Allow casting bools to integers (and vice-versa)

crates/nargo/tests/test_data/cast_bool/Nargo.toml

@@ -0,0 +1,7 @@
+
+        [package]
+        authors = [""]
+        compiler_version = "0.1"
+
+        [dependencies]
+

crates/nargo/tests/test_data/cast_bool/Prover.toml

@@ -0,0 +1,2 @@
+x = "10"
+y = "10"

crates/nargo/tests/test_data/cast_bool/Verifier.toml

@@ -0,0 +1 @@
+setpub = []

crates/nargo/tests/test_data/cast_bool/src/main.nr

@@ -0,0 +1,7 @@
+
+fn main(x : Field, y : Field) {
+    let z = x == y;
+    let t = z as u8;
+    constrain t == 1;
+}
+

crates/noirc_evaluator/src/lib.rs

@@ -365,7 +365,7 @@ impl<'a> Evaluator<'a> {
                 }
             }
             Type::PolymorphicInteger(..) => unreachable!(),
-            Type::Bool => todo!(),
+            Type::Bool(_) => todo!(),
             Type::Unit => todo!(),
             Type::Struct(_, _) => todo!(),
             Type::Tuple(_) => todo!(),

crates/noirc_evaluator/src/ssa/node.rs

@@ -216,7 +216,7 @@ impl From<ObjectType> for NumericType {
 impl From<&Type> for ObjectType {
     fn from(t: &noirc_frontend::Type) -> ObjectType {
         match t {
-            Type::Bool => ObjectType::Boolean,
+            Type::Bool(_) => ObjectType::Boolean,
             Type::FieldElement(..) => ObjectType::NativeField,
             Type::Integer(_, _ftype, sign, bit_size) => {
                 assert!(

crates/noirc_frontend/src/ast/mod.rs

@@ -98,7 +98,7 @@ pub enum UnresolvedType {
     FieldElement(IsConst, FieldElementType),
     Array(FieldElementType, ArraySize, Box<UnresolvedType>), // [4]Witness = Array(4, Witness)
     Integer(IsConst, FieldElementType, Signedness, u32),     // u32 = Integer(unsigned, 32)
-    Bool,
+    Bool(IsConst),
     Unit,
     Struct(FieldElementType, Path),
 
@@ -143,7 +143,7 @@ impl std::fmt::Display for UnresolvedType {
                 let elements = vecmap(elements, ToString::to_string);
                 write!(f, "({})", elements.join(", "))
             }
-            Bool => write!(f, "bool"),
+            Bool(is_const) => write!(f, "{}bool", is_const),
             Unit => write!(f, "()"),
             Error => write!(f, "error"),
             Unspecified => write!(f, "unspecified"),

crates/noirc_frontend/src/hir/resolution/resolver.rs

@@ -212,7 +212,7 @@ impl<'a> Resolver<'a> {
             UnresolvedType::Integer(is_const, vis, sign, bits) => {
                 Type::Integer(is_const, vis, sign, bits)
             }
-            UnresolvedType::Bool => Type::Bool,
+            UnresolvedType::Bool(is_const) => Type::Bool(is_const),
             UnresolvedType::Unit => Type::Unit,
             UnresolvedType::Unspecified => Type::Unspecified,
             UnresolvedType::Error => Type::Error,

crates/noirc_frontend/src/hir/type_check/expr.rs

@@ -63,11 +63,11 @@ pub(crate) fn type_check_expression(
 
                     arr_type
                 }
-                HirLiteral::Bool(_) => Type::Bool,
+                HirLiteral::Bool(_) => Type::Bool(IsConst::new(interner)),
                 HirLiteral::Integer(_) => {
                     let id = interner.next_type_variable_id();
                     Type::PolymorphicInteger(
-                        IsConst::Maybe(id, Rc::new(RefCell::new(None))),
+                        IsConst::new(interner),
                         Rc::new(RefCell::new(TypeBinding::Unbound(id))),
                     )
                 }
@@ -264,6 +264,8 @@ fn check_cast(from: Type, to: Type, span: Span, errors: &mut Vec<TypeCheckError>
             TypeBinding::Bound(from) => return check_cast(from.clone(), to, span, errors),
             TypeBinding::Unbound(_) => is_const,
         },
+        // TODO: Track const for bools
+        Type::Bool(is_const) => is_const,
         Type::Error => return Type::Error,
         from => {
             let msg = format!(
@@ -292,6 +294,13 @@ fn check_cast(from: Type, to: Type, span: Span, errors: &mut Vec<TypeCheckError>
 
