feat: Allow type aliases to reference other aliases (#4353)

# Description

## Problem\*

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

## Summary\*

Allows type aliases to reference other aliases, using the dependency
graph to error if there are ever cycles.

To prevent infinite recursion in the type checker, aliases now have
their own `Type::Alias` node which is used
to set the inner aliased type to `Type::Error` in the case of a cycle to
break the cycle.

## Additional Context

Example error:

```
error: Dependency cycle found
  ┌─ /home/user/Code/Noir/noir/short/src/main.nr:2:1
  │
2 │ type B = A;
  │ ---------- 'B' recursively depends on itself: B -> A -> B
  │
```

## Documentation\*

Check one:
- [ ] No documentation needed.
- [x] Documentation included in this PR.
- [ ] **[Exceptional Case]** 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.

---------

Co-authored-by: kevaundray <[email protected]>

compiler/noirc_frontend/src/hir/resolution/resolver.rs

@@ -29,7 +29,7 @@ use crate::hir::def_map::{LocalModuleId, ModuleDefId, TryFromModuleDefId, MAIN_F
 use crate::hir_def::stmt::{HirAssignStatement, HirForStatement, HirLValue, HirPattern};
 use crate::node_interner::{
     DefinitionId, DefinitionKind, DependencyId, ExprId, FuncId, GlobalId, NodeInterner, StmtId,
-    StructId, TraitId, TraitImplId, TraitMethodId,
+    StructId, TraitId, TraitImplId, TraitMethodId, TypeAliasId,
 };
 use crate::{
     hir::{def_map::CrateDefMap, resolution::path_resolver::PathResolver},
@@ -39,9 +39,9 @@ use crate::{
 use crate::{
     ArrayLiteral, ContractFunctionType, Distinctness, ForRange, FunctionDefinition,
     FunctionReturnType, FunctionVisibility, Generics, LValue, NoirStruct, NoirTypeAlias, Param,
-    Path, PathKind, Pattern, Shared, StructType, Type, TypeAliasType, TypeVariable,
-    TypeVariableKind, UnaryOp, UnresolvedGenerics, UnresolvedTraitConstraint, UnresolvedType,
-    UnresolvedTypeData, UnresolvedTypeExpression, Visibility, ERROR_IDENT,
+    Path, PathKind, Pattern, Shared, StructType, Type, TypeAlias, TypeVariable, TypeVariableKind,
+    UnaryOp, UnresolvedGenerics, UnresolvedTraitConstraint, UnresolvedType, UnresolvedTypeData,
+    UnresolvedTypeExpression, Visibility, ERROR_IDENT,
 };
 use fm::FileId;
 use iter_extended::vecmap;
@@ -573,16 +573,19 @@ impl<'a> Resolver<'a> {
         let span = path.span();
         let mut args = vecmap(args, |arg| self.resolve_type_inner(arg, new_variables));
 
-        if let Some(type_alias_type) = self.lookup_type_alias(path.clone()) {
-            let expected_generic_count = type_alias_type.generics.len();
-            let type_alias_string = type_alias_type.to_string();
-            let id = type_alias_type.id;
+        if let Some(type_alias) = self.lookup_type_alias(path.clone()) {
+            let type_alias = type_alias.borrow();
+            let expected_generic_count = type_alias.generics.len();
+            let type_alias_string = type_alias.to_string();
+            let id = type_alias.id;
 
             self.verify_generics_count(expected_generic_count, &mut args, span, || {
                 type_alias_string
             });
 
-            let result = self.interner.get_type_alias(id).get_type(&args);
+            if let Some(item) = self.current_item {
+                self.interner.add_type_alias_dependency(item, id);
+            }
 
             // Collecting Type Alias references [Location]s to be used by LSP in order
             // to resolve the definition of the type alias
@@ -593,9 +596,8 @@ impl<'a> Resolver<'a> {
             // equal to another type alias. Fixing this fully requires an analysis to create a DFG
             // of definition ordering, but for now we have an explicit check here so that we at
             // least issue an error that the type was not found instead of silently passing.
-            if result != Type::Error {
-                return result;
-            }
+            let alias = self.interner.get_type_alias(id);
+            return Type::Alias(alias, args);
         }
 
         match self.lookup_struct_or_error(path) {
@@ -752,12 +754,15 @@ impl<'a> Resolver<'a> {
         resolved_type
     }
 
-    pub fn resolve_type_aliases(
+    pub fn resolve_type_alias(
         mut self,
         unresolved: NoirTypeAlias,
+        alias_id: TypeAliasId,
     ) -> (Type, Generics, Vec<ResolverError>) {
         let generics = self.add_generics(&unresolved.generics);
         self.resolve_local_globals();
+
+        self.current_item = Some(DependencyId::Alias(alias_id));
         let typ = self.resolve_type(unresolved.typ);
 
