From 7c8064f6a6b73bcd10e708fcc0c4f20297f47b69 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Sun, 17 Nov 2019 14:32:07 -0800
Subject: [PATCH] Implement ExactSizeIterator for Chain

---
 src/chain.rs | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/src/chain.rs b/src/chain.rs
index 45b7e60..e2937cb 100644
--- a/src/chain.rs
+++ b/src/chain.rs
@@ -26,6 +26,11 @@ impl<'a> Iterator for Chain<'a> {
             Buffered { rest } => rest.next(),
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.len();
+        (len, Some(len))
+    }
 }
 
 impl DoubleEndedIterator for Chain<'_> {
@@ -46,3 +51,19 @@ impl DoubleEndedIterator for Chain<'_> {
         }
     }
 }
+
+impl ExactSizeIterator for Chain<'_> {
+    fn len(&self) -> usize {
+        match &self.state {
+            Linked { mut next } => {
+                let mut len = 0;
+                while let Some(cause) = next {
+                    next = cause.source();
+                    len += 1;
+                }
+                len
+            }
+            Buffered { rest } => rest.len(),
+        }
+    }
+}