chore: Move turbofish changes to the elaborator (#5094)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

Moving #3542,
#5041, and
#5087 to the elaborator

## Additional Context



## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

---------

Co-authored-by: jfecher <jake@aztecprotocol.com>

compiler/noirc_frontend/src/elaborator/expressions.rs

@@ -194,7 +194,7 @@ impl<'context> Elaborator<'context> {
                 let expr_id = self.interner.push_expr(ident);
                 self.interner.push_expr_location(expr_id, call_expr_span, self.file);
                 let ident = old_value.ident.clone();
-                let typ = self.type_check_variable(ident, expr_id);
+                let typ = self.type_check_variable(ident, expr_id, None);
                 self.interner.push_expr_type(expr_id, typ.clone());
                 capture_types.push(typ);
                 fmt_str_idents.push(expr_id);
@@ -291,16 +291,25 @@ impl<'context> Elaborator<'context> {
             Some(method_ref) => {
                 // Automatically add `&mut` if the method expects a mutable reference and
                 // the object is not already one.
-                if let HirMethodReference::FuncId(func_id) = &method_ref {
-                    if *func_id != FuncId::dummy_id() {
-                        let function_type = self.interner.function_meta(func_id).typ.clone();
-
-                        self.try_add_mutable_reference_to_object(
-                            &function_type,
-                            &mut object_type,
-                            &mut object,
-                        );
+                let func_id = match &method_ref {
+                    HirMethodReference::FuncId(func_id) => *func_id,
+                    HirMethodReference::TraitMethodId(method_id, _) => {
+                        let id = self.interner.trait_method_id(*method_id);
+                        let definition = self.interner.definition(id);
+                        let DefinitionKind::Function(func_id) = definition.kind else {
+                            unreachable!("Expected trait function to be a DefinitionKind::Function")
+                        };
+                        func_id
                     }
+                };
+
+                if func_id != FuncId::dummy_id() {
+                    let function_type = self.interner.function_meta(&func_id).typ.clone();
+                    self.try_add_mutable_reference_to_object(
+                        &function_type,
+                        &mut object_type,
+                        &mut object,
+                    );
                 }
 
                 // These arguments will be given to the desugared function call.
@@ -322,6 +331,7 @@ impl<'context> Elaborator<'context> {
                 let generics = method_call.generics.map(|option_inner| {
                     option_inner.into_iter().map(|generic| self.resolve_type(generic)).collect()
                 });
+                let turbofish_generics = generics.clone();
                 let method_call =
                     HirMethodCallExpression { method, object, arguments, location, generics };
 
@@ -335,7 +345,8 @@ impl<'context> Elaborator<'context> {
                     self.interner,
                 );
 
-                let func_type = self.type_check_variable(function_name, function_id);
+                let func_type =
+                    self.type_check_variable(function_name, function_id, turbofish_generics);
 
                 // Type check the new call now that it has been changed from a method call
                 // to a function call. This way we avoid duplicating code.

compiler/noirc_frontend/src/elaborator/patterns.rs

@@ -5,6 +5,7 @@ use rustc_hash::FxHashSet as HashSet;
 use crate::{
     ast::ERROR_IDENT,
     hir::{
+        def_collector::dc_crate::CompilationError,
         resolution::errors::ResolverError,
         type_check::{Source, TypeCheckError},
     },
@@ -330,9 +331,9 @@ impl<'context> Elaborator<'context> {
     ) -> (ExprId, Type) {
         let span = variable.span;
         let expr = self.resolve_variable(variable);
-        let id = self.interner.push_expr(HirExpression::Ident(expr.clone(), generics));
+        let id = self.interner.push_expr(HirExpression::Ident(expr.clone(), generics.clone()));
         self.interner.push_expr_location(id, span, self.file);
-        let typ = self.type_check_variable(expr, id);
+        let typ = self.type_check_variable(expr, id, generics);
         self.interner.push_expr_type(id, typ.clone());
         (id, typ)
     }
@@ -384,14 +385,19 @@ impl<'context> Elaborator<'context> {
         }
     }
 
-    pub(super) fn type_check_variable(&mut self, ident: HirIdent, expr_id: ExprId) -> Type {
+    pub(super) fn type_check_variable(
+        &mut self,
+        ident: HirIdent,
+        expr_id: ExprId,
+        generics: Option<Vec<Type>>,
+    ) -> Type {
         let mut bindings = TypeBindings::new();
 
