feat(experimental): Enable ownership syntax

compiler/noirc_driver/src/abi_gen.rs

@@ -136,7 +136,7 @@ pub(super) fn abi_type_from_hir_type(context: &Context, typ: &Type) -> AbiType {
         | Type::Slice(_)
         | Type::Function(_, _, _, _) => unreachable!("{typ} cannot be used in the abi"),
         Type::FmtString(_, _) => unreachable!("format strings cannot be used in the abi"),
-        Type::MutableReference(_) => unreachable!("&mut cannot be used in the abi"),
+        Type::Reference(..) => unreachable!("references cannot be used in the abi"),
     }
 }
 

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

@@ -370,7 +370,7 @@
                     unary.location,
                 ))
             }
-            UnaryOp::MutableReference => {
+            UnaryOp::Reference { mutable: _ } => {
                 Ok(self.codegen_reference(&unary.rhs)?.map(|rhs| {
                     match rhs {
                         value::Value::Normal(value) => {
@@ -527,7 +527,7 @@
     /// loop_entry(i: Field):
     ///   v2 = lt i v1
     ///   brif v2, then: loop_body, else: loop_end
     /// loop_body():
     ///   v3 = ... codegen body ...
     ///   v4 = add 1, i
     ///   br loop_entry(v4)
@@ -671,7 +671,7 @@
     /// ```text
     ///   v0 = ... codegen cond ...
     ///   brif v0, then: then_block, else: else_block
     /// then_block():
     ///   v1 = ... codegen a ...
     ///   br end_if(v1)
     /// else_block():
@@ -686,7 +686,7 @@
     /// ```text
     ///   v0 = ... codegen cond ...
     ///   brif v0, then: then_block, else: end_if
     /// then_block:
     ///   v1 = ... codegen a ...
     ///   br end_if()
     /// end_if:  // No block parameter is needed. Without an else, the unit value is always returned.

compiler/noirc_frontend/src/ast/expression.rs

@@ -383,7 +383,9 @@ impl BinaryOpKind {
 pub enum UnaryOp {
     Minus,
     Not,
-    MutableReference,
+    Reference {
+        mutable: bool,
+    },
 
     /// If implicitly_added is true, this operation was implicitly added by the compiler for a
     /// field dereference. The compiler may undo some of these implicitly added dereferences if
@@ -732,7 +734,8 @@ impl Display for UnaryOp {
         match self {
             UnaryOp::Minus => write!(f, "-"),
             UnaryOp::Not => write!(f, "!"),
-            UnaryOp::MutableReference => write!(f, "&mut"),
+            UnaryOp::Reference { mutable } if *mutable => write!(f, "&mut"),
+            UnaryOp::Reference { .. } => write!(f, "&"),
             UnaryOp::Dereference { .. } => write!(f, "*"),
         }
     }

compiler/noirc_frontend/src/ast/mod.rs

@@ -135,8 +135,8 @@
     /// A Trait as return type or parameter of function, including its generics
     TraitAsType(Path, GenericTypeArgs),
 
-    /// &mut T
-    MutableReference(Box<UnresolvedType>),
+    /// &T and &mut T
+    Reference(Box<UnresolvedType>, /*mutable*/ bool),
 
     // Note: Tuples have no visibility, instead each of their elements may have one.
     Tuple(Vec<UnresolvedType>),
@@ -311,7 +311,8 @@
                     other => write!(f, "fn[{other}]({args}) -> {ret}"),
                 }
             }
-            MutableReference(element) => write!(f, "&mut {element}"),
+            Reference(element, false) => write!(f, "&{element}"),
+            Reference(element, true) => write!(f, "&mut {element}"),
             Quoted(quoted) => write!(f, "{}", quoted),
             Unit => write!(f, "()"),
             Error => write!(f, "error"),
@@ -346,7 +347,7 @@
 impl UnresolvedType {
     pub fn is_synthesized(&self) -> bool {
         match &self.typ {
-            UnresolvedTypeData::MutableReference(ty) => ty.is_synthesized(),
+            UnresolvedTypeData::Reference(ty, _) => ty.is_synthesized(),
             UnresolvedTypeData::Named(_, _, synthesized) => *synthesized,
             _ => false,
         }
@@ -424,7 +425,7 @@
                 path_is_wildcard || an_arg_is_unresolved
             }
             UnresolvedTypeData::TraitAsType(_path, args) => args.contains_unspecified(),
-            UnresolvedTypeData::MutableReference(typ) => typ.contains_unspecified(),
+            UnresolvedTypeData::Reference(typ, _) => typ.contains_unspecified(),
             UnresolvedTypeData::Tuple(args) => args.iter().any(|arg| arg.contains_unspecified()),
             UnresolvedTypeData::Function(args, ret, env, _unconstrained) => {
                 let args_contains_unspecified = args.iter().any(|arg| arg.contains_unspecified());
@@ -604,7 +605,7 @@
             Self::Public => write!(f, "pub"),
             Self::Private => write!(f, "priv"),
             Self::CallData(id) => write!(f, "calldata{id}"),
             Self::ReturnData => write!(f, "returndata"),
         }
     }
 }

