docs: add docs on state vars docs re packing in structs

noir-projects/aztec-nr/aztec/src/macros/storage.nr

@@ -5,7 +5,9 @@ use std::meta::unquote;
 /// generating a `storage_layout()` getter on the contract struct.
 pub comptime mut global STORAGE_LAYOUT_NAME: CHashMap<Module, Quoted> = CHashMap::new();
 
-/// This function
+/// Declares the contract's storage.
+///
+/// This function:
 /// - marks the contract as having storage, so that `macros::utils::module_has_storage` will return true,
 /// - marks the struct `s` as the one describing the storage layout of a contract,
 /// - generates an `impl` block for the storage struct with an `init` function (call to `init` is then injected at the

noir-projects/aztec-nr/aztec/src/state_vars/delayed_public_mutable.nr

@@ -470,7 +470,34 @@ where
     ///
     /// ## Cost
     ///
-    /// This function performs a single merkle tree inclusion proof, which is in the order of 4k gates.
+    /// This function performs a historical public storage read (which is in the order of 4k gates), **regardless of
+    /// `T`'s packed length**. This is because [`DelayedPublicMutable::schedule_value_change`] stores not just the
+    /// value but also its hash: this function obtains the preimage from an oracle and proves that it matches the hash
+    /// from public storage.
+    ///
+    /// Because of this reason, if two mutable values are often privately read together it can be convenient to group
+    /// them in a single type `T`. Note however that this will result in them sharing the mutation delay:
+    ///
+    /// ```noir
+    /// // Bad: reading both `paused` and `fee` will require two historical public storage reads
+    /// #[storage]
+    /// struct Storage<C> {
+    ///     paused: DelayedPublicMutable<bool, INITIAL_DELAY_S, C>,
+    ///     fee: DelayedPublicMutable<Field, INITIAL_DELAY_S, C>,
+    /// }
+    ///
+    /// // Good: both `paused` and `fee` are retrieved in a single historical public storage read
+    /// #[derive(Packable)]
+    /// struct Config {
+    ///     paused: bool,
+    ///     fee: Field,
+    /// }
+    ///
+    /// #[storage]
+    /// struct Storage<Context> {
+    ///     config: DelayedPublicMutable<Config, INITIAL_DELAY_S, Context>,
+    /// }
+    /// ```
     pub fn get_current_value(self) -> T
     where
         T: Packable,

noir-projects/aztec-nr/aztec/src/state_vars/mod.nr

@@ -7,6 +7,62 @@
 //! Due to Aztec contracts being able to store both public and private state, there are many more different types of
 //! state variables, each with their nuance and use cases. Understanding these is key to understanding how a contract
 //! works.
+//!
+//! ## Packing for Efficient Access
+//!
+//! Because all state variables are fully independent, when a contract reads or writes one of them all others are left
+//! untouched. This is good for isolation, but in some cases users may want to group variables together for more
+//! efficient access, for example to access multiple values in a single storage read or write.
+//!
+//! The pattern to follow is to **group related values in a `struct`**, just like in Solidity. How values are packed
+//! inside this `struct` is governed by the [`crate::protocol::traits::Packable`] trait, which must be `#[derive]`'d or
+//! manually implemented - see the [`Packable`](crate::protocol::traits::Packable)'s docs on how to do this.
+//!
+//! ```noir
+//! // Inefficient reads and writes - each bool is assigned a distinct storage slot, so reading or writing both
+//! requires
+//! // executing `SLOAD` or `SSTORE` twice.
+//! #[storage]
+//! struct Storage<C> {
+//!     a: PublicMutable<bool, C>,
+//!     b: PublicMutable<bool, C>,
+//! }
+//!
+//! // By storing the booleans in a single struct and implementing the Packable trait with tight packing, we can now
+//! // read and write both values in a single `SLOAD` or `SSTORE` opcode.
+//! struct TwoBooleans {
+//!     a: bool,
+//!     b: bool,
+//! }
+//!
+//! impl aztec::protocol::traits::Packable for TwoBooleans {
+//!     let N: u32 = 1;
+//!
+//!     fn pack(self) -> [Field; Self::N] {
+//!         [(self.a as Field) * 2.pow_32(1) + (self.b as Field)]
+//!     }
+//!
+//!     fn unpack(packed: [Field; Self::N]) -> Self {
+//!         let b = (packed[0] as u1) != 0;
+//!         let a = (((packed[0] - b as Field) / 2.pow_32(1)) as u1) != 0;
+//!
+//!         Self { a, b }
+//!     }
+//! }
+//!
+//! #[storage]
+//! struct Storage<C> {
+//!     a_and_b: PublicMutable<TwoBooleans, C>,
+//! }
+//! ```
+//!
+//! Note that private state variables and public ones that can be read from private (like
+//! [`PublicImmutable`](crate::state_vars::PublicImmutable) and
+//! [`DelayedPublicMutable`](crate::state_vars::DelayedPublicMutable)) benefit from packing multiple values in the same
+//! `struct` even if there is no need for tight packing (e.g. if all values are `Field`s), since they often work by
+//! reading the _hash_ of the entire value. Many values in the same `struct` will result in a single hash, and
+//! therefore
+//! a single read.
 
