feat: Implement impl specialization (noir-lang#3087)

compiler/noirc_frontend/src/hir/def_collector/dc_crate.rs

@@ -394,15 +394,12 @@ fn collect_impls(
                 let module = &mut def_maps.get_mut(&crate_id).unwrap().modules[type_module.0];
 
                 for (_, method_id, method) in &unresolved.functions {
-                    let result = module.declare_function(method.name_ident().clone(), *method_id);
-
-                    if let Err((first_def, second_def)) = result {
-                        let error = DefCollectorErrorKind::Duplicate {
-                            typ: DuplicateType::Function,
-                            first_def,
-                            second_def,
-                        };
-                        errors.push((error.into(), unresolved.file_id));
+                    // If this method was already declared, remove it from the module so it cannot
+                    // be accessed with the `TypeName::method` syntax. We'll check later whether the
+                    // object types in each method overlap or not. If they do, we issue an error.
+                    // If not, that is specialization which is allowed.
+                    if module.declare_function(method.name_ident().clone(), *method_id).is_err() {
+                        module.remove_function(method.name_ident());
                     }
                 }
             // Prohibit defining impls for primitive types if we're not in the stdlib

compiler/noirc_frontend/src/hir/def_map/item_scope.rs

@@ -85,4 +85,9 @@ impl ItemScope {
     pub fn values(&self) -> &HashMap<Ident, (ModuleDefId, Visibility)> {
         &self.values
     }
+
+    pub fn remove_definition(&mut self, name: &Ident) {
+        self.types.remove(name);
+        self.values.remove(name);
+    }
 }

compiler/noirc_frontend/src/hir/def_map/module_data.rs

@@ -53,6 +53,11 @@ impl ModuleData {
         self.declare(name, id.into())
     }
 
+    pub fn remove_function(&mut self, name: &Ident) {
+        self.scope.remove_definition(name);
+        self.definitions.remove_definition(name);
+    }
+
     pub fn declare_global(&mut self, name: Ident, id: StmtId) -> Result<(), (Ident, Ident)> {
         self.declare(name, id.into())
     }

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

@@ -826,7 +826,8 @@ impl<'interner> TypeChecker<'interner> {
     ) -> Option<HirMethodReference> {
         match object_type {
             Type::Struct(typ, _args) => {
-                match self.interner.lookup_method(typ.borrow().id, method_name) {
+                let id = typ.borrow().id;
+                match self.interner.lookup_method(object_type, id, method_name, false) {
                     Some(method_id) => Some(HirMethodReference::FuncId(method_id)),
                     None => {
                         self.errors.push(TypeCheckError::UnresolvedMethodCall {

compiler/noirc_frontend/src/hir_def/types.rs

@@ -782,7 +782,7 @@ impl Type {
 
     /// `try_unify` is a bit of a misnomer since although errors are not committed,
     /// any unified bindings are on success.
-    fn try_unify(&self, other: &Type) -> Result<(), UnificationError> {
+    pub fn try_unify(&self, other: &Type) -> Result<(), UnificationError> {
         use Type::*;
         use TypeVariableKind as Kind;
 
@@ -995,7 +995,7 @@ impl Type {
     /// Instantiate this type, replacing any type variables it is quantified
     /// over with fresh type variables. If this type is not a Type::Forall,
     /// it is unchanged.
-    pub fn instantiate(&self, interner: &mut NodeInterner) -> (Type, TypeBindings) {
+    pub fn instantiate(&self, interner: &NodeInterner) -> (Type, TypeBindings) {
         match self {
             Type::Forall(typevars, typ) => {
                 let replacements = typevars

compiler/noirc_frontend/src/node_interner.rs

@@ -87,9 +87,6 @@ pub struct NodeInterner {
     // Each trait definition is possibly shared across multiple type nodes.
     // It is also mutated through the RefCell during name resolution to append
     // methods from impls to the type.
-    //
-    // TODO: We may be able to remove the Shared wrapper once traits are no longer types.
-    // We'd just lookup their methods as needed through the NodeInterner.
     traits: HashMap<TraitId, Trait>,
 
     // Trait implementation map
@@ -108,10 +105,15 @@ pub struct NodeInterner {
 
     globals: HashMap<StmtId, GlobalInfo>, // NOTE: currently only used for checking repeat globals and restricting their scope to a module
 
-    next_type_variable_id: usize,
+    next_type_variable_id: std::cell::Cell<usize>,
 
     /// A map from a struct type and method name to a function id for the method.
-    struct_methods: HashMap<(StructId, String), FuncId>,
+    /// This can resolve to potentially multiple methods if the same method name is
+    /// specialized for different generics on the same type. E.g. for `Struct<T>`, we
+    /// may have both `impl Struct<u32> { fn foo(){} }` and `impl Struct<u8> { fn foo(){} }`.
+    /// If this happens, the returned Vec will have 2 entries and we'll need to further
+    /// disambiguate them by checking the type of each function.
+    struct_methods: HashMap<(StructId, String), Vec<FuncId>>,
 
