From 4309fe33dcffa008ca45d333770901ec01130bf0 Mon Sep 17 00:00:00 2001
From: Nick Fitzgerald <fitzgen@gmail.com>
Date: Tue, 24 Feb 2026 14:35:46 -0800
Subject: [PATCH] Fix OOM-handling `SecondaryMap`'s
 serialization/deserialization so that it matches
 `cranelift_entity::SecondaryMap`

---
 cranelift/entity/src/map.rs                   |  10 ++
 .../environ/src/collections/secondary_map.rs  | 123 ++++++++++++++++--
 2 files changed, 125 insertions(+), 8 deletions(-)

diff --git a/cranelift/entity/src/map.rs b/cranelift/entity/src/map.rs
index c576b4673ee3..32aea22263d6 100644
--- a/cranelift/entity/src/map.rs
+++ b/cranelift/entity/src/map.rs
@@ -194,6 +194,16 @@ where
         Ok(())
     }
 
+    /// Get this map's underlying values as a slice.
+    pub fn as_values_slice(&self) -> &[V] {
+        &self.elems
+    }
+
+    /// Get this map's default value.
+    pub fn default_value(&self) -> &V {
+        &self.default
+    }
+
     /// Slow path for `index_mut` which resizes the vector.
     #[cold]
     fn resize_for_index_mut(&mut self, i: usize) -> &mut V {
diff --git a/crates/environ/src/collections/secondary_map.rs b/crates/environ/src/collections/secondary_map.rs
index cfa1c961c234..1ceb052792d8 100644
--- a/crates/environ/src/collections/secondary_map.rs
+++ b/crates/environ/src/collections/secondary_map.rs
@@ -1,7 +1,6 @@
 use crate::{collections::Vec, error::OutOfMemory};
 use core::{fmt, ops::Index};
 use cranelift_entity::{EntityRef, SecondaryMap as Inner};
-use serde::ser::SerializeSeq;
 
 /// Like [`cranelift_entity::SecondaryMap`] but all allocation is fallible.
 pub struct SecondaryMap<K, V>
@@ -159,16 +158,35 @@ where
 impl<K, V> serde::ser::Serialize for SecondaryMap<K, V>
 where
     K: EntityRef,
-    V: Clone + serde::ser::Serialize,
+    V: Clone + PartialEq + serde::ser::Serialize,
 {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: serde::Serializer,
     {
-        let mut seq = serializer.serialize_seq(Some(self.capacity()))?;
-        for elem in self.values() {
-            seq.serialize_element(elem)?;
+        use serde::ser::SerializeSeq as _;
+
+        // Ignore any trailing default values.
+        let mut len = self.inner.as_values_slice().len();
+        while len > 0 && &self[K::new(len - 1)] == self.inner.default_value() {
+            len -= 1;
+        }
+
+        // Plus one for the default value.
+        let mut seq = serializer.serialize_seq(Some(len + 1))?;
+
+        // Always serialize the default value first.
+        seq.serialize_element(self.inner.default_value())?;
+
+        for elem in self.values().take(len) {
+            let elem = if elem == self.inner.default_value() {
+                None
+            } else {
+                Some(elem)
+            };
+            seq.serialize_element(&elem)?;
         }
+
         seq.end()
     }
 }
@@ -176,13 +194,102 @@ where
 impl<'de, K, V> serde::de::Deserialize<'de> for SecondaryMap<K, V>
 where
     K: EntityRef,
-    V: Clone + Default + serde::de::Deserialize<'de>,
+    V: Clone + serde::de::Deserialize<'de>,
 {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
         D: serde::Deserializer<'de>,
     {
-        let values: Vec<V> = serde::de::Deserialize::deserialize(deserializer)?;
-        Ok(Self::from(values))
+        struct Visitor<K, V>(core::marker::PhantomData<fn() -> SecondaryMap<K, V>>)
+        where
+            K: EntityRef,
+            V: Clone;
+
+        impl<'de, K, V> serde::de::Visitor<'de> for Visitor<K, V>
+        where
+            K: EntityRef,
+            V: Clone + serde::de::Deserialize<'de>,
+        {
+            type Value = SecondaryMap<K, V>;
+
+            fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
+                f.write_str("struct SecondaryMap")
+            }
+
+            fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
+            where
+                A: serde::de::SeqAccess<'de>,
+            {
+                // Minus one to account for the default element, which is always
+                // the first in the sequence.
+                let size_hint = seq.size_hint().and_then(|n| n.checked_sub(1));
+
+                let Some(default) = seq.next_element::<V>()? else {
+                    return Err(serde::de::Error::custom("Default value required"));
+                };
+
+                let mut map = SecondaryMap::<K, V>::with_default(default.clone());
+
+                if let Some(n) = size_hint {
+                    map.resize(n).map_err(|oom| serde::de::Error::custom(oom))?;
+                }
+
+                let mut idx = 0;
+                while let Some(val) = seq.next_element::<Option<V>>()? {
+                    let key = K::new(idx);
+                    let val = match val {
+                        None => default.clone(),
+                        Some(val) => val,
+                    };
+
+                    map.insert(key, val)
+                        .map_err(|oom| serde::de::Error::custom(oom))?;
+
+                    idx += 1;
+                }
+
+                Ok(map)
+            }
+        }
+
+        deserializer.deserialize_seq(Visitor(core::marker::PhantomData))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::error::Result;
+    use alloc::vec;
+    use alloc::vec::Vec;
+
+    #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
+    struct K(u32);
+    crate::entity_impl!(K);
+
+    fn k(i: usize) -> K {
+        K::new(i)
+    }
+
+    #[test]
+    fn serialize_deserialize() -> Result<()> {
+        let mut map = SecondaryMap::<K, u32>::with_default(99);
+        map.insert(k(0), 33)?;
+        map.insert(k(1), 44)?;
+        map.insert(k(2), 55)?;
+        map.insert(k(3), 99)?;
+        map.insert(k(4), 99)?;
+
+        let bytes = postcard::to_allocvec(&map)?;
+        let map2: SecondaryMap<K, u32> = postcard::from_bytes(&bytes)?;
+
+        for i in 0..10 {
+            assert_eq!(map[k(i)], map2[k(i)]);
+        }
+
+        // Trailing default entries were omitted from the serialization.
+        assert_eq!(map2.keys().collect::<Vec<_>>(), vec![k(0), k(1), k(2)]);
+
+        Ok(())
     }
 }