diff --git a/examples/callbacks/src/lib.rs b/examples/callbacks/src/lib.rs
index dac5653d1b..9e30bf1d86 100644
--- a/examples/callbacks/src/lib.rs
+++ b/examples/callbacks/src/lib.rs
@@ -21,6 +21,7 @@ impl From<uniffi::UnexpectedUniFFICallbackError> for TelephoneError {
 }
 
 // SIM cards.
+#[uniffi::trait_interface]
 pub trait SimCard: Send + Sync {
     fn name(&self) -> String;
 }
@@ -39,6 +40,7 @@ fn get_sim_cards() -> Vec<Arc<dyn SimCard>> {
 }
 
 // The call-answer, callback interface.
+#[uniffi::trait_interface]
 pub trait CallAnswerer {
     fn answer(&self) -> Result<String, TelephoneError>;
 }
diff --git a/examples/sprites/tests/bindings/test_sprites.kts b/examples/sprites/tests/bindings/test_sprites.kts
index 42451f28dd..ddd983351c 100644
--- a/examples/sprites/tests/bindings/test_sprites.kts
+++ b/examples/sprites/tests/bindings/test_sprites.kts
@@ -16,7 +16,7 @@ s.destroy()
 try {
     s.moveBy(Vector(0.0, 0.0))
     assert(false) { "Should not be able to call anything after `destroy`" }
-} catch(e: IllegalStateException) {
+} catch(e: InternalException) {
     assert(true)
 }
 
diff --git a/examples/traits/src/lib.rs b/examples/traits/src/lib.rs
index 9d84bdae7f..51d81d83d8 100644
--- a/examples/traits/src/lib.rs
+++ b/examples/traits/src/lib.rs
@@ -13,6 +13,7 @@ fn press(button: Arc<dyn Button>) -> Arc<dyn Button> {
     button
 }
 
+#[uniffi::trait_interface]
 pub trait Button: Send + Sync {
     fn name(&self) -> String;
 }
diff --git a/fixtures/coverall/tests/bindings/test_coverall.py b/fixtures/coverall/tests/bindings/test_coverall.py
index ec6056b926..b4e27a450e 100644
--- a/fixtures/coverall/tests/bindings/test_coverall.py
+++ b/fixtures/coverall/tests/bindings/test_coverall.py
@@ -242,16 +242,13 @@ def test_return_objects(self):
         coveralls = None
         self.assertEqual(get_num_alive(), 0)
 
-    # FIXME: since we're now inc-refing the handle, Python will leak objects if another lower fails.
-    # For now, let's disable this test.
-    #
-    # def test_bad_objects(self):
-    #     coveralls = Coveralls("test_bad_objects")
-    #     patch = Patch(Color.RED)
-    #     # `coveralls.take_other` wants `Coveralls` not `Patch`
-    #     with self.assertRaisesRegex(TypeError, "Coveralls.*Patch"):
-    #         coveralls.take_other(patch)
-    #
+    def test_bad_objects(self):
+        coveralls = Coveralls("test_bad_objects")
+        patch = Patch(Color.RED)
+        # `coveralls.take_other` wants `Coveralls` not `Patch`
+        with self.assertRaisesRegex(TypeError, "Coveralls.*Patch"):
+            coveralls.take_other(patch)
+
     def test_dict_with_defaults(self):
         """ This does not call Rust code. """
 
diff --git a/fixtures/coverall/tests/bindings/test_coverall.rb b/fixtures/coverall/tests/bindings/test_coverall.rb
index c74a2f032f..4fe1140419 100644
--- a/fixtures/coverall/tests/bindings/test_coverall.rb
+++ b/fixtures/coverall/tests/bindings/test_coverall.rb
@@ -244,17 +244,14 @@ def test_return_objects
     assert_equal Coverall.get_num_alive(), 0
   end
 