     /// Methods on primitive types defined in the stdlib.
     primitive_methods: HashMap<(TypeMethodKey, String), FuncId>,
@@ -381,7 +383,7 @@ impl Default for NodeInterner {
             trait_implementations: HashMap::new(),
             instantiation_bindings: HashMap::new(),
             field_indices: HashMap::new(),
-            next_type_variable_id: 0,
+            next_type_variable_id: std::cell::Cell::new(0),
             globals: HashMap::new(),
             struct_methods: HashMap::new(),
             primitive_methods: HashMap::new(),
@@ -829,13 +831,13 @@ impl NodeInterner {
         *old = Node::Expression(new);
     }
 
-    pub fn next_type_variable_id(&mut self) -> TypeVariableId {
-        let id = self.next_type_variable_id;
-        self.next_type_variable_id += 1;
+    pub fn next_type_variable_id(&self) -> TypeVariableId {
+        let id = self.next_type_variable_id.get();
+        self.next_type_variable_id.set(id + 1);
         TypeVariableId(id)
     }
 
-    pub fn next_type_variable(&mut self) -> Type {
+    pub fn next_type_variable(&self) -> Type {
         Type::type_variable(self.next_type_variable_id())
     }
 
@@ -863,9 +865,10 @@ impl NodeInterner {
         self.function_definition_ids[&function]
     }
 
-    /// Add a method to a type.
-    /// This will panic for non-struct types currently as we do not support methods
-    /// for primitives. We could allow this in the future however.
+    /// Adds a non-trait method to a type.
+    ///
+    /// Returns `Some(duplicate)` if a matching method was already defined.
+    /// Returns `None` otherwise.
     pub fn add_method(
         &mut self,
         self_type: &Type,
@@ -874,8 +877,15 @@ impl NodeInterner {
     ) -> Option<FuncId> {
         match self_type {
             Type::Struct(struct_type, _generics) => {
-                let key = (struct_type.borrow().id, method_name);
-                self.struct_methods.insert(key, method_id)
+                let id = struct_type.borrow().id;
+
+                if let Some(existing) = self.lookup_method(self_type, id, &method_name, true) {
+                    return Some(existing);
+                }
+
+                let key = (id, method_name);
+                self.struct_methods.entry(key).or_default().push(method_id);
+                None
             }
             Type::Error => None,
 
@@ -899,11 +909,10 @@ impl NodeInterner {
     ) -> bool {
         self.trait_implementations.insert(key.clone(), trait_impl.clone());
         match &key.typ {
-            Type::Struct(struct_type, _generics) => {
+            Type::Struct(..) => {
                 for func_id in &trait_impl.borrow().methods {
                     let method_name = self.function_name(func_id).to_owned();
-                    let key = (struct_type.borrow().id, method_name);
-                    self.struct_methods.insert(key, *func_id);
+                    self.add_method(&key.typ, method_name, *func_id);
                 }
                 true
             }
@@ -938,9 +947,50 @@ impl NodeInterner {
         }
     }
 
-    /// Search by name for a method on the given struct
-    pub fn lookup_method(&self, id: StructId, method_name: &str) -> Option<FuncId> {
-        self.struct_methods.get(&(id, method_name.to_owned())).copied()
+    /// Search by name for a method on the given struct.
+    ///
+    /// If `check_type` is true, this will force `lookup_method` to check the type
+    /// of each candidate instead of returning only the first candidate if there is exactly one.
+    /// This is generally only desired when declaring new methods to check if they overlap any
+    /// existing methods.
+    ///
+    /// Another detail is that this method does not handle auto-dereferencing through `&mut T`.
+    /// So if an object is of type `self : &mut T` but a method only accepts `self: T` (or
+    /// vice-versa), the call will not be selected. If this is ever implemented into this method,
+    /// we can remove the `methods.len() == 1` check and the `check_type` early return.
+    pub fn lookup_method(
+        &self,
+        typ: &Type,
+        id: StructId,
+        method_name: &str,
+        check_type: bool,
+    ) -> Option<FuncId> {
+        let methods = self.struct_methods.get(&(id, method_name.to_owned()))?;
+
+        // If there is only one method, just return it immediately.
+        // It will still be typechecked later.
+        if !check_type && methods.len() == 1 {
+            return Some(methods[0]);
+        }
+
+        // When adding methods we always check they do not overlap, so there should be
+        // at most 1 matching method in this list.
+        for method in methods {
+            match self.function_meta(method).typ.instantiate(self).0 {
+                Type::Function(args, _, _) => {
+                    if let Some(object) = args.get(0) {
+                        // TODO #3089: This is dangerous! try_unify may commit type bindings even on failure
+                        if object.try_unify(typ).is_ok() {
+                            return Some(*method);
+                        }
+                    }
+                }
+                Type::Error => (),
+                other => unreachable!("Expected function type, found {other}"),
+            }
+        }
+
+        None
     }
 
     /// Looks up a given method name on the given primitive type.

tooling/nargo_cli/tests/compile_success_empty/specialization/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "specialization"
+type = "bin"
+authors = [""]
+compiler_version = "0.16.0"
+
+[dependencies]

tooling/nargo_cli/tests/compile_success_empty/specialization/src/main.nr

@@ -0,0 +1,15 @@
+struct Foo<T> {}
+
+impl Foo<u32> {
+    fn foo(_self: Self) -> Field { 1 }
+}
+
+impl Foo<u64> {
+    fn foo(_self: Self) -> Field { 2 }
+}
+
+fn main() {
+    let f1: Foo<u32> = Foo{};
+    let f2: Foo<u64> = Foo{};
+    assert(f1.foo() + f2.foo() == 3);
+}