Lazily resolve method aliases (#2201)

* Lazily resolve method aliases

* Ensure features work with aliases

lib/ruby_indexer/lib/ruby_indexer/entry.rb

@@ -399,6 +399,9 @@ def initialize(name, file_path, location, comments, owner)
       end
     end
 
+    # An unresolved method alias is an alias entry for which we aren't sure what the right hand side points to yet. For
+    # example, if we have `alias a b`, we create an unresolved alias for `a` because we aren't sure immediate what `b`
+    # is referring to
     class UnresolvedMethodAlias < Entry
       extend T::Sig
 
@@ -426,5 +429,45 @@ def initialize(new_name, old_name, owner, file_path, location, comments) # ruboc
         @owner = owner
       end
     end
+
+    # A method alias is a resolved alias entry that points to the exact method target it refers to
+    class MethodAlias < Entry
+      extend T::Sig
+
+      sig { returns(T.any(Member, MethodAlias)) }
+      attr_reader :target
+
+      sig { params(target: T.any(Member, MethodAlias), unresolved_alias: UnresolvedMethodAlias).void }
+      def initialize(target, unresolved_alias)
+        full_comments = ["Alias for #{target.name}\n"]
+        full_comments.concat(unresolved_alias.comments)
+        full_comments << "\n"
+        full_comments.concat(target.comments)
+
+        super(
+          unresolved_alias.new_name,
+          unresolved_alias.file_path,
+          unresolved_alias.location,
+          full_comments,
+        )
+
+        @target = target
+      end
+
+      sig { returns(T.nilable(Entry::Namespace)) }
+      def owner
+        @target.owner
+      end
+
+      sig { returns(T::Array[Parameter]) }
+      def parameters
+        @target.parameters
+      end
+
+      sig { returns(String) }
+      def decorated_parameters
+        @target.decorated_parameters
+      end
+    end
   end
 end

lib/ruby_indexer/lib/ruby_indexer/index.rb

@@ -140,13 +140,27 @@ def fuzzy_search(query)
       results.flat_map(&:first)
     end
 
-    sig { params(name: T.nilable(String), receiver_name: String).returns(T::Array[Entry]) }
+    sig do
+      params(
+        name: T.nilable(String),
+        receiver_name: String,
+      ).returns(T::Array[T.any(Entry::Member, Entry::MethodAlias)])
+    end
     def method_completion_candidates(name, receiver_name)
       ancestors = linearized_ancestors_of(receiver_name)
 
       candidates = name ? prefix_search(name).flatten : @entries.values.flatten
-      candidates.select! { |entry| entry.is_a?(Entry::Member) && ancestors.any?(entry.owner&.name) }
-      candidates
+      candidates.filter_map do |entry|
+        case entry
+        when Entry::Member, Entry::MethodAlias
+          entry if ancestors.any?(entry.owner&.name)
+        when Entry::UnresolvedMethodAlias
+          if ancestors.any?(entry.owner&.name)
+            resolved_alias = resolve_method_alias(entry, receiver_name)
+            resolved_alias if resolved_alias.is_a?(Entry::MethodAlias)
+          end
+        end
+      end
     end
 
     # Resolve a constant to its declaration based on its name and the nesting where the reference was found. Parameter
@@ -284,20 +298,32 @@ def follow_aliased_namespace(name, seen_names = [])
 
     # Attempts to find methods for a resolved fully qualified receiver name.
     # Returns `nil` if the method does not exist on that receiver
-    sig { params(method_name: String, receiver_name: String).returns(T.nilable(T::Array[Entry::Member])) }
+    sig do
+      params(
+        method_name: String,
+        receiver_name: String,
+      ).returns(T.nilable(T::Array[T.any(Entry::Member, Entry::MethodAlias)]))
+    end
     def resolve_method(method_name, receiver_name)
       method_entries = self[method_name]
-      ancestors = linearized_ancestors_of(receiver_name.delete_prefix("::"))
       return unless method_entries
 