 mod state_variable;
 pub use state_variable::StateVariable;

noir-projects/aztec-nr/aztec/src/state_vars/public_immutable.nr

@@ -287,11 +287,10 @@ impl<T> PublicImmutable<T, &mut PrivateContext> {
     /// ## Cost
     ///
     /// A nullifier existence request is pushed to the context, which will be verified by the kernel circuit.
-    /// Additionally, a historical public storage read at the anchor block is performed for a single storage slot,
-    /// **regardless of `T`'s packed length**. This is because [`PublicImmutable::initialize`] stores not just the
-    /// value
-    /// but also its hash: this function obtains the preimage from an oracle and proves that it matches the hash from
-    /// public storage.
+    /// Additionally, a historical public storage read at the anchor block (which is on the order of 4k gates) is
+    /// performed for a single storage slot, **regardless of `T`'s packed length**. This is because
+    /// [`PublicImmutable::initialize`] stores not just the value but also its hash: this function obtains the preimage
+    /// from an oracle and proves that it matches the hash from public storage.
     ///
     /// Because of this reason it is convenient to group together all of a contract's public immutable values that are
     /// read privately in a single type `T`:

noir-projects/noir-protocol-circuits/crates/serde/src/type_impls.nr

@@ -796,3 +796,31 @@ where
 mod impls {
     use crate::serialization::{Deserialize, Serialize};
 }
+
+#[test]
+unconstrained fn bounded_vec_serialization() {
+    // Test empty BoundedVec
+    let empty_vec: BoundedVec<Field, 3> = BoundedVec::from_array([]);
+    let serialized = empty_vec.serialize();
+    let deserialized = BoundedVec::<Field, 3>::deserialize(serialized);
+    assert_eq(empty_vec, deserialized);
+    assert_eq(deserialized.len(), 0);
+
+    // Test partially filled BoundedVec
+    let partial_vec: BoundedVec<[u32; 2], 3> = BoundedVec::from_array([[1, 2]]);
+    let serialized = partial_vec.serialize();
+    let deserialized = BoundedVec::<[u32; 2], 3>::deserialize(serialized);
+    assert_eq(partial_vec, deserialized);
+    assert_eq(deserialized.len(), 1);
+    assert_eq(deserialized.get(0), [1, 2]);
+
+    // Test full BoundedVec
+    let full_vec: BoundedVec<[u32; 2], 3> = BoundedVec::from_array([[1, 2], [3, 4], [5, 6]]);
+    let serialized = full_vec.serialize();
+    let deserialized = BoundedVec::<[u32; 2], 3>::deserialize(serialized);
+    assert_eq(full_vec, deserialized);
+    assert_eq(deserialized.len(), 3);
+    assert_eq(deserialized.get(0), [1, 2]);
+    assert_eq(deserialized.get(1), [3, 4]);
+    assert_eq(deserialized.get(2), [5, 6]);
+}