Feat/disk cache (#354)

- Bump rust version to `nightly-2025-07-02` because some of the nightly features we were using have been stabilized.
- Introduce feature `disk_cache` which enables caching to disk.  Each time an artifact is retrieved from the cache it will be read and deserialized.  On a cache miss the artifact will be created, serialized and stored to disk.
- Introduce feature `mem_cache` which enables caching to memory.  All cached artifacts are kept in memory after they are created.  The mem cache implementation avoids cloning of artifacts by extending their lifetime to `'static`.  This is `unsafe` code, but I argue that this usage is safe.
- Add a `build.rs`
  - When the feature `disk_cache` is enabled, the `build.rs` will inject env variables to the process with the git commit information, which is used to index the cached artifacts
- Replace all previous cached artifacts from `LazyStatic` methods that call the cache API
- Derive `Serialize, Deserialize` for all `*Target` types so that they can be serialized for caching to disk
- Add finer level of caching: now we cache the `CircuitData` and `VerifierData` independently.  The reason for this is that `CircuitData` is a very big artifact which is not needed for verification.  So by only accessing `VerifierData` in verification we don't pay a big overhead for reading from disk and deserializing
- Add missing artifacts to the cache: like the `CircuitData` for the `MainPod` indexed by `Params`
- Add helper types to serialize and deserialize `CircuitData`, `CommonData` and `VerifierData` with the set of gates and generators used in the recursive MainPod circuit
- Tweak the ids of our custom gates so that they remain unique when their generic parameters change
- Bugfix: several tests were using the standard `vd_set` but were using MainPod circuits with non-default parameters.  This was working before because there was a bug: the MainPod circuit was reporting that the used verifier data was the standard one instead of picking the one corresponding to it's own Params.

Summary of breaking changes:
- One and only one of the features `mem_cache` or `disk_cache` need to be enabled.  By default it's `mem_cache`
  - To enable the `disk_cache` you need to disable the default features like this: `--no-default-features --features=backend_plonky2,zk,disk_cache`
- Removed `DEFAULT_PARAMS`, instead use `Params::default()`
- Removed `STANDARD_REC_MAIN_POD_CIRCUIT_DATA`, instead use `cache_get_standard_rec_main_pod_common_circuit_data`
- The library is now using `nightly-2025-07-02`.  Some rust language features are unstable in previous versions.

src/backends/plonky2/primitives/ec/bits.rs

@@ -16,11 +16,12 @@ use plonky2::{
     plonk::{circuit_builder::CircuitBuilder, circuit_data::CommonCircuitData},
     util::serialization::{Buffer, IoResult, Read, Write},
 };
+use serde::{Deserialize, Serialize};
 
 use crate::backends::plonky2::basetypes::{D, F};
 
-#[derive(Debug)]
-struct ConditionalZeroGenerator<F: RichField + Extendable<D>, const D: usize> {
+#[derive(Debug, Default, Clone)]
+pub(crate) struct ConditionalZeroGenerator<F: RichField + Extendable<D>, const D: usize> {
     if_zero: Target,
     then_zero: Target,
     quot: Target,
@@ -78,9 +79,11 @@ impl<F: RichField + Extendable<D>, const D: usize> SimpleGenerator<F, D>
 
 /// A big integer, represented in base `2^32` with 10 digits, in little endian
 /// form.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct BigUInt320Target {
+    #[serde(with = "serde_arrays")]
     pub limbs: [Target; 10],
+    #[serde(with = "serde_arrays")]
     pub bits: [BoolTarget; 320],
 }
 
@@ -313,7 +316,7 @@ fn biguint_limbs_to_bits(builder: &mut CircuitBuilder<F, D>, limbs: &[Target]) -
 Copied from https://github.com/0xPolygonZero/plonky2/blob/82791c4809d6275682c34b926390ecdbdc2a5297/plonky2/src/gadgets/range_check.rs#L62
  */
 