-  # FIXME: since we're now inc-refing the handle, Ruby will leak objects if another lower fails.
-  # For now, let's disable this test.
-  #
-  # def test_bad_objects
-  #   coveralls = Coverall::Coveralls.new "test_bad_objects"
-  #   patch = Coverall::Patch.new Coverall::Color::RED
-  #   # `coveralls.take_other` wants `Coveralls` not `Patch`
-  #   assert_raise_message /Expected a Coveralls instance, got.*Patch/ do
-  #     coveralls.take_other patch
-  #   end
-  # end
+  def test_bad_objects
+    coveralls = Coverall::Coveralls.new "test_bad_objects"
+    patch = Coverall::Patch.new Coverall::Color::RED
+    # `coveralls.take_other` wants `Coveralls` not `Patch`
+    assert_raise_message /Expected a Coveralls instance, got.*Patch/ do
+      coveralls.take_other patch
+    end
+  end
 
   def test_bytes
     coveralls = Coverall::Coveralls.new "test_bytes"
diff --git a/fixtures/reexport-scaffolding-macro/src/lib.rs b/fixtures/reexport-scaffolding-macro/src/lib.rs
index 6bd04f2ccd..860fe2d019 100644
--- a/fixtures/reexport-scaffolding-macro/src/lib.rs
+++ b/fixtures/reexport-scaffolding-macro/src/lib.rs
@@ -6,9 +6,10 @@ mod tests {
     use cargo_metadata::Message;
     use libloading::{Library, Symbol};
     use std::ffi::CString;
-    use std::os::raw::c_void;
     use std::process::{Command, Stdio};
-    use uniffi::{FfiConverter, ForeignCallback, RustBuffer, RustCallStatus, RustCallStatusCode};
+    use uniffi::{
+        FfiConverter, ForeignCallback, Handle, RustBuffer, RustCallStatus, RustCallStatusCode,
+    };
     use uniffi_bindgen::ComponentInterface;
 
     struct UniFfiTag;
@@ -149,26 +150,27 @@ mod tests {
                 .ffi_func()
                 .name(),
         );
-        let coveralls_new: Symbol<
-            unsafe extern "C" fn(RustBuffer, &mut RustCallStatus) -> *const c_void,
-        > = get_symbol(
-            &library,
-            object_def.primary_constructor().unwrap().ffi_func().name(),
-        );
+        let coveralls_new: Symbol<unsafe extern "C" fn(RustBuffer, &mut RustCallStatus) -> Handle> =
+            get_symbol(
+                &library,
+                object_def.primary_constructor().unwrap().ffi_func().name(),
+            );
         let coveralls_get_name: Symbol<
-            unsafe extern "C" fn(*const c_void, &mut RustCallStatus) -> RustBuffer,
+            unsafe extern "C" fn(Handle, &mut RustCallStatus) -> RustBuffer,
         > = get_symbol(
             &library,
             object_def.get_method("get_name").ffi_func().name(),
         );
-        let coveralls_free: Symbol<unsafe extern "C" fn(*const c_void, &mut RustCallStatus) -> ()> =
+        let coveralls_inc_ref: Symbol<unsafe extern "C" fn(Handle, &mut RustCallStatus) -> ()> =
+            get_symbol(&library, object_def.ffi_object_inc_ref().name());
+        let coveralls_free: Symbol<unsafe extern "C" fn(Handle, &mut RustCallStatus) -> ()> =
             get_symbol(&library, object_def.ffi_object_free().name());
 
         let num_alive = unsafe { get_num_alive(&mut call_status) };
         assert_eq!(call_status.code, RustCallStatusCode::Success);
         assert_eq!(num_alive, 0);
 
-        let obj_id = unsafe {
+        let obj_handle = unsafe {
             coveralls_new(
                 <String as FfiConverter<UniFfiTag>>::lower("TestName".into()),
                 &mut call_status,
@@ -176,7 +178,10 @@ mod tests {
         };
         assert_eq!(call_status.code, RustCallStatusCode::Success);
 