             Type::FieldElement(is_const, to_vis)
         }
+        Type::Bool(dest_is_const) => {
+            if dest_is_const.is_const() && is_const.unify(&dest_is_const, span).is_err() {
+                let msg = "Cannot cast to a const type, argument to cast is non-const (not known at compile-time)".into();
+                errors.push(TypeCheckError::Unstructured { msg, span });
+            }
+            Type::Bool(dest_is_const)
+        }
         Type::Error => Type::Error,
         _ => {
             let msg = "Only integer and Field types may be casted to".into();
@@ -387,7 +396,7 @@ pub fn prefix_operand_type_rules(op: &HirUnaryOp, rhs_type: &Type) -> Result<Typ
             }
         }
         HirUnaryOp::Not => {
-            if !matches!(rhs_type, Type::Integer(..) | Type::Bool | Type::Error) {
+            if !matches!(rhs_type, Type::Integer(..) | Type::Bool(_) | Type::Error) {
                 return Err("Only Integers or Bool can be used in a Not expression".to_string());
             }
         }
@@ -428,7 +437,7 @@ pub fn infix_operand_type_rules(
             if let TypeBinding::Bound(binding) = &*int.borrow() {
                 return infix_operand_type_rules(binding, op, other, errors);
             }
-            if other.try_bind_to_polymorphic_int(int, is_const, op.span).is_ok() {
+            if other.try_bind_to_polymorphic_int(int, is_const, op.span).is_ok() || other == &Type::Error {
                 Ok(other.clone())
             } else {
                 Err(format!("Types in a binary operation should match, but found {} and {}", lhs_type, rhs_type))
@@ -453,7 +462,7 @@ pub fn infix_operand_type_rules(
             Ok(FieldElement(is_const, Private))
         }
 
-        (Bool, Bool) => Ok(Bool),
+        (Bool(is_const_x), Bool(is_const_y)) => Ok(Bool(is_const_x.and(is_const_y, op.span))),
 
         (lhs, rhs) => Err(format!("Unsupported types for binary operation: {} and {}", lhs, rhs)),
     }
@@ -469,10 +478,12 @@ fn check_if_expr(
     let then_type = type_check_expression(interner, &if_expr.consequence, errors);
 
     let expr_span = interner.expr_span(&if_expr.condition);
-    cond_type.unify(&Type::Bool, expr_span, errors, || TypeCheckError::TypeMismatch {
-        expected_typ: Type::Bool.to_string(),
-        expr_typ: cond_type.to_string(),
-        expr_span,
+    cond_type.unify(&Type::Bool(IsConst::new(interner)), expr_span, errors, || {
+        TypeCheckError::TypeMismatch {
+            expected_typ: Type::Bool(IsConst::No(None)).to_string(),
+            expr_typ: cond_type.to_string(),
+            expr_span,
+        }
     });
 
     match if_expr.alternative {
@@ -589,14 +600,15 @@ pub fn comparator_operand_type_rules(
     use HirBinaryOpKind::{Equal, NotEqual};
     use Type::*;
     match (lhs_type, rhs_type)  {
-        (Integer(_, _, sign_x, bit_width_x), Integer(_, _, sign_y, bit_width_y)) => {
+        (Integer(is_const_x, _, sign_x, bit_width_x), Integer(is_const_y, _, sign_y, bit_width_y)) => {
             if sign_x != sign_y {
                 return Err(format!("Integers must have the same signedness LHS is {:?}, RHS is {:?} ", sign_x, sign_y))
             }
             if bit_width_x != bit_width_y {
                 return Err(format!("Integers must have the same bit width LHS is {}, RHS is {} ", bit_width_x, bit_width_y))
             }
-            Ok(Bool)
+            let is_const = is_const_x.and(is_const_y, op.span);
+            Ok(Bool(is_const))
         }
         (Integer(..), FieldElement(..)) | ( FieldElement(..), Integer(..) ) => {
             Err("Cannot use an integer and a Field in a binary operation, try converting the Field into an integer first".to_string())
@@ -606,23 +618,29 @@ pub fn comparator_operand_type_rules(
             if let TypeBinding::Bound(binding) = &*int.borrow() {
                 return comparator_operand_type_rules(other, binding, op, errors);
             }
-            if other.try_bind_to_polymorphic_int(int, is_const, op.span).is_ok() {
-                Ok(Bool)
+            if other.try_bind_to_polymorphic_int(int, is_const, op.span).is_ok() || other == &Type::Error {
+                Ok(Bool(is_const.clone()))
             } else {
                 Err(format!("Types in a binary operation should match, but found {} and {}", lhs_type, rhs_type))
             }
         }
         (Integer(..), typ) | (typ,Integer(..)) => {
             Err(format!("Integer cannot be used with type {}", typ))
         }
-        (FieldElement(..), FieldElement(..)) => Ok(Bool),
+        (FieldElement(is_const_x, ..), FieldElement(is_const_y, ..)) => {
+            let is_const = is_const_x.and(is_const_y, op.span);
+            Ok(Bool(is_const))
+        }
 
