feat(traits): Add impl Trait as function return type #2397 (#3176)

Co-authored-by: Yordan Madzhunkov <ymadzhunkov@gmail.com>

compiler/noirc_frontend/src/ast/mod.rs

@@ -44,6 +44,9 @@ pub enum UnresolvedTypeData {
     /// A Named UnresolvedType can be a struct type or a type variable
     Named(Path, Vec<UnresolvedType>),
 
+    /// A Trait as return type or parameter of function, including its generics
+    TraitAsType(Path, Vec<UnresolvedType>),
+
     /// &mut T
     MutableReference(Box<UnresolvedType>),
 
@@ -112,6 +115,14 @@ impl std::fmt::Display for UnresolvedTypeData {
                     write!(f, "{}<{}>", s, args.join(", "))
                 }
             }
+            TraitAsType(s, args) => {
+                let args = vecmap(args, |arg| ToString::to_string(&arg.typ));
+                if args.is_empty() {
+                    write!(f, "impl {s}")
+                } else {
+                    write!(f, "impl {}<{}>", s, args.join(", "))
+                }
+            }
             Tuple(elements) => {
                 let elements = vecmap(elements, ToString::to_string);
                 write!(f, "({})", elements.join(", "))

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

@@ -376,6 +376,8 @@ impl<'a> Resolver<'a> {
             Unspecified => Type::Error,
             Error => Type::Error,
             Named(path, args) => self.resolve_named_type(path, args, new_variables),
+            TraitAsType(path, args) => self.resolve_trait_as_type(path, args, new_variables),
+
             Tuple(fields) => {
                 Type::Tuple(vecmap(fields, |field| self.resolve_type_inner(field, new_variables)))
             }
@@ -479,6 +481,19 @@ impl<'a> Resolver<'a> {
         }
     }
 
+    fn resolve_trait_as_type(
+        &mut self,
+        path: Path,
+        _args: Vec<UnresolvedType>,
+        _new_variables: &mut Generics,
+    ) -> Type {
+        if let Some(t) = self.lookup_trait_or_error(path) {
+            Type::TraitAsType(t)
+        } else {
+            Type::Error
+        }
+    }
+
     fn verify_generics_count(
         &mut self,
         expected_count: usize,
@@ -874,6 +889,7 @@ impl<'a> Resolver<'a> {
             | Type::Constant(_)
             | Type::NamedGeneric(_, _)
             | Type::NotConstant
+            | Type::TraitAsType(_)
             | Type::Forall(_, _) => (),
 
             Type::Array(length, element_type) => {
@@ -1430,6 +1446,17 @@ impl<'a> Resolver<'a> {
         }
     }
 
+    /// Lookup a given trait by name/path.
+    fn lookup_trait_or_error(&mut self, path: Path) -> Option<Trait> {
+        match self.lookup(path) {
+            Ok(trait_id) => Some(self.get_trait(trait_id)),
+            Err(error) => {
+                self.push_err(error);
+                None
+            }
+        }
+    }
+
     /// Looks up a given type by name.
     /// This will also instantiate any struct types found.
     fn lookup_type_or_error(&mut self, path: Path) -> Option<Type> {