-        let name_buf = unsafe { coveralls_get_name(obj_id, &mut call_status) };
+        unsafe { coveralls_inc_ref(obj_handle, &mut call_status) };
+        assert_eq!(call_status.code, RustCallStatusCode::Success);
+
+        let name_buf = unsafe { coveralls_get_name(obj_handle, &mut call_status) };
         assert_eq!(call_status.code, RustCallStatusCode::Success);
         assert_eq!(
             <String as FfiConverter<UniFfiTag>>::try_lift(name_buf).unwrap(),
@@ -187,7 +192,7 @@ mod tests {
         assert_eq!(call_status.code, RustCallStatusCode::Success);
         assert_eq!(num_alive, 1);
 
-        unsafe { coveralls_free(obj_id, &mut call_status) };
+        unsafe { coveralls_free(obj_handle, &mut call_status) };
         assert_eq!(call_status.code, RustCallStatusCode::Success);
 
         let num_alive = unsafe { get_num_alive(&mut call_status) };
diff --git a/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.rs b/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.rs
index 1bcd3a4237..6671a9624a 100644
--- a/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.rs
+++ b/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.rs
@@ -4,6 +4,7 @@ uniffi_macros::generate_and_include_scaffolding!("../../../../fixtures/uitests/s
 fn main() { /* empty main required by `trybuild` */}
 
 // This will fail to compile, because the trait is not explicit Send+Sync
+#[uniffi::trait_interface]
 pub trait Trait {
 }
 
diff --git a/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.stderr b/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.stderr
index a8e7dc90a1..3925745647 100644
--- a/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.stderr
+++ b/fixtures/uitests/tests/ui/interface_trait_not_sync_and_send.stderr
@@ -27,9 +27,9 @@ note: required by a bound in `FfiConverterArc`
   = note: this error originates in the attribute macro `::uniffi::export_for_udl` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: `(dyn ProcMacroTrait + 'static)` cannot be shared between threads safely
-  --> tests/ui/interface_trait_not_sync_and_send.rs:11:1
+  --> tests/ui/interface_trait_not_sync_and_send.rs:12:1
    |
-11 | #[uniffi::export]
+12 | #[uniffi::export]
    | ^^^^^^^^^^^^^^^^^ `(dyn ProcMacroTrait + 'static)` cannot be shared between threads safely
    |
    = help: the trait `Sync` is not implemented for `(dyn ProcMacroTrait + 'static)`
@@ -41,9 +41,9 @@ note: required by a bound in `FfiConverterArc`
    = note: this error originates in the attribute macro `uniffi::export` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error[E0277]: `(dyn ProcMacroTrait + 'static)` cannot be sent between threads safely
-  --> tests/ui/interface_trait_not_sync_and_send.rs:11:1
+  --> tests/ui/interface_trait_not_sync_and_send.rs:12:1
    |
-11 | #[uniffi::export]
+12 | #[uniffi::export]
    | ^^^^^^^^^^^^^^^^^ `(dyn ProcMacroTrait + 'static)` cannot be sent between threads safely
    |
    = help: the trait `Send` is not implemented for `(dyn ProcMacroTrait + 'static)`
diff --git a/uniffi_bindgen/src/bindings/python/gen_python/mod.rs b/uniffi_bindgen/src/bindings/python/gen_python/mod.rs
index 0d6545fc21..700dfc3ece 100644
--- a/uniffi_bindgen/src/bindings/python/gen_python/mod.rs
+++ b/uniffi_bindgen/src/bindings/python/gen_python/mod.rs
@@ -413,6 +413,10 @@ pub mod filters {
         Ok(format!("{}.lift", ffi_converter_name(as_ct)?))
     }
 
