fix: Apply self type from generic trait constraint before instantiati…

…ng identifiers (#5087)

# Description

## Problem\*

Resolves #5063 

## Summary\*

After the turbofish PRs we want the ability to instantiate a variable
from a generic trait. This was possible but it required a redundant type
annotiation like such (assume `H` has been specified with a `where`
clause):
```rust
let hasher: H = H::default()
```
Similarly to trait generics, we now add type bindings from a trait
constraint if the trait impl is assumed to exist.
We now can just do:
```rust
let hasher = H::default()
```

## Additional Context



## Documentation\*

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

# PR Checklist\*

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

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

@@ -371,7 +371,7 @@ impl<'interner> TypeChecker<'interner> {
         // 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());
 
@@ -381,6 +381,16 @@ impl<'interner> TypeChecker<'interner> {
                     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.

compiler/noirc_frontend/src/tests.rs

@@ -1430,14 +1430,14 @@ fn specify_method_types_with_turbofish() {
         }
         
         impl<T> Foo<T> {
-            fn generic_method<U>(_self: Self) where U: Default {
+            fn generic_method<U>(_self: Self) -> U where U: Default {
                 U::default()
             }
         }
         
         fn main() {
             let foo: Foo<Field> = Foo { inner: 1 };
-            foo.generic_method::<Field>();
+            let _ = foo.generic_method::<Field>();
         }
     "#;
     let errors = get_program_errors(src);

noir_stdlib/src/eddsa.nr

@@ -45,7 +45,7 @@ where H: Hasher + Default {
     // Ensure S < Subgroup Order
     assert(signature_s.lt(bjj.suborder));
     // Calculate the h = H(R, A, msg)
-    let mut hasher: H = H::default();
+    let mut hasher = H::default();
     hasher.write(signature_r8_x);
     hasher.write(signature_r8_y);
     hasher.write(pub_key_x);

test_programs/execution_success/turbofish_call_func_diff_types/src/main.nr

@@ -23,9 +23,7 @@ fn main(x: Field, y: pub Field) {
 
 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();
+    let mut hasher = 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