Allow larger preallocated capacity for smaller elements

serde/src/de/impls.rs

@@ -681,8 +681,8 @@ impl<'de> Visitor<'de> for CStringVisitor {
     where
         A: SeqAccess<'de>,
     {
-        let len = size_hint::cautious(seq.size_hint());
-        let mut values = Vec::with_capacity(len);
+        let capacity = size_hint::cautious::<u8>(seq.size_hint());
+        let mut values = Vec::<u8>::with_capacity(capacity);
 
         while let Some(value) = try!(seq.next_element()) {
             values.push(value);
@@ -936,7 +936,7 @@ macro_rules! seq_impl {
                         A: SeqAccess<'de>,
                     {
                         $clear(&mut self.0);
-                        $reserve(&mut self.0, size_hint::cautious($access.size_hint()));
+                        $reserve(&mut self.0, size_hint::cautious::<T>($access.size_hint()));
 
                         // FIXME: try to overwrite old values here? (Vec, VecDeque, LinkedList)
                         while let Some(value) = try!($access.next_element()) {
@@ -962,7 +962,7 @@ seq_impl!(
     BinaryHeap<T: Ord>,
     seq,
     BinaryHeap::clear,
-    BinaryHeap::with_capacity(size_hint::cautious(seq.size_hint())),
+    BinaryHeap::with_capacity(size_hint::cautious::<T>(seq.size_hint())),
     BinaryHeap::reserve,
     BinaryHeap::push
 );
@@ -992,7 +992,7 @@ seq_impl!(
     HashSet<T: Eq + Hash, S: BuildHasher + Default>,
     seq,
     HashSet::clear,
-    HashSet::with_capacity_and_hasher(size_hint::cautious(seq.size_hint()), S::default()),
+    HashSet::with_capacity_and_hasher(size_hint::cautious::<T>(seq.size_hint()), S::default()),
     HashSet::reserve,
     HashSet::insert
 );
@@ -1002,7 +1002,7 @@ seq_impl!(
     VecDeque<T>,
     seq,
     VecDeque::clear,
-    VecDeque::with_capacity(size_hint::cautious(seq.size_hint())),
+    VecDeque::with_capacity(size_hint::cautious::<T>(seq.size_hint())),
     VecDeque::reserve,
     VecDeque::push_back
 );
@@ -1036,7 +1036,8 @@ where
             where
                 A: SeqAccess<'de>,
             {
-                let mut values = Vec::with_capacity(size_hint::cautious(seq.size_hint()));
+                let capacity = size_hint::cautious::<T>(seq.size_hint());
+                let mut values = Vec::<T>::with_capacity(capacity);
 
                 while let Some(value) = try!(seq.next_element()) {
                     values.push(value);
@@ -1072,7 +1073,7 @@ where
             where
                 A: SeqAccess<'de>,
             {
-                let hint = size_hint::cautious(seq.size_hint());
+                let hint = size_hint::cautious::<T>(seq.size_hint());
                 if let Some(additional) = hint.checked_sub(self.0.len()) {
                     self.0.reserve(additional);
                 }
@@ -1416,7 +1417,7 @@ map_impl!(BTreeMap<K: Ord, V>, map, BTreeMap::new());
 map_impl!(
     HashMap<K: Eq + Hash, V, S: BuildHasher + Default>,
     map,
-    HashMap::with_capacity_and_hasher(size_hint::cautious(map.size_hint()), S::default())
+    HashMap::with_capacity_and_hasher(size_hint::cautious::<(K, V)>(map.size_hint()), S::default())
 );
 
 ////////////////////////////////////////////////////////////////////////////////

serde/src/private/de.rs

@@ -474,7 +474,8 @@ mod content {
         where
             V: SeqAccess<'de>,
         {
-            let mut vec = Vec::with_capacity(size_hint::cautious(visitor.size_hint()));
+            let mut vec =
+                Vec::<Content>::with_capacity(size_hint::cautious::<Content>(visitor.size_hint()));
             while let Some(e) = try!(visitor.next_element()) {
                 vec.push(e);
             }
@@ -485,7 +486,10 @@ mod content {
         where
             V: MapAccess<'de>,
         {
-            let mut vec = Vec::with_capacity(size_hint::cautious(visitor.size_hint()));
+            let mut vec =
+                Vec::<(Content, Content)>::with_capacity(
+                    size_hint::cautious::<(Content, Content)>(visitor.size_hint()),
+                );
             while let Some(kv) = try!(visitor.next_entry()) {
                 vec.push(kv);
             }
@@ -844,7 +848,10 @@ mod content {
             M: MapAccess<'de>,
         {
             let mut tag = None;
-            let mut vec = Vec::with_capacity(size_hint::cautious(map.size_hint()));
+            let mut vec = Vec::<(Content, Content)>::with_capacity(size_hint::cautious::<(
+                Content,
+                Content,
+            )>(map.size_hint()));
             while let Some(k) = try!(map.next_key_seed(TagOrContentVisitor::new(self.tag_name))) {
                 match k {
                     TagOrContent::Tag => {

serde/src/private/size_hint.rs

@@ -8,9 +8,17 @@ where
 }
 
 #[cfg(any(feature = "std", feature = "alloc"))]
-#[inline]
-pub fn cautious(hint: Option<usize>) -> usize {
-    cmp::min(hint.unwrap_or(0), 4096)
+pub fn cautious<Element>(hint: Option<usize>) -> usize {
+    const MAX_PREALLOC_BYTES: usize = 1024 * 1024;
+
+    if mem::size_of::<Element>() == 0 {
+        0
+    } else {
+        cmp::min(
+            hint.unwrap_or(0),
+            MAX_PREALLOC_BYTES / mem::size_of::<Element>(),
+        )
+    }
 }
 
 fn helper(bounds: (usize, Option<usize>)) -> Option<usize> {