try_len method

src/lib.rs

@@ -3840,6 +3840,22 @@ pub trait Itertools : Iterator {
     {
         MultiUnzip::multiunzip(self)
     }
+
+    /// Returns the length of the iterator if one exists.
+    ///
+    /// ```
+    /// use itertools::Itertools;
+    ///
+    /// assert_eq!([0; 10].into_iter().try_len(), Some(10));
+    /// assert_eq!((10..15).try_len(), Some(5));
+    /// assert_eq!((15..10).try_len(), Some(0));
+    /// assert_eq!((10..).try_len(), None);
+    /// ```
+    fn try_len(self) -> Option<usize>
+    where Self: Sized
+    {
+        size_hint::try_len(self)
+    }
 }
 
 impl<T: ?Sized> Itertools for T where T: Iterator { }

src/size_hint.rs

@@ -117,3 +117,12 @@ pub fn min(a: SizeHint, b: SizeHint) -> SizeHint {
     };
     (lower, upper)
 }
+
+/// Returns the length of the iterator if one exists.
+#[inline]
+pub fn try_len(it: impl Iterator) -> Option<usize> {
+    match it.size_hint() {
+        (lo, Some(hi)) if lo == hi => Some(lo),
+        _ => None
+    }
+}