From 4a7a188dbad077e4d0e653ce5ddb2aca3cdde3df Mon Sep 17 00:00:00 2001
From: Phlosioneer <mattmdrr2@gmail.com>
Date: Sat, 31 Mar 2018 23:51:07 -0400
Subject: [PATCH 1/2] Add size_hints for btree iterators

Continues work on adding size_hint implementations for iterators:
issue #49205
---
 src/liballoc/btree/map.rs | 35 +++++++++++++++++++++++++++++++++++
 src/liballoc/btree/set.rs |  4 ++++
 2 files changed, 39 insertions(+)

diff --git a/src/liballoc/btree/map.rs b/src/liballoc/btree/map.rs
index c604df7049e0c..cabeaf3a73f30 100644
--- a/src/liballoc/btree/map.rs
+++ b/src/liballoc/btree/map.rs
@@ -1480,6 +1480,16 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
             unsafe { Some(self.next_unchecked()) }
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.front == self.back {
+            (0, Some(0))
+        } else {
+            // There doesn't seem to be any way to get the size of the whole tree
+            // from a NodeRef.
+            (1, None)
+        }
+    }
 }
 
 #[stable(feature = "map_values_mut", since = "1.10.0")]
@@ -1612,6 +1622,16 @@ impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
             unsafe { Some(self.next_unchecked()) }
         }
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.front == self.back {
+            (0, Some(0))
+        } else {
+            // We can get the root node by using into_root_mut; but there doesn't
+            // seem to be any way to get the size of the tree from the root node.
+            (1, None)
+        }
+    }
 }
 
 impl<'a, K, V> RangeMut<'a, K, V> {
@@ -2549,4 +2569,19 @@ impl<K: Ord, V, I: Iterator<Item = (K, V)>> Iterator for MergeIter<K, V, I> {
             }
         }
     }
+
+    // Currently unused. However, BTreeMap::from_sorted_iter may some day be
+    // rewritten to pre-allocate the correct amount of nodes, in which case
+    // having this will be helpful.
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (left_lower, left_upper) = self.left.size_hint();
+        let (right_lower, right_upper) = self.right.size_hint();
+        let lower = left_lower + right_lower;
+        let upper = match (left_upper, right_upper) {
+            (Some(left), Some(right)) => left.checked_add(right),
+            (left, None) => left,
+            (None, right) => right
+        };
+        (lower, upper)
+    }
 }
diff --git a/src/liballoc/btree/set.rs b/src/liballoc/btree/set.rs
index 2aad476d3153a..ff22b82a5c251 100644
--- a/src/liballoc/btree/set.rs
+++ b/src/liballoc/btree/set.rs
@@ -987,6 +987,10 @@ impl<'a, T> Iterator for Range<'a, T> {
     fn next(&mut self) -> Option<&'a T> {
         self.iter.next().map(|(k, _)| k)
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.iter.size_hint()
+    }
 }
 
 #[stable(feature = "btree_range", since = "1.17.0")]

From 2e92759107d57910166fe1dd192b90f82482fe02 Mon Sep 17 00:00:00 2001
From: Phlosioneer <mattmdrr2@gmail.com>
Date: Mon, 16 Apr 2018 17:50:53 -0400
Subject: [PATCH 2/2] Fix bug, add FIXME

---
 src/liballoc/btree/map.rs | 26 +++++---------------------
 1 file changed, 5 insertions(+), 21 deletions(-)

diff --git a/src/liballoc/btree/map.rs b/src/liballoc/btree/map.rs
index cabeaf3a73f30..18694c06e7227 100644
--- a/src/liballoc/btree/map.rs
+++ b/src/liballoc/btree/map.rs
@@ -1481,15 +1481,7 @@ impl<'a, K, V> Iterator for Range<'a, K, V> {
         }
     }
 
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        if self.front == self.back {
-            (0, Some(0))
-        } else {
-            // There doesn't seem to be any way to get the size of the whole tree
-            // from a NodeRef.
-            (1, None)
-        }
-    }
+    // FIXME(#49205): Add size_hint.
 }
 
 #[stable(feature = "map_values_mut", since = "1.10.0")]
@@ -1623,15 +1615,7 @@ impl<'a, K, V> Iterator for RangeMut<'a, K, V> {
         }
     }
 
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        if self.front == self.back {
-            (0, Some(0))
-        } else {
-            // We can get the root node by using into_root_mut; but there doesn't
-            // seem to be any way to get the size of the tree from the root node.
-            (1, None)
-        }
-    }
+    // FIXME(#49205): Add size_hint.
 }
 
 impl<'a, K, V> RangeMut<'a, K, V> {
@@ -2576,11 +2560,11 @@ impl<K: Ord, V, I: Iterator<Item = (K, V)>> Iterator for MergeIter<K, V, I> {
     fn size_hint(&self) -> (usize, Option<usize>) {
         let (left_lower, left_upper) = self.left.size_hint();
         let (right_lower, right_upper) = self.right.size_hint();
-        let lower = left_lower + right_lower;
+        let lower = left_lower.saturating_add(right_lower);
         let upper = match (left_upper, right_upper) {
             (Some(left), Some(right)) => left.checked_add(right),
-            (left, None) => left,
-            (None, right) => right
+            (_, None) => None,
+            (None, _) => None
         };
         (lower, upper)
     }