+    pub fn check_fn(as_ct: &impl AsCodeType) -> Result<String, askama::Error> {
+        Ok(format!("{}.check", ffi_converter_name(as_ct)?))
+    }
+
     pub fn lower_fn(as_ct: &impl AsCodeType) -> Result<String, askama::Error> {
         Ok(format!("{}.lower", ffi_converter_name(as_ct)?))
     }
diff --git a/uniffi_bindgen/src/bindings/python/templates/BooleanHelper.py b/uniffi_bindgen/src/bindings/python/templates/BooleanHelper.py
index 6775e9e132..52ca6a2646 100644
--- a/uniffi_bindgen/src/bindings/python/templates/BooleanHelper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/BooleanHelper.py
@@ -1,16 +1,20 @@
-class _UniffiConverterBool(_UniffiConverterPrimitive):
+class _UniffiConverterBool:
     @classmethod
     def check(cls, value):
-        return not not value
+        pass
+
+    @classmethod
+    def lower(cls, value):
+        return 1 if value else 0
+
+    @staticmethod
+    def lift(value):
+        return value != 0
 
     @classmethod
     def read(cls, buf):
         return cls.lift(buf.read_u8())
 
     @classmethod
-    def write_unchecked(cls, value, buf):
+    def write(cls, value, buf):
         buf.write_u8(value)
-
-    @staticmethod
-    def lift(value):
-        return value != 0
diff --git a/uniffi_bindgen/src/bindings/python/templates/BytesHelper.py b/uniffi_bindgen/src/bindings/python/templates/BytesHelper.py
index 196b5b29fa..f649b5e41a 100644
--- a/uniffi_bindgen/src/bindings/python/templates/BytesHelper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/BytesHelper.py
@@ -7,10 +7,13 @@ def read(buf):
         return buf.read(size)
 
     @staticmethod
-    def write(value, buf):
+    def check(value):
         try:
             memoryview(value)
         except TypeError:
             raise TypeError("a bytes-like object is required, not {!r}".format(type(value).__name__))
+
+    @staticmethod
+    def write(value, buf):
         buf.write_i32(len(value))
         buf.write(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py b/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py
index 2d98eba5db..6257407c22 100644
--- a/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py
+++ b/uniffi_bindgen/src/bindings/python/templates/CallbackInterfaceRuntime.py
@@ -24,6 +24,10 @@ def read(cls, buf):
         handle = buf.read_u64()
         cls.lift(handle)
 
+    @classmethod
+    def check(cls, cb):
+        pass
+
     @classmethod
     def lower(cls, cb):
         handle = cls._slab.insert(cb)
diff --git a/uniffi_bindgen/src/bindings/python/templates/CustomType.py b/uniffi_bindgen/src/bindings/python/templates/CustomType.py
index 5be6155b84..f7e626c163 100644
--- a/uniffi_bindgen/src/bindings/python/templates/CustomType.py
+++ b/uniffi_bindgen/src/bindings/python/templates/CustomType.py
@@ -17,6 +17,10 @@ def read(buf):
     def lift(value):
         return {{ builtin|ffi_converter_name }}.lift(value)
 
+    @staticmethod
+    def check(value):
+        return {{ builtin|ffi_converter_name }}.check(value)
+
     @staticmethod
     def lower(value):
         return {{ builtin|ffi_converter_name }}.lower(value)
@@ -51,6 +55,11 @@ def lift(value):
         builtin_value = {{ builtin|lift_fn }}(value)
         return {{ config.into_custom.render("builtin_value") }}
 
+    @staticmethod
+    def check(value):
+        builtin_value = {{ config.from_custom.render("value") }}
+        return {{ builtin|check_fn }}(builtin_value)
+
     @staticmethod
     def lower(value):
         builtin_value = {{ config.from_custom.render("value") }}
diff --git a/uniffi_bindgen/src/bindings/python/templates/DurationHelper.py b/uniffi_bindgen/src/bindings/python/templates/DurationHelper.py
index 10974e009d..3bb06e1099 100644
--- a/uniffi_bindgen/src/bindings/python/templates/DurationHelper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/DurationHelper.py
@@ -12,10 +12,14 @@ def read(buf):
         return datetime.timedelta(seconds=seconds, microseconds=microseconds)
 