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Clone)]
 pub struct LowHighGenerator {
     integer: Target,
     n_log: usize,

src/backends/plonky2/primitives/ec/curve.rs

@@ -57,6 +57,7 @@ pub fn ec_field_sqrt(x: &ECField) -> Option<ECField> {
     ]);
     // Compute x^((r-1)/2) = x^(p*((1+p)/2)*(1+p^2))
     let x1 = x.frobenius();
+    #[allow(clippy::manual_div_ceil)]
     let x2 = x1.exp_u64((1 + GoldilocksField::ORDER) / 2);
     let den = x2 * x2.repeated_frobenius(2);
     Some(num / den)
@@ -440,7 +441,7 @@ impl Mul<Point> for &BigUint {
 
 type FieldTarget = OEFTarget<5, QuinticExtension<GoldilocksField>>;
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Default, Debug, Serialize, Deserialize)]
 pub struct PointTarget {
     pub x: FieldTarget,
     pub u: FieldTarget,
@@ -470,8 +471,8 @@ impl PointTarget {
     }
 }
 
-#[derive(Clone, Debug)]
-struct PointSquareRootGenerator {
+#[derive(Clone, Default, Debug)]
+pub(crate) struct PointSquareRootGenerator {
     pub orig: PointTarget,
     pub sqrt: PointTarget,
 }

src/backends/plonky2/primitives/ec/field.rs

@@ -15,6 +15,7 @@ use plonky2::{
     plonk::{circuit_builder::CircuitBuilder, circuit_data::CommonCircuitData},
     util::serialization::{Buffer, IoError, Read, Write},
 };
+use serde::{Deserialize, Serialize};
 
 //use super::gates::field::NNFMulGate;
 use crate::{
@@ -83,8 +84,9 @@ pub trait CircuitBuilderNNF<
 }
 
 /// Target type modelled on OEF.
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct OEFTarget<const DEG: usize, NNF: OEF<DEG>> {
+    #[serde(with = "serde_arrays")]
     pub components: [Target; DEG],
     _phantom_data: PhantomData<NNF>,
 }
@@ -106,8 +108,8 @@ impl<const DEG: usize, NNF: OEF<DEG>> Default for OEFTarget<DEG, NNF> {
 
 /// Quotient generator for OEF targets. Allows us to automagically
 /// generate quotients as witnesses.
-#[derive(Debug, Default)]
-struct QuotientGeneratorOEF<const DEG: usize, NNF: OEF<DEG>> {
+#[derive(Debug, Default, Clone)]
+pub(crate) struct QuotientGeneratorOEF<const DEG: usize, NNF: OEF<DEG>> {
     numerator: OEFTarget<DEG, NNF>,
     denominator: OEFTarget<DEG, NNF>,
     quotient: OEFTarget<DEG, NNF>,
@@ -121,7 +123,11 @@ impl<
     > SimpleGenerator<F, D> for QuotientGeneratorOEF<DEG, NNF>
 {
     fn id(&self) -> String {
-        "QuotientGeneratorOEF".to_string()
+        format!(
+            "QuotientGeneratorOEF<{}, {}>",
+            DEG,
+            std::any::type_name::<NNF>()
+        )
     }
 
     fn dependencies(&self) -> Vec<Target> {

src/backends/plonky2/primitives/ec/gates/curve.rs

@@ -25,7 +25,7 @@ use crate::backends::plonky2::primitives::ec::{
 /// operation when all its witness wire values are zero (so that when the gate is partially used,
 /// the unused slots still pass the constraints). This is the reason why this gate doesn't add the
 /// final offset: if it did, the constraints wouldn't pass on the zero witness values.
-#[derive(Debug, Clone)]
+#[derive(Debug, Default, Clone)]
 pub struct ECAddHomogOffset;
 
