Rollup merge of rust-lang#81152 - lzutao:intersperse_fold, r=m-ou-se

Fix intersperse_fold

Here is a standalone playground link in case anybody wants to modify code: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=626b4d044fb74f044a36098ad907e40f

Fixes rust-lang#81145

cc rust-lang#79479 ``@jonas-schievink``

library/core/src/iter/adapters/intersperse.rs

@@ -160,7 +160,7 @@ where
 }
 
 fn intersperse_fold<I, B, F, G>(
-    mut iter: Peekable<I>,
+    mut iter: I,
     init: B,
     mut f: F,
     mut separator: G,
@@ -173,10 +173,11 @@ where
 {
     let mut accum = init;
 
-    // Use `peek()` first to avoid calling `next()` on an empty iterator.
-    if !needs_sep || iter.peek().is_some() {
+    if !needs_sep {
         if let Some(x) = iter.next() {
             accum = f(accum, x);
+        } else {
+            return accum;
         }
     }
 

library/core/tests/iter.rs

@@ -3562,6 +3562,47 @@ fn test_intersperse_size_hint() {
     assert_eq!([].iter().intersperse(&()).size_hint(), (0, Some(0)));
 }
 
+#[test]
+fn test_intersperse_fold() {
+    let v = (1..4).intersperse(9).fold(Vec::new(), |mut acc, x| {
+        acc.push(x);
+        acc
+    });
+    assert_eq!(v.as_slice(), [1, 9, 2, 9, 3]);
+
+    let mut iter = (1..4).intersperse(9);
+    assert_eq!(iter.next(), Some(1));
+    let v = iter.fold(Vec::new(), |mut acc, x| {
+        acc.push(x);
+        acc
+    });
+    assert_eq!(v.as_slice(), [9, 2, 9, 3]);
+
+    struct NoneAtStart(i32); // Produces: None, Some(2), Some(3), None, ...
+    impl Iterator for NoneAtStart {
+        type Item = i32;
+        fn next(&mut self) -> Option<i32> {
+            self.0 += 1;
+            Some(self.0).filter(|i| i % 3 != 1)
+        }
+    }
+
+    let v = NoneAtStart(0).intersperse(1000).fold(0, |a, b| a + b);
+    assert_eq!(v, 0);
+}
+
+#[test]
+fn test_intersperse_collect_string() {
+    let contents = vec![1, 2, 3];
+
+    let contents_string = contents
+        .into_iter()
+        .map(|id| id.to_string())
+        .intersperse(", ".to_owned())
+        .collect::<String>();
+    assert_eq!(contents_string, "1, 2, 3");
+}
+
 #[test]
 fn test_fold_specialization_intersperse() {
     let mut iter = (1..2).intersperse(0);