     @staticmethod
-    def write(value, buf):
+    def check(value):
         seconds = value.seconds + value.days * 24 * 3600
-        nanoseconds = value.microseconds * 1000
         if seconds < 0:
             raise ValueError("Invalid duration, must be non-negative")
+
+    @staticmethod
+    def write(value, buf):
+        seconds = value.seconds + value.days * 24 * 3600
+        nanoseconds = value.microseconds * 1000
         buf.write_i64(seconds)
         buf.write_u32(nanoseconds)
diff --git a/uniffi_bindgen/src/bindings/python/templates/EnumTemplate.py b/uniffi_bindgen/src/bindings/python/templates/EnumTemplate.py
index 84d089baf9..2a347f873e 100644
--- a/uniffi_bindgen/src/bindings/python/templates/EnumTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/EnumTemplate.py
@@ -81,6 +81,25 @@ def read(buf):
         {%- endfor %}
         raise InternalError("Raw enum value doesn't match any cases")
 
+    @staticmethod
+    def check(value):
+        {%- if e.variants().is_empty() %}
+        pass
+        {%- else %}
+        {%- for variant in e.variants() %}
+        {%- if e.is_flat() %}
+        if value == {{ type_name }}.{{ variant.name()|enum_variant_py }}:
+        {%- else %}
+        if value.is_{{ variant.name()|var_name }}():
+        {%- endif %}
+            {%- for field in variant.fields() %}
+            {{ field|check_fn }}(value.{{ field.name()|var_name }})
+            {%- endfor %}
+            return
+        {%- endfor %}
+        {%- endif %}
+
+    @staticmethod
     def write(value, buf):
         {%- for variant in e.variants() %}
         {%- if e.is_flat() %}
diff --git a/uniffi_bindgen/src/bindings/python/templates/ErrorTemplate.py b/uniffi_bindgen/src/bindings/python/templates/ErrorTemplate.py
index 26a1e6452a..b4c9849626 100644
--- a/uniffi_bindgen/src/bindings/python/templates/ErrorTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/ErrorTemplate.py
@@ -59,6 +59,20 @@ def read(buf):
         {%- endfor %}
         raise InternalError("Raw enum value doesn't match any cases")
 
+    @staticmethod
+    def check(value):
+        {%- if e.variants().is_empty() %}
+        pass
+        {%- else %}
+        {%- for variant in e.variants() %}
+        if isinstance(value, {{ type_name }}.{{ variant.name()|class_name }}):
+            {%- for field in variant.fields() %}
+            {{ field|check_fn }}(value.{{ field.name()|var_name }})
+            {%- endfor %}
+            return
+        {%- endfor %}
+        {%- endif %}
+
     @staticmethod
     def write(value, buf):
         {%- for variant in e.variants() %}
diff --git a/uniffi_bindgen/src/bindings/python/templates/Float32Helper.py b/uniffi_bindgen/src/bindings/python/templates/Float32Helper.py
index a52107a638..49a1a7286e 100644
--- a/uniffi_bindgen/src/bindings/python/templates/Float32Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/Float32Helper.py
@@ -4,5 +4,5 @@ def read(buf):
         return buf.read_float()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_float(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/Float64Helper.py b/uniffi_bindgen/src/bindings/python/templates/Float64Helper.py
index 772f5080e9..e2084c7b13 100644
--- a/uniffi_bindgen/src/bindings/python/templates/Float64Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/Float64Helper.py
@@ -4,5 +4,5 @@ def read(buf):
         return buf.read_double()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_double(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/ForeignExecutorTemplate.py b/uniffi_bindgen/src/bindings/python/templates/ForeignExecutorTemplate.py
index 9da7bad33b..166fa95b42 100644
--- a/uniffi_bindgen/src/bindings/python/templates/ForeignExecutorTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/ForeignExecutorTemplate.py
@@ -12,9 +12,12 @@ class {{ ffi_converter_name }}:
     _slab = UniffiSlab()
 
