fix: enforce correctness of decompositions performed at compile time (#…

…6278)

# Description

## Problem\*

Resolves #6244

## Summary\*

We were not checking that a radix decomposition simplification performed
at compile time was valid, resulting in a truncation to fit the result
array. This PR prevents the simplification in this case so we can
instead error at runtime.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

compiler/noirc_evaluator/src/ssa/ir/instruction/call.rs

@@ -60,9 +60,7 @@ pub(super) fn simplify_call(
                 } else {
                     unreachable!("ICE: Intrinsic::ToRadix return type must be array")
                 };
-                let result_array = constant_to_radix(endian, field, 2, limb_count, dfg);
-
-                SimplifyResult::SimplifiedTo(result_array)
+                constant_to_radix(endian, field, 2, limb_count, dfg)
             } else {
                 SimplifyResult::None
             }
@@ -79,10 +77,7 @@ pub(super) fn simplify_call(
                 } else {
                     unreachable!("ICE: Intrinsic::ToRadix return type must be array")
                 };
-
-                let result_array = constant_to_radix(endian, field, radix, limb_count, dfg);
-
-                SimplifyResult::SimplifiedTo(result_array)
+                constant_to_radix(endian, field, radix, limb_count, dfg)
             } else {
                 SimplifyResult::None
             }
@@ -611,22 +606,29 @@ fn constant_to_radix(
     radix: u32,
     limb_count: u32,
     dfg: &mut DataFlowGraph,
-) -> ValueId {
+) -> SimplifyResult {
     let bit_size = u32::BITS - (radix - 1).leading_zeros();
     let radix_big = BigUint::from(radix);
     assert_eq!(BigUint::from(2u128).pow(bit_size), radix_big, "ICE: Radix must be a power of 2");
     let big_integer = BigUint::from_bytes_be(&field.to_be_bytes());
 
     // Decompose the integer into its radix digits in little endian form.
     let decomposed_integer = big_integer.to_radix_le(radix);
-    let mut limbs = vecmap(0..limb_count, |i| match decomposed_integer.get(i as usize) {
-        Some(digit) => FieldElement::from_be_bytes_reduce(&[*digit]),
-        None => FieldElement::zero(),
-    });
-    if endian == Endian::Big {
-        limbs.reverse();
+    if limb_count < decomposed_integer.len() as u32 {
+        // `field` cannot be represented as `limb_count` bits.
+        // defer error to acir_gen.
+        SimplifyResult::None
+    } else {
+        let mut limbs = vecmap(0..limb_count, |i| match decomposed_integer.get(i as usize) {
+            Some(digit) => FieldElement::from_be_bytes_reduce(&[*digit]),
+            None => FieldElement::zero(),
+        });
+        if endian == Endian::Big {
+            limbs.reverse();
+        }
+        let result_array = make_constant_array(dfg, limbs, Type::unsigned(bit_size));
+        SimplifyResult::SimplifiedTo(result_array)
     }
-    make_constant_array(dfg, limbs, Type::unsigned(bit_size))
 }
 
 fn to_u8_vec(dfg: &DataFlowGraph, values: im::Vector<Id<Value>>) -> Vec<u8> {

test_programs/execution_failure/invalid_comptime_bits_decomposition/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "invalid_comptime_bits_decomposition"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.30.0"
+
+[dependencies]

test_programs/execution_failure/invalid_comptime_bits_decomposition/src/main.nr

@@ -0,0 +1,5 @@
+fn main() -> pub [u1; 1] {
+    let large_number: Field = 2;
+
+    large_number.to_be_bits()
+}

test_programs/execution_failure/invalid_comptime_bytes_decomposition/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "invalid_comptime_bytes_decomposition"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.30.0"
+
+[dependencies]

test_programs/execution_failure/invalid_comptime_bytes_decomposition/src/main.nr

@@ -0,0 +1,4 @@
+fn main() -> pub [u8; 1] {
+    let large_number: Field = 256;
+    large_number.to_be_bytes()
+}