Improve InvalidCircuitSize error message

Resolves #792

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Improve InvalidCircuitSize error [#792]
+
 ## [0.17.0] - 2023-11-1
 
 ### Added
@@ -525,6 +529,7 @@ is necessary since `rkyv/validation` was required as a bound.
 - Proof system module.
 
 <!-- ISSUES -->
+[#792]: https://github.com/dusk-network/plonk/issues/792
 [#784]: https://github.com/dusk-network/plonk/issues/784
 [#773]: https://github.com/dusk-network/plonk/issues/773
 [#774]: https://github.com/dusk-network/plonk/issues/774

src/composer.rs

@@ -81,7 +81,8 @@ pub trait Composer: Sized + Index<Witness, Output = BlsScalar> {
     /// PLONK runtime controller
     fn runtime(&mut self) -> &mut Runtime;
 
-    /// Initialize the constraint system with dummy gates
+    /// Initialize the constraint system with the witnesses for 0 and 1 and
+    /// append two dummy gates
     fn initialized() -> Self {
         #[allow(deprecated)]
         let mut slf = Self::uninitialized();
@@ -1051,7 +1052,7 @@ pub trait Composer: Sized + Index<Witness, Output = BlsScalar> {
         o
     }
 
-    /// Prove a circuit with a builder initialized with `constraints` capacity.
+    /// Prove a circuit with a builder initialized with dummy gates
     fn prove<C>(constraints: usize, circuit: &C) -> Result<Self, Error>
     where
         C: Circuit,
@@ -1061,8 +1062,12 @@ pub trait Composer: Sized + Index<Witness, Output = BlsScalar> {
         circuit.circuit(&mut builder)?;
 
         // assert that the circuit has the expected amount of constraints
-        if builder.constraints() != constraints {
-            return Err(Error::InvalidCircuitSize);
+        let description_size = builder.constraints();
+        if description_size != constraints {
+            return Err(Error::InvalidCircuitSize(
+                description_size,
+                constraints,
+            ));
         }
 
         builder.runtime().event(RuntimeEvent::ProofFinished);

src/error.rs

@@ -37,7 +37,7 @@ pub enum Error {
     CircuitAlreadyPreprocessed,
     /// This error occurs when the circuit for the proof has a different size
     /// than the prover circuit description
-    InvalidCircuitSize,
+    InvalidCircuitSize(usize, usize),
 
     // Preprocessing errors
     /// This error occurs when an error triggers during the preprocessing
@@ -127,8 +127,8 @@ impl std::fmt::Display for Error {
             Self::CircuitAlreadyPreprocessed => {
                 write!(f, "circuit has already been preprocessed")
             }
-            Self::InvalidCircuitSize => {
-                write!(f, "circuit size doesn't match with circuit description")
+            Self::InvalidCircuitSize(description_size, circuit_size) => {
+                write!(f, "circuit size {circuit_size} doesn't match with size of circuit description {description_size}")
             }
             Self::DegreeIsZero => {
                 write!(f, "cannot create PublicParameters with max degree 0")

tests/size.rs

@@ -47,18 +47,15 @@ fn size() {
     let pp = PublicParameters::setup(CAPACITY, rng)
         .expect("Creation of public parameter shouldn't fail");
 
-    // compiling the default version of TestSize, which only one gate: sum = 0
+    // compiling the default version of TestSize circuit, with only one gate in
+    // addition to the 4 dummy gates
     let (prover, _verifier) = Compiler::compile::<TestSize>(&pp, LABEL)
         .expect("It should be possible to compile the prover and verifier");
 
     // Create circuit with more gates
-    let pi: Vec<BlsScalar> = [BlsScalar::one(); 5].into();
-    let sum = pi.iter().sum();
-    let circuit = TestSize::new(pi, sum);
+    let witnesses: Vec<BlsScalar> = [BlsScalar::one(); 4].into();
+    let sum = witnesses.iter().sum();
+    let circuit = TestSize::new(witnesses, sum);
     let result = prover.prove(rng, &circuit);
-    assert_eq!(
-        result,
-        Err(Error::InvalidCircuitSize),
-        "proof creation for different sized circuit shouldn't be possible"
-    );
+    assert!(result.is_err_and(|e| e == Error::InvalidCircuitSize(9, 5)));
 }