fix: allow using Self for function calls (#5629)

# Description

## Problem

Resolves #1791

## Summary

When resolving a path (any path), we now check if the first segment is
`Self` (a similar check is done in another places of our codebase). If
so, resolve the next path segments relative to that type. Only struct
types are supported here... in theory a trait type can also show up, but
that case is handled as a special case in the compiler. I'll create a
follow-up PR removing that special case and seeing if it works.

## Additional Context

None.

## 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/elaborator/scope.rs

@@ -1,6 +1,6 @@
 use noirc_errors::{Location, Spanned};
 
-use crate::ast::ERROR_IDENT;
+use crate::ast::{PathKind, ERROR_IDENT};
 use crate::hir::def_map::{LocalModuleId, ModuleId};
 use crate::hir::resolution::path_resolver::{PathResolver, StandardPathResolver};
 use crate::hir::scope::{Scope as GenericScope, ScopeTree as GenericScopeTree};
@@ -43,7 +43,34 @@ impl<'context> Elaborator<'context> {
     }
 
     pub(super) fn resolve_path(&mut self, path: Path) -> Result<ModuleDefId, ResolverError> {
-        let resolver = StandardPathResolver::new(self.module_id());
+        let mut module_id = self.module_id();
+        let mut path = path;
+
+        if path.kind == PathKind::Plain && path.first_name() == SELF_TYPE_NAME {
+            if let Some(Type::Struct(struct_type, _)) = &self.self_type {
+                let struct_type = struct_type.borrow();
+                if path.segments.len() == 1 {
+                    return Ok(ModuleDefId::TypeId(struct_type.id));
+                }
+
+                module_id = struct_type.id.module_id();
+                path = Path {
+                    segments: path.segments[1..].to_vec(),
+                    kind: PathKind::Plain,
+                    span: path.span(),
+                };
+            }
+        }
+
+        self.resolve_path_in_module(path, module_id)
+    }
+
+    fn resolve_path_in_module(
+        &mut self,
+        path: Path,
+        module_id: ModuleId,
+    ) -> Result<ModuleDefId, ResolverError> {
+        let resolver = StandardPathResolver::new(module_id);
         let path_resolution;
 
         if self.interner.track_references {

compiler/noirc_frontend/src/tests.rs

@@ -2729,3 +2729,83 @@ fn incorrect_generic_count_on_type_alias() {
     assert_eq!(actual, 1);
     assert_eq!(expected, 0);
 }
+
+#[test]
+fn uses_self_type_for_struct_function_call() {
+    let src = r#"
+    struct S { }
+
+    impl S {
+        fn one() -> Field {
+            1
+        }
+
+        fn two() -> Field {
+            Self::one() + Self::one()
+        }
+    }
+
+    fn main() {}
+    "#;
+    assert_no_errors(src);
+}
+
+#[test]
+fn uses_self_type_inside_trait() {
+    let src = r#"
+    trait Foo {
+        fn foo() -> Self {
+            Self::bar()
+        }
+
+        fn bar() -> Self;
+    }
+
+    impl Foo for Field {
+        fn bar() -> Self {
+            1
+        }
+    }
+
+    fn main() {
+        let _: Field = Foo::foo();
+    }
+    "#;
+    assert_no_errors(src);
+}
+
+#[test]
+fn uses_self_type_in_trait_where_clause() {
+    let src = r#"
+    trait Trait {
+        fn trait_func() -> bool;
+    }
+
+    trait Foo where Self: Trait {
+        fn foo(self) -> bool {
+            self.trait_func()
+        }
+    }
+
+    struct Bar {
+
+    }
+
+    impl Foo for Bar {
+
+    }
+
+    fn main() {}
+    "#;
+
+    let errors = get_program_errors(src);
+    assert_eq!(errors.len(), 1);
+
+    let CompilationError::TypeError(TypeCheckError::UnresolvedMethodCall { method_name, .. }) =
+        &errors[0].0
+    else {
+        panic!("Expected an unresolved method call error, got {:?}", errors[0].0);
+    };
+
+    assert_eq!(method_name, "trait_func");
+}