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

@@ -35,6 +35,7 @@ impl<'interner> TypeChecker<'interner> {
             }
         }
     }
+
     /// Infers a type for a given expression, and return this type.
     /// As a side-effect, this function will also remember this type in the NodeInterner
     /// for the given expr_id key.
@@ -50,7 +51,7 @@ impl<'interner> TypeChecker<'interner> {
                 // E.g. `fn foo<T>(t: T, field: Field) -> T` has type `forall T. fn(T, Field) -> T`.
                 // We must instantiate identifiers at every call site to replace this T with a new type
                 // variable to handle generic functions.
-                let t = self.interner.id_type(ident.id);
+                let t = self.interner.id_type_substitute_trait_as_type(ident.id);
                 let (typ, bindings) = t.instantiate(self.interner);
                 self.interner.store_instantiation_bindings(*expr_id, bindings);
                 typ
@@ -131,7 +132,6 @@ impl<'interner> TypeChecker<'interner> {
             HirExpression::Index(index_expr) => self.check_index_expression(expr_id, index_expr),
             HirExpression::Call(call_expr) => {
                 self.check_if_deprecated(&call_expr.func);
-
                 let function = self.check_expression(&call_expr.func);
                 let args = vecmap(&call_expr.arguments, |arg| {
                     let typ = self.check_expression(arg);
@@ -839,6 +839,14 @@ impl<'interner> TypeChecker<'interner> {
                     }
                 }
             }
+            Type::TraitAsType(_trait) => {
+                self.errors.push(TypeCheckError::UnresolvedMethodCall {
+                    method_name: method_name.to_string(),
+                    object_type: object_type.clone(),
+                    span: self.interner.expr_span(expr_id),
+                });
+                None
+            }
             Type::NamedGeneric(_, _) => {
                 let func_meta = self.interner.function_meta(
                     &self.current_function.expect("unexpected method outside a function"),

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

@@ -15,7 +15,7 @@ pub use errors::TypeCheckError;
 
 use crate::{
     hir_def::{expr::HirExpression, stmt::HirStatement},
-    node_interner::{ExprId, FuncId, NodeInterner, StmtId},
+    node_interner::{ExprId, FuncId, NodeInterner, StmtId, TraitImplKey},
     Type,
 };
 
@@ -63,30 +63,41 @@ pub fn type_check_func(interner: &mut NodeInterner, func_id: FuncId) -> Vec<Type
     // Check declared return type and actual return type
     if !can_ignore_ret {
         let (expr_span, empty_function) = function_info(interner, function_body_id);
-
         let func_span = interner.expr_span(function_body_id); // XXX: We could be more specific and return the span of the last stmt, however stmts do not have spans yet
-        function_last_type.unify_with_coercions(
-            &declared_return_type,
-            *function_body_id,
-            interner,
-            &mut errors,
-            || {
-                let mut error = TypeCheckError::TypeMismatchWithSource {
+        if let Type::TraitAsType(t) = &declared_return_type {
+            let key = TraitImplKey { typ: function_last_type.follow_bindings(), trait_id: t.id };
+            if interner.get_trait_implementation(&key).is_none() {
+                let error = TypeCheckError::TypeMismatchWithSource {
                     expected: declared_return_type.clone(),
                     actual: function_last_type.clone(),
                     span: func_span,
                     source: Source::Return(meta.return_type, expr_span),
                 };
-
-                if empty_function {
-                    error = error.add_context(
+                errors.push(error);
+            }
+        } else {
+            function_last_type.unify_with_coercions(
+                &declared_return_type,
+                *function_body_id,
+                interner,
+                &mut errors,
+                || {
+                    let mut error = TypeCheckError::TypeMismatchWithSource {
+                        expected: declared_return_type.clone(),
+                        actual: function_last_type.clone(),
+                        span: func_span,
+                        source: Source::Return(meta.return_type, expr_span),
+                    };
+
+                    if empty_function {
+                        error = error.add_context(
                         "implicitly returns `()` as its body has no tail or `return` expression",
                     );
-                }
-
-                error
-            },
-        );
+                    }
+                    error
+                },
+            );
+        }
     }
 
     errors

compiler/noirc_frontend/src/hir_def/traits.rs

@@ -34,7 +34,7 @@ pub struct TraitType {
 /// Represents a trait in the type system. Each instance of this struct
 /// will be shared across all Type::Trait variants that represent
 /// the same trait.
-#[derive(Clone, Debug)]
+#[derive(Debug, Eq, Clone)]
 pub struct Trait {
     /// A unique id representing this trait type. Used to check if two
     /// struct traits are equal.

