Implement some trivial size_hints for various iterators

This also implements ExactSizeIterator where applicable.

Addresses most of the Iterator traits mentioned in #23708.

3 files changed, 14 insertions, 0 deletions

diff --git a/src/libcore/char.rs b/src/libcore/char.rs
index 1638f9710f5..2f56238a463 100644
--- a/src/libcore/char.rs
+++ b/src/libcore/char.rs
@@ -902,6 +902,11 @@ impl<I: Iterator<Item = u8>> Iterator for DecodeUtf8<I> {
             }
         })
     }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.0.size_hint()
+    }
 }
 
 #[unstable(feature = "decode_utf8", issue = "33906")]
diff --git a/src/libcore/iter/traits.rs b/src/libcore/iter/traits.rs
index 0267fcd3754..5e4622f804a 100644
--- a/src/libcore/iter/traits.rs
+++ b/src/libcore/iter/traits.rs
@@ -901,6 +901,10 @@ impl<I, T, E> Iterator for ResultShunt<I, E>
             None => None,
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
 }
 
 #[stable(feature = "iter_arith_traits_result", since="1.16.0")]
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index 570f745f833..1ca69f3f503 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -1188,6 +1188,11 @@ impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
                     None => None,
                 }
             }
+
+            #[inline]
+            fn size_hint(&self) -> (usize, Option<usize>) {
+                self.iter.size_hint()
+            }
         }
 
         let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };