From 6d40df8d14745a4deebf37b5a9f600319f9d489b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Ole=20Kr=C3=BCger?= <ole.kruger@trili.tech>
Date: Thu, 19 Sep 2024 11:17:40 +0100
Subject: [PATCH] RISC-V: Specialise impl Serialise/Deserialise for 1-element
 regions

---
 src/riscv/Cargo.lock                          |  1 +
 src/riscv/lib/Cargo.toml                      |  1 +
 .../lib/src/state_backend/owned_backend.rs    | 29 +++++++++++++++++++
 src/rust_deps/Cargo.lock                      |  1 +
 4 files changed, 32 insertions(+)

diff --git a/src/riscv/Cargo.lock b/src/riscv/Cargo.lock
index e71f172ea70c..2d8c43966385 100644
--- a/src/riscv/Cargo.lock
+++ b/src/riscv/Cargo.lock
@@ -1348,6 +1348,7 @@ dependencies = [
  "rand 0.8.5",
  "rustc_apfloat",
  "serde",
+ "serde_json",
  "sha2 0.10.8",
  "strum 0.26.2",
  "tempfile",
diff --git a/src/riscv/lib/Cargo.toml b/src/riscv/lib/Cargo.toml
index e6f00483caa3..dcb668da301a 100644
--- a/src/riscv/lib/Cargo.toml
+++ b/src/riscv/lib/Cargo.toml
@@ -16,6 +16,7 @@ num_enum.workspace = true
 paste.workspace = true
 rustc_apfloat.workspace = true
 serde.workspace = true
+serde_json.workspace = true
 sha2.workspace = true
 strum.workspace = true
 tezos_crypto_rs.workspace = true
diff --git a/src/riscv/lib/src/state_backend/owned_backend.rs b/src/riscv/lib/src/state_backend/owned_backend.rs
index cfa80bfcfa85..66f848e4db3d 100644
--- a/src/riscv/lib/src/state_backend/owned_backend.rs
+++ b/src/riscv/lib/src/state_backend/owned_backend.rs
@@ -175,6 +175,13 @@ impl ManagerSerialise for Owned {
         region: &Self::Region<E, LEN>,
         serializer: S,
     ) -> Result<S::Ok, S::Error> {
+        // A special encoding for single-element regions helps clean up encoding for serialisation
+        // formats that contain structures. For example, JSON, where single-element regions would
+        // be represented as array singletons.
+        if LEN == 1 {
+            return region[0].serialize(serializer);
+        }
+
         // We're serialising this as a fixed-sized tuple because otherwise `bincode` would prefix
         // the length of this array, which is not needed.
         let mut serializer = serializer.serialize_tuple(LEN)?;
@@ -203,6 +210,18 @@ impl ManagerDeserialise for Owned {
     >(
         deserializer: D,
     ) -> Result<Self::Region<E, LEN>, D::Error> {
+        // A special encoding for single-element regions helps clean up encoding for serialisation
+        // formats that contain structures. For example, JSON, where single-element regions would
+        // be represented as array singletons.
+        if LEN == 1 {
+            let values = unsafe {
+                let mut values: [MaybeUninit<E>; LEN] = mem::zeroed();
+                values[0].write(E::deserialize(deserializer)?);
+                values.map(|value| value.assume_init())
+            };
+            return Ok(values);
+        }
+
         struct Inner<E, const LEN: usize>(PhantomData<E>);
 
         impl<'de, E: serde::Deserialize<'de> + Sized, const LEN: usize> serde::de::Visitor<'de>
@@ -341,4 +360,14 @@ pub mod test_helpers {
             assert_eq!(bytes, bytes_after);
         });
     }
+
+    /// Ensure that [`Cell`] serialises in a way that represents the underlying element
+    /// directly instead of wrapping it into an array (as it is an array under the hood).
+    #[test]
+    fn cell_direct_serialise() {
+        let cell: Cell<u64, Owned> = Cell::bind([42]);
+        let json_value = serde_json::to_value(cell).unwrap();
+        let expected_json_value = serde_json::json!(42);
+        assert_eq!(json_value, expected_json_value);
+    }
 }
diff --git a/src/rust_deps/Cargo.lock b/src/rust_deps/Cargo.lock
index 12de255b8486..72b8b5a8a285 100644
--- a/src/rust_deps/Cargo.lock
+++ b/src/rust_deps/Cargo.lock
@@ -2232,6 +2232,7 @@ dependencies = [
  "paste",
  "rustc_apfloat",
  "serde",
+ "serde_json",
  "sha2 0.10.8",
  "strum 0.26.2",
  "tezos-smart-rollup-constants",
-- 
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