T, P> FoldHelper for Punctuated<T, P> {
-        type Item = T;
-        fn lift<V, F>(self, fold: &mut V, mut f: F) -> Self
-        where
-            V: Fold + ?Sized,
-            F: FnMut(&mut V, Self::Item) -> Self::Item,
-        {
-            self.into_pairs()
-                .map(|pair| match pair {
-                    Pair::Punctuated(t, p) => Pair::Punctuated(f(fold, t), p),
-                    Pair::End(t) => Pair::End(f(fold, t)),
-                })
-                .collect()
-        }
-    }
 }

src/gen_helper.rs

@@ -1073,6 +1073,29 @@ impl<T, P> IndexMut<usize> for Punctuated<T, P> {
     }
 }
 
+#[cfg(all(feature = "fold", any(feature = "full", feature = "derive")))]
+pub(crate) fn fold<T, P, V, F>(
+    punctuated: Punctuated<T, P>,
+    fold: &mut V,
+    mut f: F,
+) -> Punctuated<T, P>
+where
+    V: ?Sized,
+    F: FnMut(&mut V, T) -> T,
+{
+    Punctuated {
+        inner: punctuated
+            .inner
+            .into_iter()
+            .map(|(t, p)| (f(fold, t), p))
+            .collect(),
+        last: match punctuated.last {
+            Some(t) => Some(Box::new(f(fold, *t))),
+            None => None,
+        },
+    }
+}
+
 #[cfg(feature = "printing")]
 mod printing {
     use crate::punctuated::{Pair, Punctuated};