         // <= and friends are technically valid for booleans, just not very useful
-        (Bool, Bool) => Ok(Bool),
+        (Bool(is_const_x), Bool(is_const_y)) => {
+            let is_const = is_const_x.and(is_const_y, op.span);
+            Ok(Bool(is_const))
+        }
 
         // Avoid reporting errors multiple times
-        (Error, _) | (_,Error) => Ok(Bool),
-        (Unspecified, _) | (_,Unspecified) => Ok(Bool),
+        (Error, _) | (_,Error) => Ok(Bool(IsConst::Yes(None))),
+        (Unspecified, _) | (_,Unspecified) => Ok(Bool(IsConst::Yes(None))),
 
         // Special-case == and != for arrays
         (Array(_, x_size, x_type), Array(_, y_size, y_type)) if matches!(op.kind, Equal | NotEqual) => {
@@ -637,7 +655,9 @@ pub fn comparator_operand_type_rules(
                 return Err(format!("Can only compare arrays of the same length. Here LHS is of length {}, and RHS is {} ", 
                     x_size, y_size));
             }
-            Ok(Bool)
+
+            // We could check if all elements of all arrays are const but I am lazy
+            Ok(Bool(IsConst::No(Some(op.span))))
         }
         (lhs, rhs) => Err(format!("Unsupported types for comparison: {} and {}", lhs, rhs)),
     }

crates/noirc_frontend/src/hir/type_check/stmt.rs

@@ -5,6 +5,7 @@ use crate::hir_def::stmt::{
 };
 use crate::hir_def::types::Type;
 use crate::node_interner::{ExprId, NodeInterner, StmtId};
+use crate::IsConst;
 
 use super::{errors::TypeCheckError, expr::type_check_expression};
 
@@ -200,10 +201,12 @@ fn type_check_constrain_stmt(
     let expr_type = type_check_expression(interner, &stmt.0, errors);
     let expr_span = interner.expr_span(&stmt.0);
 
-    expr_type.unify(&Type::Bool, expr_span, errors, &mut || TypeCheckError::TypeMismatch {
-        expr_typ: expr_type.to_string(),
-        expected_typ: Type::Bool.to_string(),
-        expr_span,
+    expr_type.unify(&Type::Bool(IsConst::new(interner)), expr_span, errors, &mut || {
+        TypeCheckError::TypeMismatch {
+            expr_typ: expr_type.to_string(),
+            expected_typ: Type::Bool(IsConst::No(None)).to_string(),
+            expr_span,
+        }
     });
 }
 

crates/noirc_frontend/src/hir_def/types.rs

@@ -1,6 +1,6 @@
 use std::{cell::RefCell, collections::HashMap, rc::Rc};
 
-use crate::hir::type_check::TypeCheckError;
+use crate::{hir::type_check::TypeCheckError, node_interner::NodeInterner};
 use noirc_abi::{AbiFEType, AbiType};
 use noirc_errors::Span;
 
