fix(frontend): Correctly monomorphize turbofish functions (#5049)

# Description

## Problem\*

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

Working towards #4514 and fully
resolving #4710

## Summary\*

We need to accurately monomorphize functions with turbofish operators
where no function parameters or the return type using the specified
generic.

Without this change the following would pass:
```
fn main(x: Field, y: pub Field) {
    let mut hasher = PoseidonHasher::default();
    hasher.write(x);
    hasher.write(y);
    let poseidon_expected_hash = hasher.finish();
    assert(hash_simple_array::<PoseidonHasher>([x, y]) == poseidon_expected_hash);

    let mut hasher = Poseidon2Hasher::default();
    hasher.write(x);
    hasher.write(y);
    let poseidon2_expected_hash = hasher.finish();
    assert(hash_simple_array::<Poseidon2Hasher>([x, y]) == poseidon_expected_hash);
    assert(hash_simple_array::<Poseidon2Hasher>([x, y]) != poseidon2_expected_hash);
}

fn hash_simple_array<H>(input: [Field; 2]) -> Field where H: Hasher + Default {
    let mut hasher: H = H::default();
    hasher.write(input[0]);
    hasher.write(input[1]);
    hasher.finish()
}
```

Essentially any future invocations of `hash_simple_array` would use the
`PoseidonHasher`. We now correctly monomorphize the functions to use the
correct type.

## 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: Jake Fecher <jake@aztecprotocol.com>
Co-authored-by: Jake Fecher <jfecher11@gmail.com>
Co-authored-by: Tom French <tom@tomfren.ch>

compiler/noirc_frontend/src/monomorphization/mod.rs

@@ -55,9 +55,12 @@ struct LambdaContext {
 struct Monomorphizer<'interner> {
     /// Functions are keyed by their unique ID and expected type so that we can monomorphize
     /// a new version of the function for each type.
+    /// We also key by any turbofish generics that are specified.
+    /// This is necessary for a case where we may have a trait generic that can be instantiated
+    /// outside of a function parameter or return value.
     ///
     /// Using nested HashMaps here lets us avoid cloning HirTypes when calling .get()
-    functions: HashMap<node_interner::FuncId, HashMap<HirType, FuncId>>,
+    functions: HashMap<node_interner::FuncId, HashMap<(HirType, Vec<HirType>), FuncId>>,
 
     /// Unlike functions, locals are only keyed by their unique ID because they are never
     /// duplicated during monomorphization. Doing so would allow them to be used polymorphically
@@ -198,10 +201,15 @@ impl<'interner> Monomorphizer<'interner> {
         id: node_interner::FuncId,
         expr_id: node_interner::ExprId,
         typ: &HirType,
+        turbofish_generics: Vec<HirType>,
         trait_method: Option<TraitMethodId>,
     ) -> Definition {
         let typ = typ.follow_bindings();
-        match self.functions.get(&id).and_then(|inner_map| inner_map.get(&typ)) {
+        match self
+            .functions
+            .get(&id)
+            .and_then(|inner_map| inner_map.get(&(typ.clone(), turbofish_generics.clone())))
+        {
             Some(id) => Definition::Function(*id),
             None => {
                 // Function has not been monomorphized yet
@@ -222,7 +230,8 @@ impl<'interner> Monomorphizer<'interner> {
                         Definition::Builtin(opcode)
                     }
                     FunctionKind::Normal => {
-                        let id = self.queue_function(id, expr_id, typ, trait_method);
+                        let id =
+                            self.queue_function(id, expr_id, typ, turbofish_generics, trait_method);
                         Definition::Function(id)
                     }
                     FunctionKind::Oracle => {
@@ -249,8 +258,14 @@ impl<'interner> Monomorphizer<'interner> {
     }
 
     /// Prerequisite: typ = typ.follow_bindings()
-    fn define_function(&mut self, id: node_interner::FuncId, typ: HirType, new_id: FuncId) {
-        self.functions.entry(id).or_default().insert(typ, new_id);
+    fn define_function(
+        &mut self,
+        id: node_interner::FuncId,
+        typ: HirType,
+        turbofish_generics: Vec<HirType>,
+        new_id: FuncId,
+    ) {
+        self.functions.entry(id).or_default().insert((typ, turbofish_generics), new_id);
     }
 
     fn compile_main(
@@ -393,7 +408,7 @@ impl<'interner> Monomorphizer<'interner> {
         use ast::Literal::*;
 
         let expr = match self.interner.expression(&expr) {
-            HirExpression::Ident(ident, _) => self.ident(ident, expr)?,
+            HirExpression::Ident(ident, generics) => self.ident(ident, expr, generics)?,
             HirExpression::Literal(HirLiteral::Str(contents)) => Literal(Str(contents)),
             HirExpression::Literal(HirLiteral::FmtStr(contents, idents)) => {
                 let fields = try_vecmap(idents, |ident| self.expr(ident))?;
@@ -825,6 +840,7 @@ impl<'interner> Monomorphizer<'interner> {
         &mut self,
         ident: HirIdent,
         expr_id: node_interner::ExprId,
+        generics: Option<Vec<HirType>>,
     ) -> Result<ast::Expression, MonomorphizationError> {
         let typ = self.interner.id_type(expr_id);
 
@@ -838,7 +854,13 @@ impl<'interner> Monomorphizer<'interner> {
                 let mutable = definition.mutable;
                 let location = Some(ident.location);
                 let name = definition.name.clone();
-                let definition = self.lookup_function(*func_id, expr_id, &typ, None);
+                let definition = self.lookup_function(
+                    *func_id,
+                    expr_id,
+                    &typ,
+                    generics.unwrap_or_default(),
+                    None,
+                );
                 let typ = Self::convert_type(&typ, ident.location)?;
                 let ident = ast::Ident { location, mutable, definition, name, typ: typ.clone() };
                 let ident_expression = ast::Expression::Ident(ident);
@@ -1063,10 +1085,11 @@ impl<'interner> Monomorphizer<'interner> {
             }
         };
 