+      ancestors = linearized_ancestors_of(receiver_name.delete_prefix("::"))
       ancestors.each do |ancestor|
-        found = method_entries.select do |entry|
-          next unless entry.is_a?(Entry::Member)
-
-          entry.owner&.name == ancestor
+        found = method_entries.filter_map do |entry|
+          case entry
+          when Entry::Member, Entry::MethodAlias
+            entry if entry.owner&.name == ancestor
+          when Entry::UnresolvedMethodAlias
+            # Resolve aliases lazily as we find them
+            if entry.owner&.name == ancestor
+              resolved_alias = resolve_method_alias(entry, receiver_name)
+              resolved_alias if resolved_alias.is_a?(Entry::MethodAlias)
+            end
+          end
         end
 
-        return T.cast(found, T::Array[Entry::Member]) if found.any?
+        return found if found.any?
       end
 
       nil
@@ -582,5 +608,26 @@ def direct_or_aliased_constant(full_name, seen_names)
     def search_top_level(name, seen_names)
       @entries[name]&.map { |e| e.is_a?(Entry::UnresolvedAlias) ? resolve_alias(e, seen_names) : e }
     end
+
+    # Attempt to resolve a given unresolved method alias. This method returns the resolved alias if we managed to
+    # identify the target or the same unresolved alias entry if we couldn't
+    sig do
+      params(
+        entry: Entry::UnresolvedMethodAlias,
+        receiver_name: String,
+      ).returns(T.any(Entry::MethodAlias, Entry::UnresolvedMethodAlias))
+    end
+    def resolve_method_alias(entry, receiver_name)
+      return entry if entry.new_name == entry.old_name
+
+      target_method_entries = resolve_method(entry.old_name, receiver_name)
+      return entry unless target_method_entries
+
+      resolved_alias = Entry::MethodAlias.new(T.must(target_method_entries.first), entry)
+      original_entries = T.must(@entries[entry.new_name])
+      original_entries.delete(entry)
+      original_entries << resolved_alias
+      resolved_alias
+    end
   end
 end

lib/ruby_indexer/test/index_test.rb

@@ -1351,5 +1351,126 @@ class << self
       assert_equal(1, results.length)
       assert_equal("Zwq", results.first.name)
     end
+
+    def test_resolving_method_aliases
+      index(<<~RUBY)
+        class Foo
+          def bar(a, b, c)
+          end
+
+          alias double_alias bar
+        end
+
+        class Bar < Foo
+          def hello(b); end
+
+          alias baz bar
+          alias_method :qux, :hello
+          alias double double_alias
+        end
+      RUBY
+
+      # baz
+      methods = @index.resolve_method("baz", "Bar")
+      refute_nil(methods)
+
+      entry = T.must(methods.first)
+      assert_kind_of(Entry::MethodAlias, entry)
+      assert_equal("bar", entry.target.name)
+      assert_equal("Foo", T.must(entry.target.owner).name)
+
+      # qux
+      methods = @index.resolve_method("qux", "Bar")
+      refute_nil(methods)
+
+      entry = T.must(methods.first)
+      assert_kind_of(Entry::MethodAlias, entry)
+      assert_equal("hello", entry.target.name)
+      assert_equal("Bar", T.must(entry.target.owner).name)
+
+      # double
+      methods = @index.resolve_method("double", "Bar")
+      refute_nil(methods)
+
+      entry = T.must(methods.first)
+      assert_kind_of(Entry::MethodAlias, entry)
+
+      target = entry.target
+      assert_equal("double_alias", target.name)
+      assert_kind_of(Entry::MethodAlias, target)
+      assert_equal("Foo", T.must(target.owner).name)
+
+      final_target = target.target
+      assert_equal("bar", final_target.name)
+      assert_kind_of(Entry::Method, final_target)
+      assert_equal("Foo", T.must(final_target.owner).name)
+    end
+
+    def test_resolving_circular_method_aliases
+      index(<<~RUBY)
+        class Foo
+          alias bar bar
+        end
+      RUBY
+
+      # It's not possible to resolve an alias that points to itself
+      methods = @index.resolve_method("bar", "Foo")
+      assert_nil(methods)
+
+      entry = T.must(@index["bar"].first)
+      assert_kind_of(Entry::UnresolvedMethodAlias, entry)
+    end
+
+    def test_unresolable_method_aliases
+      index(<<~RUBY)
+        class Foo
+          alias bar baz
+        end
+      RUBY
+
+      # `baz` does not exist, so resolving `bar` is not possible
+      methods = @index.resolve_method("bar", "Foo")
+      assert_nil(methods)
+
+      entry = T.must(@index["bar"].first)
+      assert_kind_of(Entry::UnresolvedMethodAlias, entry)
+    end
+
+    def test_only_aliases_for_the_right_owner_are_resolved
+      index(<<~RUBY)
+        class Foo
+          attr_reader :name
+          alias_method :decorated_name, :name
+        end
+
+        class Bar
+          alias_method :decorated_name, :to_s
+        end
+      RUBY
+
+      methods = @index.resolve_method("decorated_name", "Foo")
+      refute_nil(methods)
+
+      entry = T.must(methods.first)
+      assert_kind_of(Entry::MethodAlias, entry)
+
+      target = entry.target
+      assert_equal("name", target.name)
+      assert_kind_of(Entry::Accessor, target)
+      assert_equal("Foo", T.must(target.owner).name)
+
+      other_decorated_name = T.must(@index["decorated_name"].find { |e| e.is_a?(Entry::UnresolvedMethodAlias) })
+      assert_kind_of(Entry::UnresolvedMethodAlias, other_decorated_name)
+    end
+
+    def test_completion_does_not_include_unresolved_aliases
+      index(<<~RUBY)
+        class Foo
+          alias_method :bar, :missing
+        end
+      RUBY
+
+      assert_empty(@index.method_completion_candidates("bar", "Foo"))
+    end
   end
 end