compiler/noirc_frontend/src/ast/visitor.rs

@@ -393,7 +393,7 @@ pub trait Visitor {
         true
     }
 
-    fn visit_mutable_reference_type(&mut self, _: &UnresolvedType, _: Span) -> bool {
+    fn visit_reference_type(&mut self, _: &UnresolvedType, _mutable: bool, _: Span) -> bool {
         true
     }
 
@@ -1382,8 +1382,8 @@ impl UnresolvedType {
                     generic_type_args.accept(visitor);
                 }
             }
-            UnresolvedTypeData::MutableReference(unresolved_type) => {
-                if visitor.visit_mutable_reference_type(unresolved_type, self.location.span) {
+            UnresolvedTypeData::Reference(unresolved_type, mutable) => {
+                if visitor.visit_reference_type(unresolved_type, *mutable, self.location.span) {
                     unresolved_type.accept(visitor);
                 }
             }

compiler/noirc_frontend/src/elaborator/enums.rs

@@ -938,7 +938,7 @@
             | Type::NamedGeneric(_, _)
             | Type::CheckedCast { .. }
             | Type::Function(_, _, _, _)
-            | Type::MutableReference(_)
+            | Type::Reference(..)
             | Type::Forall(_, _)
             | Type::Constant(_, _)
             | Type::Quoted(_)
@@ -967,7 +967,7 @@
 
     /// Compiles the cases and fallback cases for integer and range patterns.
     ///
     /// Integers have an infinite number of constructors, so we specialise the
     /// compilation of integer and range patterns.
     fn compile_int_cases(
         &mut self,

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -38,7 +38,7 @@ use crate::{
     token::{FmtStrFragment, Tokens},
 };
 
-use super::{Elaborator, LambdaContext, UnsafeBlockStatus};
+use super::{Elaborator, LambdaContext, UnsafeBlockStatus, UnstableFeature};
 
 impl Elaborator<'_> {
     pub(crate) fn elaborate_expression(&mut self, expr: Expression) -> (ExprId, Type) {
@@ -344,8 +344,12 @@ impl Elaborator<'_> {
 
         let operator = prefix.operator;
 
-        if let UnaryOp::MutableReference = operator {
-            self.check_can_mutate(rhs, rhs_location);
+        if let UnaryOp::Reference { mutable } = operator {
+            if mutable {
+                self.check_can_mutate(rhs, rhs_location);
+            } else {
+                self.use_unstable_feature(UnstableFeature::Ownership, location);
+            }
         }
 
         let expr =

compiler/noirc_frontend/src/elaborator/mod.rs

@@ -588,7 +588,7 @@ impl<'context> Elaborator<'context> {
         for (mut constraint, expr_id, select_impl) in context.trait_constraints {
             let location = self.interner.expr_location(&expr_id);
 
-            if matches!(&constraint.typ, Type::MutableReference(_)) {
+            if matches!(&constraint.typ, Type::Reference(..)) {
                 let (_, dereferenced_typ) =
                     self.insert_auto_dereferences(expr_id, constraint.typ.clone());
                 constraint.typ = dereferenced_typ;
@@ -1078,7 +1078,7 @@ impl<'context> Elaborator<'context> {
                 self.mark_type_as_used(from);
                 self.mark_type_as_used(to);
             }
-            Type::MutableReference(typ) => {
+            Type::Reference(typ, _) => {
                 self.mark_type_as_used(typ);
             }
             Type::InfixExpr(left, _op, right, _) => {
@@ -1461,8 +1461,8 @@ impl<'context> Elaborator<'context> {
         self.self_type = Some(self_type.clone());
         let self_type_location = trait_impl.object_type.location;
 
-        if matches!(self_type, Type::MutableReference(_)) {
-            self.push_err(DefCollectorErrorKind::MutableReferenceInTraitImpl {
+        if matches!(self_type, Type::Reference(..)) {
+            self.push_err(DefCollectorErrorKind::ReferenceInTraitImpl {
                 location: self_type_location,
             });
         }
@@ -1755,7 +1755,7 @@ impl<'context> Elaborator<'context> {
                 );
                 self.check_type_is_not_more_private_then_item(name, visibility, env, location);
             }
-            Type::MutableReference(typ) | Type::Array(_, typ) | Type::Slice(typ) => {
+            Type::Reference(typ, _) | Type::Array(_, typ) | Type::Slice(typ) => {
                 self.check_type_is_not_more_private_then_item(name, visibility, typ, location);
             }
             Type::InfixExpr(left, _op, right, _) => {
@@ -2168,6 +2168,11 @@ impl<'context> Elaborator<'context> {
         }
     }
 