         (typ, generics, self.errors)
@@ -1120,6 +1125,17 @@ impl<'a> Resolver<'a> {
                     }
                 }
             }
+            Type::Alias(alias, generics) => {
+                for (i, generic) in generics.iter().enumerate() {
+                    if let Type::NamedGeneric(type_variable, name) = generic {
+                        if alias.borrow().generic_is_numeric(i) {
+                            found.insert(name.to_string(), type_variable.clone());
+                        }
+                    } else {
+                        Self::find_numeric_generics_in_type(generic, found);
+                    }
+                }
+            }
             Type::MutableReference(element) => Self::find_numeric_generics_in_type(element, found),
             Type::String(length) => {
                 if let Type::NamedGeneric(type_variable, name) = length.as_ref() {
@@ -1791,7 +1807,7 @@ impl<'a> Resolver<'a> {
         }
     }
 
-    fn lookup_type_alias(&mut self, path: Path) -> Option<&TypeAliasType> {
+    fn lookup_type_alias(&mut self, path: Path) -> Option<Shared<TypeAlias>> {
         self.lookup(path).ok().map(|id| self.interner.get_type_alias(id))
     }
 

compiler/noirc_frontend/src/hir/resolution/type_aliases.rs

@@ -17,17 +17,17 @@ pub(crate) fn resolve_type_aliases(
     crate_id: CrateId,
 ) -> Vec<(CompilationError, FileId)> {
     let mut errors: Vec<(CompilationError, FileId)> = vec![];
-    for (type_id, unresolved_typ) in type_aliases {
+    for (alias_id, unresolved_typ) in type_aliases {
         let path_resolver = StandardPathResolver::new(ModuleId {
             local_id: unresolved_typ.module_id,
             krate: crate_id,
         });
         let file = unresolved_typ.file_id;
         let (typ, generics, resolver_errors) =
             Resolver::new(&mut context.def_interner, &path_resolver, &context.def_maps, file)
-                .resolve_type_aliases(unresolved_typ.type_alias_def);
+                .resolve_type_alias(unresolved_typ.type_alias_def, alias_id);
         errors.extend(resolver_errors.iter().cloned().map(|e| (e.into(), file)));
-        context.def_interner.set_type_alias(type_id, typ, generics);
+        context.def_interner.set_type_alias(alias_id, typ, generics);
     }
     errors
 }

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

@@ -844,6 +844,10 @@ impl<'interner> TypeChecker<'interner> {
                     })
                 }
             }
+            (Alias(alias, args), other) | (other, Alias(alias, args)) => {
+                let alias = alias.borrow().get_type(args);
+                self.comparator_operand_type_rules(&alias, other, op, span)
+            }
             (Integer(sign_x, bit_width_x), Integer(sign_y, bit_width_y)) => {
                 if sign_x != sign_y {
                     return Err(TypeCheckError::IntegerSignedness {
@@ -1141,6 +1145,10 @@ impl<'interner> TypeChecker<'interner> {
                     })
                 }
             }
+            (Alias(alias, args), other) | (other, Alias(alias, args)) => {
+                let alias = alias.borrow().get_type(args);
+                self.infix_operand_type_rules(&alias, op, other, span)
+            }
             (Integer(sign_x, bit_width_x), Integer(sign_y, bit_width_y)) => {
                 if sign_x != sign_y {
                     return Err(TypeCheckError::IntegerSignedness {

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

@@ -93,7 +93,7 @@ impl<'interner> TypeChecker<'interner> {
         match pattern {
             HirPattern::Identifier(ident) => self.interner.push_definition_type(ident.id, typ),
             HirPattern::Mutable(pattern, _) => self.bind_pattern(pattern, typ),
-            HirPattern::Tuple(fields, location) => match typ {
+            HirPattern::Tuple(fields, location) => match typ.follow_bindings() {
                 Type::Tuple(field_types) if field_types.len() == fields.len() => {
                     for (field, field_type) in fields.iter().zip(field_types) {
                         self.bind_pattern(field, field_type);
@@ -120,12 +120,12 @@ impl<'interner> TypeChecker<'interner> {
                     source: Source::Assignment,
                 });
 
-                if let Type::Struct(struct_type, generics) = struct_type {
+                if let Type::Struct(struct_type, generics) = struct_type.follow_bindings() {
                     let struct_type = struct_type.borrow();
 
                     for (field_name, field_pattern) in fields {
                         if let Some((type_field, _)) =
-                            struct_type.get_field(&field_name.0.contents, generics)
+                            struct_type.get_field(&field_name.0.contents, &generics)
                         {
                             self.bind_pattern(field_pattern, type_field);
                         }