fix: Implement .len() in Acir-Gen

crates/nargo_cli/tests/test_data/array_len/Prover.toml

@@ -1,2 +1,3 @@
 len3 = [1, 2, 3]
 len4 = [1, 2, 3, 4]
+x = 123

crates/nargo_cli/tests/test_data/array_len/src/main.nr

@@ -12,12 +12,17 @@ fn nested_call<N>(b: [Field; N]) -> Field {
     len_plus_1(b)
 }
 
-fn main(len3: [u8; 3], len4: [Field; 4]) {
+fn main(x: Field, len3: [u8; 3], len4: [Field; 4]) {
     assert(len_plus_1(len3) == 4);
     assert(len_plus_1(len4) == 5);
     assert(add_lens(len3, len4) == 7);
     assert(nested_call(len4) == 5);
 
     // std::array::len returns a comptime value
     assert(len4[len3.len()] == 4);
+
+    // Regression for #1023, ensure .len still works after calling to_le_bytes on a witness.
+    // This was needed because normally .len is evaluated before acir-gen where to_le_bytes
+    // on a witness is only evaluated during/after acir-gen.
+    assert(x.to_le_bytes(8).len() != 0);
 }

crates/noirc_evaluator/src/ssa_refactor/acir_gen/acir_ir/acir_variable.rs

@@ -55,6 +55,11 @@ impl AcirType {
         }
     }
 
+    /// Returns a field type
+    pub(crate) fn field() -> Self {
+        AcirType::NumericType(NumericType::NativeField)
+    }
+
     /// Returns a boolean type
     fn boolean() -> Self {
         AcirType::NumericType(NumericType::Unsigned { bit_size: 1 })

crates/noirc_evaluator/src/ssa_refactor/acir_gen/mod.rs

@@ -933,6 +933,14 @@ impl Context {
 
                 Ok(Self::convert_vars_to_values(out_vars, dfg, result_ids))
             }
+            Intrinsic::ArrayLen => {
+                let len = match self.convert_value(arguments[0], dfg) {
+                    AcirValue::Var(_, _) =>  unreachable!("Non-array passed to array.len() method"),
+                    AcirValue::Array(values) => (values.len() as u128).into(),
+                    AcirValue::DynamicArray(array) => (array.len as u128).into(),
+                };
+                Ok(vec![AcirValue::Var(self.acir_context.add_constant(len), AcirType::field())])
+            }
             _ => todo!("expected a black box function"),
         }
     }