Sync the size_hint changes from serde

Upstream serde has a new implementation of the cautious size hint
function, which takes into consideration the size of the elements.
Use the same implementation in serde_with.

serde-rs/serde#2495

Author: jonasbb

serde_with/src/content/de.rs

@@ -283,7 +283,7 @@ impl<'de> Visitor<'de> for ContentVisitor<'de> {
     where
         V: SeqAccess<'de>,
     {
-        let mut vec = Vec::with_capacity(size_hint_cautious(visitor.size_hint()));
+        let mut vec = Vec::with_capacity(size_hint_cautious::<Content<'_>>(visitor.size_hint()));
         while let Some(e) = visitor.next_element()? {
             vec.push(e);
         }
@@ -294,7 +294,9 @@ impl<'de> Visitor<'de> for ContentVisitor<'de> {
     where
         V: MapAccess<'de>,
     {
-        let mut vec = Vec::with_capacity(size_hint_cautious(visitor.size_hint()));
+        let mut vec = Vec::with_capacity(size_hint_cautious::<(Content<'_>, Content<'_>)>(
+            visitor.size_hint(),
+        ));
         while let Some(kv) = visitor.next_entry()? {
             vec.push(kv);
         }

serde_with/src/de/impls.rs

@@ -400,7 +400,7 @@ macro_rules! seq_impl {
                         A: SeqAccess<'de>,
                     {
                         #[allow(clippy::redundant_closure_call)]
-                        let mut values = ($with_capacity)(utils::size_hint_cautious(seq.size_hint()));
+                        let mut values = ($with_capacity)(utils::size_hint_cautious::<T>(seq.size_hint()));
 
                         while let Some(value) = seq
                             .next_element()?
@@ -465,7 +465,7 @@ macro_rules! map_impl {
                         A: MapAccess<'de>,
                     {
                         #[allow(clippy::redundant_closure_call)]
-                        let mut values = ($with_capacity)(utils::size_hint_cautious(map.size_hint()));
+                        let mut values = ($with_capacity)(utils::size_hint_cautious::<(K, V)>(map.size_hint()));
 
                         while let Some((key, value)) = (map.next_entry())?.map(|(k, v): (DeserializeAsWrap::<K, KU>, DeserializeAsWrap::<V, VU>)| (k.into_inner(), v.into_inner())) {
                             values.insert(key, value);

serde_with/src/utils.rs

@@ -5,8 +5,17 @@ use crate::prelude::*;
 /// Re-Implementation of `serde::private::de::size_hint::cautious`
 #[cfg(feature = "alloc")]
 #[inline]
-pub(crate) fn size_hint_cautious(hint: Option<usize>) -> usize {
-    core::cmp::min(hint.unwrap_or(0), 4096)
+pub(crate) fn size_hint_cautious<Element>(hint: Option<usize>) -> usize {
+    const MAX_PREALLOC_BYTES: usize = 1024 * 1024;
+
+    if core::mem::size_of::<Element>() == 0 {
+        0
+    } else {
+        core::cmp::min(
+            hint.unwrap_or(0),
+            MAX_PREALLOC_BYTES / core::mem::size_of::<Element>(),
+        )
+    }
 }
 
 /// Re-Implementation of `serde::private::de::size_hint::from_bounds`