Result not Option, inline, no Sized bound, improve docs

src/lib.rs

@@ -3842,18 +3842,23 @@ pub trait Itertools : Iterator {
     }
 
     /// Returns the length of the iterator if one exists.
-    /// Relies on the [`size_hint`] of the iterator being correct.
+    /// Otherwise return `self.size_hint()`.
+    ///
+    /// Fallible [`ExactSizeIterator::len`].
+    ///
+    /// Inherits guarantees and restrictions from [`Iterator::size_hint`].
     ///
     /// ```
     /// 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);
+    /// assert_eq!([0; 10].iter().try_len(), Ok(10));
+    /// assert_eq!((10..15).try_len(), Ok(5));
+    /// assert_eq!((15..10).try_len(), Ok(0));
+    /// assert_eq!((10..).try_len(), Err((usize::MAX, None)));
+    /// assert_eq!((10..15).filter(|x| x % 2 == 0).try_len(), Err((0, Some(5))));
     /// ```
-    fn try_len(&self) -> Option<usize>
-    where Self: Sized
+    #[inline]
+    fn try_len(&self) -> Result<usize, size_hint::SizeHint>
     {
         size_hint::try_len(self.size_hint())
     }

src/size_hint.rs

@@ -120,9 +120,9 @@ pub fn min(a: SizeHint, b: SizeHint) -> SizeHint {
 
 /// Returns the length of the iterator if one exists.
 #[inline]
-pub fn try_len(sh: SizeHint) -> Option<usize> {
+pub fn try_len(sh: SizeHint) -> Result<usize, SizeHint> {
     match sh {
-        (lo, Some(hi)) if lo == hi => Some(lo),
-        _ => None
+        (lo, Some(hi)) if lo == hi => Ok(lo),
+        _ => Err(sh)
     }
 }