-        let func_id = match self.lookup_function(func_id, expr_id, &function_type, Some(method)) {
-            Definition::Function(func_id) => func_id,
-            _ => unreachable!(),
-        };
+        let func_id =
+            match self.lookup_function(func_id, expr_id, &function_type, vec![], Some(method)) {
+                Definition::Function(func_id) => func_id,
+                _ => unreachable!(),
+            };
 
         let the_trait = self.interner.get_trait(method.trait_id);
         let location = self.interner.expr_location(&expr_id);
@@ -1292,10 +1315,11 @@ impl<'interner> Monomorphizer<'interner> {
         id: node_interner::FuncId,
         expr_id: node_interner::ExprId,
         function_type: HirType,
+        turbofish_generics: Vec<HirType>,
         trait_method: Option<TraitMethodId>,
     ) -> FuncId {
         let new_id = self.next_function_id();
-        self.define_function(id, function_type.clone(), new_id);
+        self.define_function(id, function_type.clone(), turbofish_generics, new_id);
 
         let bindings = self.interner.get_instantiation_bindings(expr_id);
         let bindings = self.follow_bindings(bindings);

test_programs/execution_success/trait_method_mut_self/src/main.nr

@@ -12,14 +12,6 @@ fn main(x: Field, y: pub Field) {
 
     pass_trait_by_mut_ref(&mut a_mut_ref, y);
     assert(a_mut_ref.x == y);
-
-    let mut hasher = Poseidon2Hasher::default();
-    hasher.write(x);
-    hasher.write(y);
-    let expected_hash = hasher.finish();
-    // Check that we get the same result when using the hasher in a 
-    // method that purely uses trait methods without a supplied implementation.
-    assert(hash_simple_array::<Poseidon2Hasher>([x, y]) == expected_hash);
 }
 
 trait SomeTrait {
@@ -58,17 +50,3 @@ fn pass_trait_by_mut_ref<T>(a_mut_ref: &mut T, value: Field) where T: SomeTrait
     // We auto add a mutable reference to the object type if the method call expects a mutable self
     a_mut_ref.set_value(value);
 }
-
-fn hash_simple_array<H>(input: [Field; 2]) -> Field where H: Hasher + Default {
-    // Check that we can call a trait method instead of a trait implementation
-    // TODO: Need to remove the need for this type annotation
-    // TODO: Curently, without the annotation we will get `Expression type is ambiguous` when trying to use the `hasher`
-    let mut hasher: H = H::default();
-    // Regression that the object is converted to a mutable reference type `&mut _`.
-    // Otherwise will see `Expected type &mut _, found type H`.
-    // Then we need to make sure to also auto dereference later in the type checking process
-    // when searching for a matching impl or else we will get `No matching impl found for `&mut H: Hasher`
-    hasher.write(input[0]);
-    hasher.write(input[1]);
-    hasher.finish()
-}

test_programs/execution_success/turbofish_call_func_diff_types/Nargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "turbofish_call_func_diff_types"
+type = "bin"
+authors = [""]
+compiler_version = ">=0.29.0"
+
+[dependencies]

test_programs/execution_success/turbofish_call_func_diff_types/Prover.toml

@@ -0,0 +1,2 @@
+x = "5"
+y = "10"

test_programs/execution_success/turbofish_call_func_diff_types/src/main.nr

@@ -0,0 +1,36 @@
+use dep::std::hash::Hasher;
+use dep::std::hash::poseidon2::Poseidon2Hasher;
+use dep::std::hash::poseidon::PoseidonHasher;
+
+fn main(x: Field, y: pub Field) {
+    let mut hasher = PoseidonHasher::default();
+    hasher.write(x);
+    hasher.write(y);
+    let poseidon_expected_hash = hasher.finish();
+    // Check that we get the same result when using the hasher in a 
+    // method that purely uses trait methods without a supplied implementation.
+    assert(hash_simple_array::<PoseidonHasher>([x, y]) == poseidon_expected_hash);
+
+    // Now let's do the same logic but with a different `Hasher` supplied to the turbofish operator
+    // We want to make sure that we have correctly monomorphized a function with a trait generic
+    // where the generic is not used on any function parameters or the return value.
+    let mut hasher = Poseidon2Hasher::default();
+    hasher.write(x);
+    hasher.write(y);
+    let poseidon2_expected_hash = hasher.finish();
+    assert(hash_simple_array::<Poseidon2Hasher>([x, y]) == poseidon2_expected_hash);
+}
+
+fn hash_simple_array<H>(input: [Field; 2]) -> Field where H: Hasher + Default {
+    // Check that we can call a trait method instead of a trait implementation
+    // TODO(https://github.com/noir-lang/noir/issues/5063): Need to remove the need for this type annotation
+    // Curently, without the annotation we will get `Expression type is ambiguous` when trying to use the `hasher`
+    let mut hasher: H = H::default();
+    // Regression that the object is converted to a mutable reference type `&mut _`.
+    // Otherwise will see `Expected type &mut _, found type H`.
+    // Then we need to make sure to also auto dereference later in the type checking process
+    // when searching for a matching impl or else we will get `No matching impl found for `&mut H: Hasher`
+    hasher.write(input[0]);
+    hasher.write(input[1]);
+    hasher.finish()
+}