+    /// Return true if the given unstable feature is enabled
+    fn unstable_feature_enabled(&self, feature: UnstableFeature) -> bool {
+        self.options.enabled_unstable_features.contains(&feature)
+    }
+
     /// Run the given function using the resolver and return true if any errors (not warnings)
     /// occurred while running it.
     pub fn errors_occurred_in<T>(&mut self, f: impl FnOnce(&mut Self) -> T) -> (bool, T) {

compiler/noirc_frontend/src/elaborator/options.rs

@@ -3,14 +3,14 @@ use std::str::FromStr;
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum UnstableFeature {
     Enums,
-    ArrayOwnership,
+    Ownership,
 }
 
 impl std::fmt::Display for UnstableFeature {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {
             Self::Enums => write!(f, "enums"),
-            Self::ArrayOwnership => write!(f, "array-ownership"),
+            Self::Ownership => write!(f, "ownership"),
         }
     }
 }
@@ -21,7 +21,7 @@ impl FromStr for UnstableFeature {
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         match s {
             "enums" => Ok(Self::Enums),
-            "array-ownership" => Ok(Self::ArrayOwnership),
+            "ownership" => Ok(Self::Ownership),
             other => Err(format!("Unknown unstable feature '{other}'")),
         }
     }

compiler/noirc_frontend/src/elaborator/statements.rs

@@ -437,8 +437,8 @@ impl Elaborator<'_> {
                 let (mut lvalue, mut lvalue_type, mut mutable) = self.elaborate_lvalue(*array);
 
                 // Before we check that the lvalue is an array, try to dereference it as many times
-                // as needed to unwrap any &mut wrappers.
-                while let Type::MutableReference(element) = lvalue_type.follow_bindings() {
+                // as needed to unwrap any `&` or `&mut` wrappers.
+                while let Type::Reference(element, _) = lvalue_type.follow_bindings() {
                     let element_type = element.as_ref().clone();
                     lvalue =
                         HirLValue::Dereference { lvalue: Box::new(lvalue), element_type, location };
@@ -482,7 +482,9 @@ impl Elaborator<'_> {
                 let lvalue = Box::new(lvalue);
 
                 let element_type = Type::type_variable(self.interner.next_type_variable_id());
-                let expected_type = Type::MutableReference(Box::new(element_type.clone()));
+
+                // Always expect a mutable reference here since we're storing to it
+                let expected_type = Type::Reference(Box::new(element_type.clone()), true);
 
                 self.unify(&reference_type, &expected_type, || TypeCheckError::TypeMismatch {
                     expected_typ: expected_type.to_string(),
@@ -539,9 +541,8 @@ impl Elaborator<'_> {
                     }
                 }
             }
-            // If the lhs is a mutable reference we automatically transform
-            // lhs.field into (*lhs).field
-            Type::MutableReference(element) => {
+            // If the lhs is a reference we automatically transform `lhs.field` into `(*lhs).field`
+            Type::Reference(element, mutable) => {
                 if let Some(mut dereference_lhs) = dereference_lhs {
                     dereference_lhs(self, lhs_type.clone(), element.as_ref().clone());
                     return self.check_field_access(
@@ -553,7 +554,7 @@ impl Elaborator<'_> {
                 } else {
                     let (element, index) =
                         self.check_field_access(element, field_name, location, dereference_lhs)?;
-                    return Some((Type::MutableReference(Box::new(element)), index));
+                    return Some((Type::Reference(Box::new(element), *mutable), index));
                 }
             }
             _ => (),

compiler/noirc_frontend/src/elaborator/types.rs

@@ -12,6 +12,7 @@ use crate::{
         Signedness, UnaryOp, UnresolvedGeneric, UnresolvedGenerics, UnresolvedType,
         UnresolvedTypeData, UnresolvedTypeExpression, WILDCARD_TYPE,
     },
+    elaborator::UnstableFeature,
     hir::{
         def_collector::dc_crate::CompilationError,
         def_map::{ModuleDefId, fully_qualified_module_path},
@@ -141,8 +142,11 @@ impl Elaborator<'_> {
                     }
                 }
             }
-            MutableReference(element) => {
-                Type::MutableReference(Box::new(self.resolve_type_inner(*element, kind)))
+            Reference(element, mutable) => {
+                if !mutable {
+                    self.use_unstable_feature(UnstableFeature::Ownership, location);
+                }
+                Type::Reference(Box::new(self.resolve_type_inner(*element, kind)), mutable)
             }
             Parenthesized(typ) => self.resolve_type_inner(*typ, kind),
             Resolved(id) => self.interner.get_quoted_type(id).clone(),
@@ -845,7 +849,7 @@ impl Elaborator<'_> {
     /// Insert as many dereference operations as necessary to automatically dereference a method
     /// call object to its base value type T.
     pub(super) fn insert_auto_dereferences(&mut self, object: ExprId, typ: Type) -> (ExprId, Type) {
-        if let Type::MutableReference(element) = typ.follow_bindings() {
+        if let Type::Reference(element, _mut) = typ.follow_bindings() {
             let location = self.interner.id_location(object);
 
