fix!: disallow casting numeric to bool

.github/benchmark_projects.yml

@@ -1,4 +1,4 @@
-define: &AZ_COMMIT ad837fe0687a4c9e58c43c7295ea31e39420060e
+define: &AZ_COMMIT 42d5984235d2ffcaf5545e88e77eeb698e147416
 projects:
   private-kernel-inner:
     repo: AztecProtocol/aztec-packages

EXTERNAL_NOIR_LIBRARIES.yml

@@ -1,4 +1,4 @@
-define: &AZ_COMMIT ad837fe0687a4c9e58c43c7295ea31e39420060e
+define: &AZ_COMMIT 42d5984235d2ffcaf5545e88e77eeb698e147416
 libraries:
   noir_check_shuffle:
     repo: noir-lang/noir_check_shuffle

compiler/noirc_frontend/src/elaborator/types.rs

@@ -1194,7 +1194,21 @@ impl Elaborator<'_> {
         match to {
             Type::Integer(sign, bits) => Type::Integer(sign, bits),
             Type::FieldElement => Type::FieldElement,
-            Type::Bool => Type::Bool,
+            Type::Bool => {
+                let from_is_numeric = match from_follow_bindings {
+                    Type::Integer(..) | Type::FieldElement => true,
+                    Type::TypeVariable(ref var) => var.is_integer() || var.is_integer_or_field(),
+                    _ => false,
+                };
+                if from_is_numeric {
+                    self.push_err(TypeCheckError::CannotCastNumericToBool {
+                        typ: from_follow_bindings,
+                        location,
+                    });
+                }
+
+                Type::Bool
+            }
             Type::Error => Type::Error,
             _ => {
                 self.push_err(TypeCheckError::UnsupportedCast { location });

compiler/noirc_frontend/src/hir/type_check/errors.rs

@@ -70,6 +70,8 @@ pub enum TypeCheckError {
     InvalidCast { from: Type, location: Location, reason: String },
     #[error("Casting value of type {from} to a smaller type ({to})")]
     DownsizingCast { from: Type, to: Type, location: Location, reason: String },
+    #[error("Cannot cast `{typ}` as `bool`")]
+    CannotCastNumericToBool { typ: Type, location: Location },
     #[error("Expected a function, but found a(n) {found}")]
     ExpectedFunction { found: Type, location: Location },
     #[error("Type {lhs_type} has no member named {field_name}")]
@@ -258,6 +260,7 @@ impl TypeCheckError {
             | TypeCheckError::ArityMisMatch { location, .. }
             | TypeCheckError::InvalidCast { location, .. }
             | TypeCheckError::DownsizingCast { location, .. }
+            | TypeCheckError::CannotCastNumericToBool { location, .. }
             | TypeCheckError::ExpectedFunction { location, .. }
             | TypeCheckError::AccessUnknownMember { location, .. }
             | TypeCheckError::ParameterCountMismatch { location, .. }
@@ -459,6 +462,10 @@ impl<'a> From<&'a TypeCheckError> for Diagnostic {
             TypeCheckError::DownsizingCast { location, reason, .. } => {
                 Diagnostic::simple_warning(error.to_string(), reason.clone(), *location)
             }
+            TypeCheckError::CannotCastNumericToBool { typ: _, location } => {
+                let secondary = "compare with zero instead: ` != 0`".to_string();
+                Diagnostic::simple_error(error.to_string(), secondary, *location)
+            }
 