compiler/noirc_frontend/src/hir_def/types.rs

@@ -14,7 +14,10 @@ use noirc_printable_type::PrintableType;
 
 use crate::{node_interner::StructId, Ident, Signedness};
 
-use super::expr::{HirCallExpression, HirExpression, HirIdent};
+use super::{
+    expr::{HirCallExpression, HirExpression, HirIdent},
+    traits::Trait,
+};
 
 #[derive(Debug, PartialEq, Eq, Clone, Hash)]
 pub enum Type {
@@ -62,6 +65,8 @@ pub enum Type {
     /// different argument types each time.
     TypeVariable(TypeVariable, TypeVariableKind),
 
+    TraitAsType(Trait),
+
     /// NamedGenerics are the 'T' or 'U' in a user-defined generic function
     /// like `fn foo<T, U>(...) {}`. Unlike TypeVariables, they cannot be bound over.
     NamedGeneric(TypeVariable, Rc<String>),
@@ -483,7 +488,8 @@ impl Type {
             | Type::Constant(_)
             | Type::NamedGeneric(_, _)
             | Type::NotConstant
-            | Type::Forall(_, _) => false,
+            | Type::Forall(_, _)
+            | Type::TraitAsType(_) => false,
 
             Type::Array(length, elem) => {
                 elem.contains_numeric_typevar(target_id) || named_generic_id_matches_target(length)
@@ -560,6 +566,9 @@ impl std::fmt::Display for Type {
                     write!(f, "{}<{}>", s.borrow(), args.join(", "))
                 }
             }
+            Type::TraitAsType(tr) => {
+                write!(f, "impl {}", tr.name)
+            }
             Type::Tuple(elements) => {
                 let elements = vecmap(elements, ToString::to_string);
                 write!(f, "({})", elements.join(", "))
@@ -1057,6 +1066,7 @@ impl Type {
                 let fields = vecmap(fields, |field| field.substitute(type_bindings));
                 Type::Tuple(fields)
             }
+            Type::TraitAsType(_) => todo!(),
             Type::Forall(typevars, typ) => {
                 // Trying to substitute a variable defined within a nested Forall
                 // is usually impossible and indicative of an error in the type checker somewhere.
@@ -1096,6 +1106,7 @@ impl Type {
                 let field_occurs = fields.occurs(target_id);
                 len_occurs || field_occurs
             }
+            Type::TraitAsType(_) => todo!(),
             Type::Struct(_, generic_args) => generic_args.iter().any(|arg| arg.occurs(target_id)),
             Type::Tuple(fields) => fields.iter().any(|field| field.occurs(target_id)),
             Type::NamedGeneric(binding, _) | Type::TypeVariable(binding, _) => {
@@ -1147,7 +1158,6 @@ impl Type {
                 Struct(def.clone(), args)
             }
             Tuple(args) => Tuple(vecmap(args, |arg| arg.follow_bindings())),
-
             TypeVariable(var, _) | NamedGeneric(var, _) => {
                 if let TypeBinding::Bound(typ) = &*var.borrow() {
                     return typ.follow_bindings();
@@ -1166,10 +1176,14 @@ impl Type {
 
             // Expect that this function should only be called on instantiated types
             Forall(..) => unreachable!(),
-
-            FieldElement | Integer(_, _) | Bool | Constant(_) | Unit | Error | NotConstant => {
-                self.clone()
-            }
+            TraitAsType(_)
+            | FieldElement
+            | Integer(_, _)
+            | Bool
+            | Constant(_)
+            | Unit
+            | Error
+            | NotConstant => self.clone(),
         }
     }
 }
@@ -1270,6 +1284,7 @@ impl From<&Type> for PrintableType {
                 let fields = vecmap(fields, |(name, typ)| (name, typ.into()));
                 PrintableType::Struct { fields, name: struct_type.name.to_string() }
             }
+            Type::TraitAsType(_) => unreachable!(),
             Type::Tuple(_) => todo!("printing tuple types is not yet implemented"),
             Type::TypeVariable(_, _) => unreachable!(),
             Type::NamedGeneric(..) => unreachable!(),

compiler/noirc_frontend/src/monomorphization/mod.rs

@@ -222,9 +222,15 @@ impl<'interner> Monomorphizer<'interner> {
         let modifiers = self.interner.function_modifiers(&f);
         let name = self.interner.function_name(&f).to_owned();
 