lib/ruby_lsp/listeners/completion.rb

@@ -299,7 +299,7 @@ def complete_methods(node, name)
             text_edit: Interface::TextEdit.new(range: range, new_text: entry_name),
             kind: Constant::CompletionItemKind::METHOD,
             data: {
-              owner_name: T.cast(entry, RubyIndexer::Entry::Member).owner&.name,
+              owner_name: entry.owner&.name,
             },
           )
         end
@@ -309,7 +309,7 @@ def complete_methods(node, name)
 
       sig do
         params(
-          entry: RubyIndexer::Entry::Member,
+          entry: T.any(RubyIndexer::Entry::Member, RubyIndexer::Entry::MethodAlias),
           node: Prism::CallNode,
         ).returns(Interface::CompletionItem)
       end

test/requests/completion_test.rb

@@ -1048,6 +1048,31 @@ def self.yeah
     end
   end
 
+  def test_completion_for_aliased_methods
+    source = +<<~RUBY
+      class Parent
+        def bar(a); end
+      end
+
+      class Child < Parent
+        alias baz bar
+
+        def do_something
+          b
+        end
+      end
+    RUBY
+
+    with_server(source, stub_no_typechecker: true) do |server, uri|
+      server.process_message(id: 1, method: "textDocument/completion", params: {
+        textDocument: { uri: uri },
+        position: { line: 8, character: 5 },
+      })
+      result = server.pop_response.response
+      assert_equal(["bar", "baz"], result.map(&:label))
+    end
+  end
+
   private
 
   def with_file_structure(server, &block)

test/requests/definition_expectations_test.rb

@@ -675,6 +675,33 @@ def baz
     end
   end
 
+  def test_definition_for_aliased_methods
+    source = <<~RUBY
+      class Parent
+        def bar; end
+      end
+
+      class Child < Parent
+        alias baz bar
+
+        def do_something
+          baz
+        end
+      end
+    RUBY
+
+    with_server(source) do |server, uri|
+      server.process_message(
+        id: 1,
+        method: "textDocument/definition",
+        params: { textDocument: { uri: uri }, position: { character: 4, line: 8 } },
+      )
+      response = server.pop_response.response
+
+      assert_equal(5, response[0].range.start.line)
+    end
+  end
+
   private
 
   def create_definition_addon