Support go to definition on method declaration names

lib/ruby_lsp/listeners/definition.rb

@@ -22,6 +22,7 @@ def initialize(response_builder, global_state, language_id, uri, node_context, d
         dispatcher.register(
           self,
           :on_call_node_enter,
+          :on_def_node_enter,
           :on_block_argument_node_enter,
           :on_constant_read_node_enter,
           :on_constant_path_node_enter,
@@ -69,6 +70,20 @@ def on_call_node_enter(node)
         handle_method_definition(message, inferrer_receiver_type)
       end
 
+      # On the name of a method declaration, the declaration itself is the definition. Returning it lets editors fall
+      # back to their alternative action, such as showing references in VS Code
+      #: (Prism::DefNode node) -> void
+      def on_def_node_enter(node)
+        # Sorbet can handle go to definition for methods declared on typed true or higher
+        return if @sorbet_level.true_or_higher?
+
+        @response_builder << Interface::LocationLink.new(
+          target_uri: @uri.to_s,
+          target_range: range_from_location(node.location),
+          target_selection_range: range_from_location(node.name_loc),
+        )
+      end
+
       #: (Prism::StringNode node) -> void
       def on_string_node_enter(node)
         with_enclosing_call(node) do |enclosing_call, name|

lib/ruby_lsp/requests/definition.rb

@@ -24,6 +24,7 @@ def initialize(document, global_state, position, dispatcher, sorbet_level)
           char_position,
           node_types: [
             Prism::CallNode,
+            Prism::DefNode,
             Prism::ConstantReadNode,
             Prism::ConstantPathNode,
             Prism::BlockArgumentNode,
@@ -109,7 +110,8 @@ def position_outside_target?(position, target)
           Prism::InstanceVariableAndWriteNode,
           Prism::InstanceVariableOperatorWriteNode,
           Prism::InstanceVariableOrWriteNode,
-          Prism::InstanceVariableWriteNode
+          Prism::InstanceVariableWriteNode,
+          Prism::DefNode
 
           !covers_position?(target.name_loc, position)
         when Prism::CallNode

test/requests/definition_expectations_test.rb

@@ -1333,6 +1333,99 @@ def baz
     end
   end
 
+  def test_definition_on_method_name_returns_the_method_itself
+    source = <<~RUBY
+      class Foo
+        def bar(baz)
+          baz
+        end
+      end
+    RUBY
+
+    with_server(source) do |server, uri|
+      server.process_message(
+        id: 1,
+        method: "textDocument/definition",
+        params: { textDocument: { uri: uri }, position: { character: 7, line: 1 } },
+      )
+
+      response = server.pop_response.response
+      assert_equal(1, response.length)
+
+      link = response.first
+      assert_equal(uri.to_s, link.target_uri)
+
+      selection_range = link.target_selection_range
+      assert_equal([[1, 6], [1, 9]], [selection_range.start, selection_range.end].map { |p| [p.line, p.character] })
+      assert_equal([1, 3], [link.target_range.start.line, link.target_range.end.line])
+    end
+  end
+
+  def test_definition_on_singleton_method_name_returns_the_method_itself
+    source = <<~RUBY
+      class Foo
+        def self.bar; end
+      end
+    RUBY
+
+    with_server(source) do |server, uri|
+      server.process_message(
+        id: 1,
+        method: "textDocument/definition",
+        params: { textDocument: { uri: uri }, position: { character: 12, line: 1 } },
+      )
+
+      response = server.pop_response.response
+      assert_equal(1, response.length)
+
+      selection_range = response.first.target_selection_range
+      assert_equal([[1, 11], [1, 14]], [selection_range.start, selection_range.end].map { |p| [p.line, p.character] })
+    end
+  end
+
+  def test_definition_inside_method_declaration_outside_its_name_returns_nothing
+    source = <<~RUBY
+      class Foo
+        def bar(baz)
+          baz
+        end
+      end
+    RUBY
+
+    with_server(source) do |server, uri|
+      # The `def` keyword, the parameter and the local variable in the body
+      [{ character: 3, line: 1 }, { character: 11, line: 1 }, { character: 5, line: 2 }].each do |position|
+        server.process_message(
+          id: 1,
+          method: "textDocument/definition",
+          params: { textDocument: { uri: uri }, position: position },
+        )
+
+        assert_empty(server.pop_response.response)
+      end
+    end
+  end
+
+  def test_definition_on_method_name_is_disabled_for_typed_true
+    source = <<~RUBY
+      # typed: true
+      class Foo
+        def bar
+        end
+      end
+    RUBY
+
+    with_server(source) do |server, uri|
+      server.process_message(
+        id: 1,
+        method: "textDocument/definition",
+        params: { textDocument: { uri: uri }, position: { character: 7, line: 2 } },
+      )
+
+      assert_empty(server.pop_response.response)
+    end
+  end
+
   def test_definition_for_instance_variables_is_disabled_on_typed_strict
     source = <<~RUBY
       # typed: strict