-        let return_type = self.convert_type(meta.return_type());
+        let body_expr_id = *self.interner.function(&f).as_expr();
+        let body_return_type = self.interner.id_type(body_expr_id);
+        let return_type = self.convert_type(match meta.return_type() {
+            Type::TraitAsType(_) => &body_return_type,
+            _ => meta.return_type(),
+        });
+
         let parameters = self.parameters(meta.parameters);
-        let body = self.expr(*self.interner.function(&f).as_expr());
+        let body = self.expr(body_expr_id);
         let unconstrained = modifiers.is_unconstrained
             || matches!(modifiers.contract_function_type, Some(ContractFunctionType::Open));
 
@@ -381,8 +387,8 @@ impl<'interner> Monomorphizer<'interner> {
                 }
             }
 
-            HirExpression::MethodCall(_) => {
-                unreachable!("Encountered HirExpression::MethodCall during monomorphization")
+            HirExpression::MethodCall(hir_method_call) => {
+                unreachable!("Encountered HirExpression::MethodCall during monomorphization {hir_method_call:?}")
             }
             HirExpression::Error => unreachable!("Encountered Error node during monomorphization"),
         }
@@ -635,7 +641,6 @@ impl<'interner> Monomorphizer<'interner> {
                 let location = Some(ident.location);
                 let name = definition.name.clone();
                 let typ = self.interner.id_type(expr_id);
-
                 let definition = self.lookup_function(*func_id, expr_id, &typ);
                 let typ = self.convert_type(&typ);
                 let ident = ast::Ident { location, mutable, definition, name, typ: typ.clone() };
@@ -686,7 +691,6 @@ impl<'interner> Monomorphizer<'interner> {
                 ast::Type::FmtString(size, fields)
             }
             HirType::Unit => ast::Type::Unit,
-
             HirType::Array(length, element) => {
                 let element = Box::new(self.convert_type(element.as_ref()));
 
@@ -696,7 +700,9 @@ impl<'interner> Monomorphizer<'interner> {
                     ast::Type::Slice(element)
                 }
             }
-
+            HirType::TraitAsType(_) => {
+                unreachable!("All TraitAsType should be replaced before calling convert_type");
+            }
             HirType::NamedGeneric(binding, _) => {
                 if let TypeBinding::Bound(binding) = &*binding.borrow() {
                     return self.convert_type(binding);
@@ -780,8 +786,7 @@ impl<'interner> Monomorphizer<'interner> {
         }
     }
 
-    fn is_function_closure(&self, raw_func_id: node_interner::ExprId) -> bool {
-        let t = self.convert_type(&self.interner.id_type(raw_func_id));
+    fn is_function_closure(&self, t: ast::Type) -> bool {
         if self.is_function_closure_type(&t) {
             true
         } else if let ast::Type::Tuple(elements) = t {
@@ -850,6 +855,7 @@ impl<'interner> Monomorphizer<'interner> {
         let func: Box<ast::Expression>;
         let return_type = self.interner.id_type(id);
         let return_type = self.convert_type(&return_type);
+
         let location = call.location;
 
         if let ast::Expression::Ident(ident) = original_func.as_ref() {
@@ -863,8 +869,9 @@ impl<'interner> Monomorphizer<'interner> {
         }
 
         let mut block_expressions = vec![];
-
-        let is_closure = self.is_function_closure(call.func);
+        let func_type = self.interner.id_type(call.func);
+        let func_type = self.convert_type(&func_type);
+        let is_closure = self.is_function_closure(func_type);
         if is_closure {
             let local_id = self.next_local_id();