From e6c300892dce9202e4f21359129569536945dfea Mon Sep 17 00:00:00 2001
From: The 8472 <git@infinite-source.de>
Date: Sun, 15 Jun 2025 21:47:48 +0200
Subject: [PATCH] small iter.intersperse.fold() optimization

No need to call into fold when the first item is already None,
this avoids some redundant work for empty iterators.

"But it uses Fuse" one might want to protest, but Fuse is specialized
and may call into the inner iterator anyway.
---
 library/core/src/iter/adapters/intersperse.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/library/core/src/iter/adapters/intersperse.rs b/library/core/src/iter/adapters/intersperse.rs
index c97a59b614f9d..843479e2a27a7 100644
--- a/library/core/src/iter/adapters/intersperse.rs
+++ b/library/core/src/iter/adapters/intersperse.rs
@@ -223,7 +223,16 @@ where
 {
     let mut accum = init;
 
-    let first = if started { next_item.take() } else { iter.next() };
+    let first = if started {
+        next_item.take()
+    } else {
+        let n = iter.next();
+        // skip invoking fold() for empty iterators
+        if n.is_none() {
+            return accum;
+        }
+        n
+    };
     if let Some(x) = first {
         accum = f(accum, x);
     }