             TypeCheckError::ExpectedFunction { location, .. }
             | TypeCheckError::AccessUnknownMember { location, .. }

noir_stdlib/src/convert.nr

@@ -175,15 +175,23 @@ comptime fn generate_as_primitive_impls(_: FunctionDefinition) -> Quoted {
     let mut impls = &[];
     for type1 in types {
         for type2 in types {
-            let trait_impl = quote {
+            let body = if type1 == type2 {
+                quote { self }
+            } else if type1 == quote { bool } {
+                quote { self != 0 }
+            } else {
+                quote { self as $type1 }
+            };
+
+            impls = impls.push_back(
+                quote {
                 impl AsPrimitive<$type1> for $type2 {
                     fn as_(self) -> $type1 {
-                        self as $type1
+                        $body
                     }
                 }
-            };
-
-            impls = impls.push_back(trait_impl);
+            },
+            );
         }
     }
     impls.join(quote {})

test_programs/compile_failure/cast_numeric_to_bool/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "cast_numeric_to_bool"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.33.0"
+
+[dependencies]

test_programs/compile_failure/cast_numeric_to_bool/src/main.nr

@@ -0,0 +1,10 @@
+fn main() {
+    let x = 1;
+    let _ = x as bool;
+
+    let x: i32 = 1;
+    let _ = x as bool;
+
+    let x: u64 = 1;
+    let _ = x as bool;
+}

test_programs/compile_success_empty/brillig_cast/src/main.nr

@@ -4,20 +4,13 @@
 fn main() {
     // Safety: testing context
     unsafe {
-        bool_casts();
         field_casts();
         uint_casts();
         int_casts();
         mixed_casts();
     }
 }
 
-unconstrained fn bool_casts() {
-    assert(false == 0 as bool);
-    assert(true == 1 as bool);
-    assert(true == 3 as bool);
-}
-
 unconstrained fn field_casts() {
     assert(5 as u8 as Field == 5);
     assert(256 as u8 as Field == 0);
@@ -39,6 +32,5 @@ unconstrained fn int_casts() {
 unconstrained fn mixed_casts() {
     assert(100 as u32 as i32 as u32 == 100);
     assert(257 as u8 as u32 == 1);
-    assert(1 as u8 as bool == true);
     assert(true as i8 == 1);
 }

test_programs/execution_success/assert/src/main.nr

@@ -1,6 +1,6 @@
 fn main(x: Field) {
     assert(x == 1);
-    assert(1 == conditional(x as bool));
+    assert(1 == conditional(x != 0));
 }
 
 fn conditional(x: bool) -> Field {

test_programs/execution_success/brillig_not/src/main.nr

@@ -4,8 +4,8 @@
 fn main(x: Field, y: Field) {
     // Safety: testing context
     unsafe {
-        assert(false == not_operator(x as bool));
-        assert(true == not_operator(y as bool));
+        assert(false == not_operator(x != 0));
+        assert(true == not_operator(y != 0));
     }
 }
 

test_programs/execution_success/merkle_insert/src/main.nr

@@ -16,7 +16,7 @@ fn compute_merkle_root<let N: u32>(leaf: Field, index: Field, hash_path: [Field;
     let index_bits: [u1; N] = index.to_le_bits();
     let mut current = leaf;
     for i in 0..N {
-        let path_bit = index_bits[i] as bool;
+        let path_bit = index_bits[i] != 0;
         let (hash_left, hash_right) = if path_bit {
             (hash_path[i], current)
         } else {

test_programs/execution_success/regression_8329/src/main.nr

@@ -1,9 +1,9 @@
 fn main(x: u1, y: u1, z: u1) -> pub u1 {
     let p = y - z;
-    if p as bool {
+    if p != 0 {
         let a = x / z;
         let b = a - z;
-        if b as bool {
+        if b != 0 {
             let _ = a / b;
         }
     }

test_programs/execution_success/simple_shield/src/main.nr

@@ -29,7 +29,7 @@ fn compute_merkle_root<let N: u32>(leaf: Field, index: Field, hash_path: [Field;
     let index_bits: [u1; N] = index.to_le_bits();
     let mut current = leaf;
     for i in 0..N {
-        let path_bit = index_bits[i] as bool;
+        let path_bit = index_bits[i] != 0;
         let (hash_left, hash_right) = if path_bit {
             (hash_path[i], current)
         } else {