@@ -47,7 +47,7 @@ pub enum Type {
     Array(FieldElementType, ArraySize, Box<Type>), // [4]Witness = Array(4, Witness)
     Integer(IsConst, FieldElementType, Signedness, u32), // u32 = Integer(unsigned, 32)
     PolymorphicInteger(IsConst, TypeVariable),
-    Bool,
+    Bool(IsConst),
     Unit,
     Struct(FieldElementType, Rc<RefCell<StructType>>),
     Tuple(Vec<Type>),
@@ -86,6 +86,11 @@ pub enum SpanKind {
 }
 
 impl IsConst {
+    pub fn new(interner: &mut NodeInterner) -> Self {
+        let id = interner.next_type_variable_id();
+        Self::Maybe(id, Rc::new(RefCell::new(None)))
+    }
+
     fn set_span(&mut self, new_span: Span) {
         match self {
             IsConst::Yes(span) | IsConst::No(span) => *span = Some(new_span),
@@ -254,7 +259,7 @@ impl std::fmt::Display for Type {
                 let elements = vecmap(elements, ToString::to_string);
                 write!(f, "({})", elements.join(", "))
             }
-            Type::Bool => write!(f, "bool"),
+            Type::Bool(is_const) => write!(f, "{}bool", is_const),
             Type::Unit => write!(f, "()"),
             Type::Error => write!(f, "error"),
             Type::Unspecified => write!(f, "unspecified"),
@@ -455,6 +460,8 @@ impl Type {
                 }
             }
 
+            (Bool(const_a), Bool(const_b)) => const_a.unify(const_b, span),
+
             (other_a, other_b) => {
                 if other_a == other_b {
                     Ok(())
@@ -545,6 +552,8 @@ impl Type {
                 }
             }
 
+            (Bool(const_a), Bool(const_b)) => const_a.is_subtype_of(const_b, span),
+
             (other_a, other_b) => {
                 if other_a == other_b {
                     Ok(())
@@ -567,32 +576,21 @@ impl Type {
             Type::FieldElement(..)
             | Type::Integer(..)
             | Type::PolymorphicInteger(..)
-            | Type::Bool
+            | Type::Bool(_)
             | Type::Error
             | Type::Unspecified
             | Type::Unit => 1,
         }
     }
 
-    pub fn can_be_used_in_constrain(&self) -> bool {
-        matches!(
-            self,
-            Type::FieldElement(..)
-                | Type::Integer(..)
-                | Type::PolymorphicInteger(..)
-                | Type::Array(_, _, _)
-                | Type::Error
-                | Type::Bool
-        )
-    }
-
     pub fn is_fixed_sized_array(&self) -> bool {
         let (sized, _) = match self.array() {
             None => return false,
             Some(arr) => arr,
         };
         sized.is_fixed()
     }
+
     pub fn is_variable_sized_array(&self) -> bool {
         let (sized, _) = match self.array() {
             None => return false,
@@ -652,7 +650,7 @@ impl Type {
                 TypeBinding::Bound(typ) => typ.as_abi_type(),
                 TypeBinding::Unbound(_) => Type::default_int_type(None).as_abi_type(),
             },
-            Type::Bool => panic!("currently, cannot have a bool in the entry point function"),
+            Type::Bool(_) => panic!("currently, cannot have a bool in the entry point function"),
             Type::Error => unreachable!(),
             Type::Unspecified => unreachable!(),
             Type::Unit => unreachable!(),

crates/noirc_frontend/src/lexer/token.rs

@@ -377,6 +377,7 @@ pub enum Keyword {
     // Field types
     Pub,
     Const,
+    Bool,
     //
     SetPub,
     //
@@ -409,6 +410,7 @@ impl fmt::Display for Keyword {
             Keyword::Pub => write!(f, "pub"),
             Keyword::Field => write!(f, "Field"),
             Keyword::Const => write!(f, "const"),
+            Keyword::Bool => write!(f, "bool"),
         }
     }
 }
@@ -442,6 +444,7 @@ impl Keyword {
 
             // Native Types
             "Field" => Keyword::Field,
+            "bool" => Keyword::Bool,
 
             "true" => return Some(Token::Bool(true)),
             "false" => return Some(Token::Bool(false)),

crates/noirc_frontend/src/parser/parser.rs

@@ -376,6 +376,7 @@ where
         struct_type(visibility_parser.clone()),
         array_type(visibility_parser, recursive_type_parser.clone()),
         tuple_type(recursive_type_parser),
+        bool_type(),
     ))
 }
 
@@ -411,6 +412,10 @@ where
         .map(|(vis, is_const)| UnresolvedType::FieldElement(is_const, vis))
 }
 
+fn bool_type() -> impl NoirParser<UnresolvedType> {
+    maybe_const().then_ignore(keyword(Keyword::Bool)).map(UnresolvedType::Bool)
+}
+
 fn int_type<P>(visibility_parser: P) -> impl NoirParser<UnresolvedType>
 where
     P: NoirParser<FieldElementType>,