             let object = self.interner.push_expr(HirExpression::Prefix(HirPrefixExpression {
@@ -1304,17 +1308,34 @@ impl Elaborator<'_> {
                     _ => Ok((rhs_type.clone(), true)),
                 }
             }
-            crate::ast::UnaryOp::MutableReference => {
-                Ok((Type::MutableReference(Box::new(rhs_type.follow_bindings())), false))
+            crate::ast::UnaryOp::Reference { mutable } => {
+                let typ = Type::Reference(Box::new(rhs_type.follow_bindings()), *mutable);
+                Ok((typ, false))
             }
             crate::ast::UnaryOp::Dereference { implicitly_added: _ } => {
                 let element_type = self.interner.next_type_variable();
-                let expected = Type::MutableReference(Box::new(element_type.clone()));
-                self.unify(rhs_type, &expected, || TypeCheckError::TypeMismatch {
-                    expr_typ: rhs_type.to_string(),
-                    expected_typ: expected.to_string(),
-                    expr_location: location,
-                });
+                let make_expected =
+                    |mutable| Type::Reference(Box::new(element_type.clone()), mutable);
+
+                let immutable = make_expected(false);
+                let ownership_enabled = self.unstable_feature_enabled(UnstableFeature::Ownership);
+
+                // Both `&mut T` and `&T` should coerce to an expected `&T`.
+                if !rhs_type.try_reference_coercion(&immutable) {
+                    self.unify(rhs_type, &immutable, || {
+                        let expected_typ = if ownership_enabled {
+                            immutable.to_string()
+                        } else {
+                            make_expected(true).to_string()
+                        };
+
+                        TypeCheckError::TypeMismatch {
+                            expr_typ: rhs_type.to_string(),
+                            expected_typ,
+                            expr_location: location,
+                        }
+                    });
+                }
                 Ok((element_type, false))
             }
         }
@@ -1437,9 +1458,9 @@ impl Elaborator<'_> {
             Type::NamedGeneric(_, _) => {
                 self.lookup_method_in_trait_constraints(object_type, method_name, location)
             }
-            // Mutable references to another type should resolve to methods of their element type.
+            // References to another type should resolve to methods of their element type.
             // This may be a data type or a primitive type.
-            Type::MutableReference(element) => {
+            Type::Reference(element, _mutable) => {
                 self.lookup_method(&element, method_name, location, check_self_param)
             }
 
@@ -1836,12 +1857,12 @@ impl Elaborator<'_> {
         if let Some(expected_object_type) = expected_object_type {
             let actual_type = object_type.follow_bindings();
 
-            if matches!(expected_object_type.follow_bindings(), Type::MutableReference(_)) {
-                if !matches!(actual_type, Type::MutableReference(_)) {
+            if let Type::Reference(_, mutable) = expected_object_type.follow_bindings() {
+                if !matches!(actual_type, Type::Reference(..)) {
                     let location = self.interner.id_location(*object);
                     self.check_can_mutate(*object, location);
 
-                    let new_type = Type::MutableReference(Box::new(actual_type));
+                    let new_type = Type::Reference(Box::new(actual_type), mutable);
                     *object_type = new_type.clone();
 
                     // First try to remove a dereference operator that may have been implicitly
@@ -1852,7 +1873,7 @@ impl Elaborator<'_> {
                     *object = new_object.unwrap_or_else(|| {
                         let new_object =
                             self.interner.push_expr(HirExpression::Prefix(HirPrefixExpression {
-                                operator: UnaryOp::MutableReference,
+                                operator: UnaryOp::Reference { mutable },
                                 rhs: *object,
                                 trait_method_id: None,
                             }));
@@ -1863,7 +1884,7 @@ impl Elaborator<'_> {
                 }
             // Otherwise if the object type is a mutable reference and the method is not, insert as
             // many dereferences as needed.
-            } else if matches!(actual_type, Type::MutableReference(_)) {
+            } else if matches!(actual_type, Type::Reference(..)) {
                 let (new_object, new_type) = self.insert_auto_dereferences(*object, actual_type);
                 *object_type = new_type;
                 *object = new_object;