Infer attr_accessor writer signatures

lib/tapioca/gem/listeners/methods.rb

@@ -69,6 +69,7 @@ def compile_method(tree, symbol_name, constant, method, visibility = RBI::Public
 
           begin
             signature = signature_of!(method)
+            signature ||= inferred_attr_writer_signature(method, constant)
             method = signature.method if signature #: UnboundMethod
 
             case @pipeline.method_definition_in_gem(method.name, constant)
@@ -192,6 +193,66 @@ def method_names_by_visibility(mod)
           }
         end
 
+        #: (UnboundMethod method, Module[top] constant) -> untyped
+        def inferred_attr_writer_signature(method, constant)
+          reader_method = attr_reader_for_writer(method, constant)
+          return unless reader_method
+
+          reader_signature = signature_of(reader_method)
+          return unless reader_signature
+
+          build_attr_writer_signature(method, reader_method, reader_signature)
+        end
+
+        #: (UnboundMethod method, Module[top] constant) -> UnboundMethod?
+        def attr_reader_for_writer(method, constant)
+          method_name = method.name.to_s
+          return unless method_name.end_with?("=")
+          return unless method.parameters == [[:req]]
+
+          reader_method = T.let(constant.instance_method(method_name.delete_suffix("=").to_sym), UnboundMethod)
+          reader_method = original_method(reader_method)
+          return unless same_source_location?(method, reader_method)
+          return unless method_owned_by_constant?(reader_method, constant)
+
+          reader_method
+        rescue NameError
+          nil
+        end
+
+        #: (UnboundMethod writer_method, UnboundMethod reader_method, untyped reader_signature) -> untyped
+        def build_attr_writer_signature(writer_method, reader_method, reader_signature)
+          return unless reader_signature.arg_types.empty?
+          return unless reader_signature.kwarg_types.empty?
+          return if reader_signature.rest_type
+          return if reader_signature.keyrest_type
+          return if reader_signature.block_type
+
+          T::Private::Methods::Signature.new(
+            method: writer_method,
+            method_name: writer_method.name,
+            raw_arg_types: { reader_method.name => reader_signature.return_type },
+            raw_return_type: reader_signature.return_type,
+            bind: nil,
+            mode: reader_signature.mode,
+            check_level: reader_signature.check_level,
+            on_failure: reader_signature.on_failure,
+            override_allow_incompatible: reader_signature.override_allow_incompatible,
+            defined_raw: reader_signature.defined_raw,
+          )
+        end
+
+        #: (UnboundMethod method) -> UnboundMethod
+        def original_method(method)
+          T.let(signature_of(method)&.method || method, UnboundMethod)
+        end
+
+        #: (UnboundMethod method, UnboundMethod other_method) -> bool
+        def same_source_location?(method, other_method)
+          source_location = method.source_location
+          !!source_location && source_location == other_method.source_location
+        end
+
         #: (Module[top] constant, String method_name) -> bool
         def struct_method?(constant, method_name)
           return false unless T::Props::ClassMethods === constant

sorbet/rbi/shims/sorbet.rbi

@@ -60,7 +60,9 @@ module T::Private
       def finalized=(finalized); end
     end
 
-    class Signature; end
+    class Signature
+      def initialize(method:, method_name:, raw_arg_types:, raw_return_type:, bind:, mode:, check_level:, on_failure:, parameters: T.unsafe(nil), override_allow_incompatible: T.unsafe(nil), defined_raw: T.unsafe(nil)); end
+    end
   end
 
   module DeclState

spec/tapioca/gem/pipeline_spec.rb

@@ -1933,6 +1933,83 @@ def foo; end
       assert_equal(output, compile)
     end
 
+    it "compiles attr_reader/attr_writer/attr_accessor with signatures" do
+      add_ruby_file("bar.rb", <<~RUBY)
+        class Bar
+          sig { returns(String) }
+          attr_reader(:a)
+
+          sig { returns(Integer) }
+          attr_accessor(:b)
+
+          sig { params(c: Symbol).void }
+          attr_writer(:c)
+        end
+      RUBY
+
+      output = template(<<~RBI)
+        class Bar
+          sig { returns(::String) }
+          def a; end
+
+          sig { returns(::Integer) }
+          def b; end
+
+          sig { params(b: ::Integer).returns(::Integer) }
+          def b=(b); end
+
+          sig { params(c: ::Symbol).void }
+          def c=(c); end
+        end
+      RBI
+
+      assert_equal(output, compile)
+    end
+
+    it "does not infer attr_accessor writer signatures for separate attr_readers and attr_writers" do
+      add_ruby_file("bar.rb", <<~RUBY)
+        class Bar
+          sig { returns(Integer) }
+          attr_reader(:foo)
+
+          attr_writer(:foo)
+        end
+      RUBY
+
+      output = template(<<~RBI)
+        class Bar
+          sig { returns(::Integer) }
+          def foo; end
+
+          def foo=(_arg0); end
+        end
+      RBI
+
+      assert_equal(output, compile)
+    end
+
+    it "does not infer attr_accessor writer signatures for manually defined writers" do
+      add_ruby_file("bar.rb", <<~RUBY)
+        class Bar
+          sig { returns(Integer) }
+          def foo; end
+
+          def foo=(foo); end
+        end
+      RUBY
+
+      output = template(<<~RBI)
+        class Bar
+          sig { returns(::Integer) }
+          def foo; end
+
+          def foo=(foo); end
+        end
+      RBI
+
+      assert_equal(output, compile)
+    end
+
     it "ignores methods with invalid names" do
       add_ruby_file("bar.rb", <<~RUBY)
         class Bar
@@ -3407,7 +3484,8 @@ def baz=(baz); end
           sig { returns(T::Array[::Integer]) }
           def foo; end
 
-          def foo=(_arg0); end
+          sig { params(foo: T::Array[::Integer]).returns(T::Array[::Integer]) }
+          def foo=(foo); end
 
           sig { override.returns(::Integer) }
           def something; end
@@ -4703,7 +4781,9 @@ def bar(a, b:); end
           sig { returns(::String) }
           def foo; end
 
-          def foo=(_arg0); end
+          sig { params(foo: ::String).returns(::String) }
+          def foo=(foo); end
+
           def qux; end
 
           class << self