 impl SimpleGate for ECAddHomogOffset {

src/backends/plonky2/primitives/ec/gates/generic.rs

@@ -66,7 +66,7 @@ pub struct RecursiveGateAdapter<const D: usize, G: SimpleGate> {
     _gate: PhantomData<G>,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Default, Clone)]
 pub struct RecursiveGenerator<const D: usize, G: SimpleGate> {
     row: usize,
     index: usize,
@@ -175,7 +175,7 @@ where
     G::F: RichField + Extendable<D> + Extendable<1>,
 {
     fn id(&self) -> String {
-        G::ID.to_string()
+        format!("GateAdapter<{}>", std::any::type_name::<G>())
     }
 
     fn serialize(
@@ -336,7 +336,7 @@ where
     }
 
     fn id(&self) -> String {
-        format!("Generator<{},{}>", D, G::ID)
+        format!("RecursiveGenerator<{}, {}>", D, std::any::type_name::<G>())
     }
 
     fn dependencies(&self) -> Vec<Target> {
@@ -374,7 +374,11 @@ where
     F: RichField + Extendable<D>,
 {
     fn id(&self) -> String {
-        format!("Recursive<{},{}>", D, G::ID)
+        format!(
+            "RecursiveGateAdapter<{}, {}>",
+            D,
+            std::any::type_name::<G>()
+        )
     }
 
     fn serialize(&self, dst: &mut Vec<u8>, _common_data: &CommonCircuitData<F, D>) -> IoResult<()> {

src/backends/plonky2/primitives/ec/schnorr.rs

@@ -101,7 +101,7 @@ impl<'de> Deserialize<'de> for Signature {
 }
 
 /// Targets for Schnorr signature over ecGFp5.
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct SignatureTarget {
     pub s: BigUInt320Target,
     pub e: BigUInt320Target,

src/backends/plonky2/primitives/merkletree/circuit.rs

@@ -23,6 +23,7 @@ use plonky2::{
     },
     plonk::circuit_builder::CircuitBuilder,
 };
+use serde::{Deserialize, Serialize};
 
 use crate::{
     backends::plonky2::{
@@ -35,7 +36,7 @@ use crate::{
     middleware::{EMPTY_HASH, EMPTY_VALUE, F, HASH_SIZE},
 };
 
-#[derive(Clone, Debug)]
+#[derive(Clone, Debug, Serialize, Deserialize)]
 pub struct MerkleClaimAndProofTarget {
     pub(crate) max_depth: usize,
     // `enabled` determines if the merkleproof verification is enabled
@@ -194,6 +195,7 @@ impl MerkleClaimAndProofTarget {
     }
 }
 
+#[derive(Clone, Serialize, Deserialize)]
 pub struct MerkleProofExistenceTarget {
     max_depth: usize,
     // `enabled` determines if the merkleproof verification is enabled

src/backends/plonky2/primitives/merkletree/mod.rs

@@ -289,7 +289,7 @@ impl MerkleTree {
     }
 
     /// returns an iterator over the leaves of the tree
-    pub fn iter(&self) -> Iter {
+    pub fn iter(&self) -> Iter<'_> {
         Iter {
             state: vec![&self.root],
         }

src/backends/plonky2/primitives/signature/circuit.rs

@@ -20,6 +20,7 @@ use plonky2::{
         proof::{ProofWithPublicInputs, ProofWithPublicInputsTarget},
     },
 };
+use serde::{Deserialize, Serialize};
 
 use crate::{
     backends::plonky2::{
@@ -38,6 +39,7 @@ use crate::{
 // TODO: This is a very simple wrapper over the signature verification implemented on
 // `SignatureTarget`.  I think we can remove this and use it directly.  Also we're not using the
 // `enabled` flag, so it should be straight-forward to remove this.
+#[derive(Clone, Serialize, Deserialize)]
 pub struct SignatureVerifyTarget {
     // `enabled` determines if the signature verification is enabled
     pub(crate) enabled: BoolTarget,