chore(acir_gen): Improve error message for radix decomposition

EXTERNAL_NOIR_LIBRARIES.yml

@@ -113,7 +113,7 @@ libraries:
     repo: AztecProtocol/aztec-packages
     ref: *AZ_COMMIT
     path: noir-projects/noir-protocol-circuits/crates/rollup-lib
-    timeout: 625
+    timeout: 700
     critical: false
     # Use 1 test threads for rollup-lib because each test requires a lot of memory, and multiple ones in parallel exceed the maximum memory limit.
     nargo_args: "--test-threads 1"

compiler/noirc_evaluator/src/acir/acir_context/generated_acir/mod.rs

@@ -378,6 +378,10 @@ impl<F: AcirField> GeneratedAcir<F> {
         }
 
         self.assert_is_zero(input_expr - &composed_limbs);
+        let assertion_payload = self.generate_assertion_message_payload(format!(
+            "Field failed to decompose into specified {limb_count} limbs"
+        ));
+        self.assertion_payloads.insert(self.last_acir_opcode_location(), assertion_payload);
 
         Ok(limb_witnesses)
     }

test_programs/noir_test_success/to_radix_error/Nargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "to_radix_error"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/noir_test_success/to_radix_error/src/main.nr

@@ -0,0 +1,29 @@
+fn constrained_le_decompose(x: Field) -> [u8; 16] {
+    let x_bytes: [u8; 16] = x.to_le_bytes();
+    println(x);
+    println(x_bytes);
+    x_bytes
+}
+
+unconstrained fn brillig_le_decompose(x: Field) -> [u8; 16] {
+    constrained_le_decompose(x)
+}
+
+#[test]
+fn test_acir_field_fits() {
+    let _ = constrained_le_decompose(0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF);
+}
+
+#[test(should_fail_with = "Field failed to decompose into specified 16 limbs")]
+fn test_acir_field_too_big() {
+    // One greater than `0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF`
+    let _ = constrained_le_decompose(0x100000000000000000000000000000000);
+}
+
+// We do not constrain a decomposition in Brillig. We instead just write up to the number of specified limbs,
+// thus this does not fail in Brillig where the number of limbs determines the correctness of the byte decomposition.
+#[test]
+fn test_brillig() {
+    // Safety: testing context
+    let _ = unsafe { brillig_le_decompose(0x100000000000000000000000000000000) };
+}

tooling/debugger/ignored-noir-tests.txt

@@ -12,3 +12,4 @@ test_add
 test_add_and_show
 test_sort
 test_sort_comptime
+test_acir_field_too_big

tooling/debugger/ignored-tests.txt

@@ -16,3 +16,4 @@ array_rc_regression_7842
 mutate_array_copy
 regression_8229
 regression_8926
+to_radix_error