feat: Add assert_max_bit_size method to Field (#4016)

# Description


resolves #4015 

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*



## Additional Context



## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[Exceptional Case]** Documentation to be submitted in a separate
PR.

# PR Checklist\*

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

---------

Co-authored-by: Tom French <[email protected]>
Co-authored-by: Tom French <[email protected]>

compiler/noirc_evaluator/src/ssa/acir_gen/acir_ir/acir_variable.rs

@@ -135,6 +135,13 @@ impl AcirContext {
         self.add_data(constant_data)
     }
 
+    /// Returns the constant represented by the given variable.
+    ///
+    /// Panics: if the variable does not represent a constant.
+    pub(crate) fn constant(&self, var: AcirVar) -> FieldElement {
+        self.vars[&var].as_constant().expect("ICE - expected the variable to be a constant value")
+    }
+
     /// Adds a Variable to the context, whose exact value is resolved at
     /// runtime.
     pub(crate) fn add_variable(&mut self) -> AcirVar {

compiler/noirc_evaluator/src/ssa/acir_gen/mod.rs

@@ -1712,6 +1712,9 @@ impl Context {
 
                 Ok(Self::convert_vars_to_values(vars, dfg, result_ids))
             }
+            Intrinsic::ApplyRangeConstraint => {
+                unreachable!("ICE: `Intrinsic::ApplyRangeConstraint` calls should be transformed into an `Instruction::RangeCheck`");
+            }
             Intrinsic::ToRadix(endian) => {
                 let field = self.convert_value(arguments[0], dfg).into_var()?;
                 let radix = self.convert_value(arguments[1], dfg).into_var()?;

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

@@ -40,6 +40,7 @@ pub(crate) enum Intrinsic {
     SlicePopFront,
     SliceInsert,
     SliceRemove,
+    ApplyRangeConstraint,
     StrAsBytes,
     ToBits(Endian),
     ToRadix(Endian),
@@ -61,6 +62,7 @@ impl std::fmt::Display for Intrinsic {
             Intrinsic::SliceInsert => write!(f, "slice_insert"),
             Intrinsic::SliceRemove => write!(f, "slice_remove"),
             Intrinsic::StrAsBytes => write!(f, "str_as_bytes"),
+            Intrinsic::ApplyRangeConstraint => write!(f, "apply_range_constraint"),
             Intrinsic::ToBits(Endian::Big) => write!(f, "to_be_bits"),
             Intrinsic::ToBits(Endian::Little) => write!(f, "to_le_bits"),
             Intrinsic::ToRadix(Endian::Big) => write!(f, "to_be_radix"),
@@ -78,7 +80,7 @@ impl Intrinsic {
     /// If there are no side effects then the `Intrinsic` can be removed if the result is unused.
     pub(crate) fn has_side_effects(&self) -> bool {
         match self {
-            Intrinsic::AssertConstant => true,
+            Intrinsic::AssertConstant | Intrinsic::ApplyRangeConstraint => true,
 
             Intrinsic::Sort
             | Intrinsic::ArrayLen
@@ -106,6 +108,7 @@ impl Intrinsic {
             "arraysort" => Some(Intrinsic::Sort),
             "array_len" => Some(Intrinsic::ArrayLen),
             "assert_constant" => Some(Intrinsic::AssertConstant),
+            "apply_range_constraint" => Some(Intrinsic::ApplyRangeConstraint),
             "slice_push_back" => Some(Intrinsic::SlicePushBack),
             "slice_push_front" => Some(Intrinsic::SlicePushFront),
             "slice_pop_back" => Some(Intrinsic::SlicePopBack),

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

@@ -233,6 +233,20 @@ pub(super) fn simplify_call(
                 SimplifyResult::None
             }
         }
+        Intrinsic::ApplyRangeConstraint => {
+            let value = arguments[0];
+            let max_bit_size = dfg.get_numeric_constant(arguments[1]);
+            if let Some(max_bit_size) = max_bit_size {
+                let max_bit_size = max_bit_size.to_u128() as u32;
+                SimplifyResult::SimplifiedToInstruction(Instruction::RangeCheck {
+                    value,
+                    max_bit_size,
+                    assert_message: Some("call to assert_max_bit_size".to_owned()),
+                })
+            } else {
+                SimplifyResult::None
+            }
+        }
         Intrinsic::BlackBox(bb_func) => simplify_black_box_func(bb_func, arguments, dfg),
         Intrinsic::Sort => simplify_sort(dfg, arguments),
         Intrinsic::AsField => {

noir_stdlib/src/field.nr

@@ -15,6 +15,15 @@ impl Field {
     #[builtin(to_be_bits)]
     fn __to_be_bits(_self: Self, _bit_size: u32) -> [u1] {}
 
+    #[builtin(apply_range_constraint)]
+    fn __assert_max_bit_size(_self: Self, _bit_size: u32) {}
+
+    pub fn assert_max_bit_size(self: Self, bit_size: u32) {
+        crate::assert_constant(bit_size);
+        assert(bit_size < modulus_num_bits() as u32);
+        self.__assert_max_bit_size(bit_size);
+    }
+
     pub fn to_le_bytes(self: Self, byte_size: u32) -> [u8] {
         self.to_le_radix(256, byte_size)
     }

test_programs/execution_success/unsafe_range_constraint/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "unsafe_range_constraint"
+version = "0.1.0"
+type = "bin"
+authors = [""]
+
+[dependencies]

test_programs/execution_success/unsafe_range_constraint/Prover.toml

@@ -0,0 +1 @@
+x = "3"

test_programs/execution_success/unsafe_range_constraint/src/main.nr

@@ -0,0 +1,5 @@
+// Test that we can apply a range constraint to a field using 
+// a builtin.
+fn main(x: Field) {
+    x.assert_max_bit_size(48);
+}