         // Add type bindings from any constraints that were used.
         // We need to do this first since otherwise instantiating the type below
         // will replace each trait generic with a fresh type variable, rather than
         // the type used in the trait constraint (if it exists). See #4088.
-        if let ImplKind::TraitMethod(_, constraint, _) = &ident.impl_kind {
+        if let ImplKind::TraitMethod(_, constraint, assumed) = &ident.impl_kind {
             let the_trait = self.interner.get_trait(constraint.trait_id);
             assert_eq!(the_trait.generics.len(), constraint.trait_generics.len());
 
@@ -401,6 +407,16 @@ impl<'context> Elaborator<'context> {
                     bindings.insert(param.id(), (param.clone(), arg.clone()));
                 }
             }
+
+            // If the trait impl is already assumed to exist we should add any type bindings for `Self`.
+            // Otherwise `self` will be replaced with a fresh type variable, which will require the user
+            // to specify a redundant type annotation.
+            if *assumed {
+                bindings.insert(
+                    the_trait.self_type_typevar_id,
+                    (the_trait.self_type_typevar.clone(), constraint.typ.clone()),
+                );
+            }
         }
 
         // An identifiers type may be forall-quantified in the case of generic functions.
@@ -409,10 +425,21 @@ impl<'context> Elaborator<'context> {
         // variable to handle generic functions.
         let t = self.interner.id_type_substitute_trait_as_type(ident.id);
 
+        let definition = self.interner.try_definition(ident.id);
+        let function_generic_count = definition.map_or(0, |definition| match &definition.kind {
+            DefinitionKind::Function(function) => {
+                self.interner.function_modifiers(function).generic_count
+            }
+            _ => 0,
+        });
+
+        let span = self.interner.expr_span(&expr_id);
+        let location = self.interner.expr_location(&expr_id);
         // This instantiates a trait's generics as well which need to be set
         // when the constraint below is later solved for when the function is
         // finished. How to link the two?
-        let (typ, bindings) = t.instantiate_with_bindings(bindings, self.interner);
+        let (typ, bindings) =
+            self.instantiate(t, bindings, generics, function_generic_count, span, location);
 
         // Push any trait constraints required by this definition to the context
         // to be checked later when the type of this variable is further constrained.
@@ -447,6 +474,39 @@ impl<'context> Elaborator<'context> {
         typ
     }
 
+    fn instantiate(
+        &mut self,
+        typ: Type,
+        bindings: TypeBindings,
+        turbofish_generics: Option<Vec<Type>>,
+        function_generic_count: usize,
+        span: Span,
+        location: Location,
+    ) -> (Type, TypeBindings) {
+        match turbofish_generics {
+            Some(turbofish_generics) => {
+                if turbofish_generics.len() != function_generic_count {
+                    let type_check_err = TypeCheckError::IncorrectTurbofishGenericCount {
+                        expected_count: function_generic_count,
+                        actual_count: turbofish_generics.len(),
+                        span,
+                    };
+                    self.errors.push((CompilationError::TypeError(type_check_err), location.file));
+                    typ.instantiate_with_bindings(bindings, self.interner)
+                } else {
+                    // Fetch the count of any implicit generics on the function, such as
+                    // for a method within a generic impl.
+                    let implicit_generic_count = match &typ {
+                        Type::Forall(generics, _) => generics.len() - function_generic_count,
+                        _ => 0,
+                    };
+                    typ.instantiate_with(turbofish_generics, self.interner, implicit_generic_count)
+                }
+            }
+            None => typ.instantiate_with_bindings(bindings, self.interner),
+        }
+    }
+
     fn get_ident_from_path(&mut self, path: Path) -> (HirIdent, usize) {
         let location = Location::new(path.span(), self.file);
 

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

@@ -399,8 +399,6 @@ impl<'interner> TypeChecker<'interner> {
         // variable to handle generic functions.
         let t = self.interner.id_type_substitute_trait_as_type(ident.id);
 
-        let span = self.interner.expr_span(expr_id);
-
         let definition = self.interner.try_definition(ident.id);
         let function_generic_count = definition.map_or(0, |definition| match &definition.kind {
             DefinitionKind::Function(function) => {
@@ -409,6 +407,7 @@ impl<'interner> TypeChecker<'interner> {
             _ => 0,
         });
 
+        let span = self.interner.expr_span(expr_id);
         // This instantiates a trait's generics as well which need to be set
         // when the constraint below is later solved for when the function is
         // finished. How to link the two?