     @classmethod
-    def lower(cls, eventloop):
+    def check(cls, eventloop):
         if not isinstance(eventloop, asyncio.BaseEventLoop):
             raise TypeError("_uniffi_executor_callback: Expected EventLoop instance")
+
+    @classmethod
+    def lower(cls, eventloop):
         return cls._slab.insert(eventloop)
 
     @classmethod
diff --git a/uniffi_bindgen/src/bindings/python/templates/Int16Helper.py b/uniffi_bindgen/src/bindings/python/templates/Int16Helper.py
index 99f19dc1c0..befa563384 100644
--- a/uniffi_bindgen/src/bindings/python/templates/Int16Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/Int16Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_i16()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_i16(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/Int32Helper.py b/uniffi_bindgen/src/bindings/python/templates/Int32Helper.py
index 3b142c8749..70644f6717 100644
--- a/uniffi_bindgen/src/bindings/python/templates/Int32Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/Int32Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_i32()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_i32(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/Int64Helper.py b/uniffi_bindgen/src/bindings/python/templates/Int64Helper.py
index 6e94379cbf..232f127bd6 100644
--- a/uniffi_bindgen/src/bindings/python/templates/Int64Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/Int64Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_i64()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_i64(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/Int8Helper.py b/uniffi_bindgen/src/bindings/python/templates/Int8Helper.py
index 732530e3cb..c1de1625e7 100644
--- a/uniffi_bindgen/src/bindings/python/templates/Int8Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/Int8Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_i8()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_i8(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/MapTemplate.py b/uniffi_bindgen/src/bindings/python/templates/MapTemplate.py
index 387227ed09..6f413bb7e6 100644
--- a/uniffi_bindgen/src/bindings/python/templates/MapTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/MapTemplate.py
@@ -2,6 +2,12 @@
 {%- let value_ffi_converter = value_type|ffi_converter_name %}
 
 class {{ ffi_converter_name }}(_UniffiConverterRustBuffer):
+    @classmethod
+    def check(cls, items):
+        for (key, value) in items.items():
+            {{ key_ffi_converter }}.check(key)
+            {{ value_ffi_converter }}.check(value)
+
     @classmethod
     def write(cls, items, buf):
         buf.write_i32(len(items))
diff --git a/uniffi_bindgen/src/bindings/python/templates/ObjectTemplate.py b/uniffi_bindgen/src/bindings/python/templates/ObjectTemplate.py
index 92e71a3dc2..e23970eaf6 100644
--- a/uniffi_bindgen/src/bindings/python/templates/ObjectTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/ObjectTemplate.py
@@ -39,7 +39,6 @@ def _make_instance_(cls, handle):
     @classmethod
     def {{ cons.name()|fn_name }}(cls, {% call py::arg_list_decl(cons) %}):
         {%- call py::setup_args_extra_indent(cons) %}
-        # Call the (fallible) function before creating any half-baked object instances.
         uniffi_handle = {% call py::to_ffi_call(cons) %}
         return cls._make_instance_(uniffi_handle)
 {% endfor %}
@@ -94,6 +93,15 @@ def lift(value: UniffiHandle):
         return {{ impl_name }}._make_instance_(value)
         {%- endif %}
 
+    @staticmethod
+    def check(value: {{ type_name }}):
+        {%- if obj.is_trait_interface() %}
+        pass
+        {%- else %}
+        if not isinstance(value, {{ impl_name }}):
+            raise TypeError("Expected {{ impl_name }} instance, {} found".format(type(value).__name__))
+        {%- endif %}
+
     @staticmethod
     def lower(value: {{ type_name }}):
         {%- if obj.is_trait_interface() %}
@@ -105,8 +113,6 @@ def lower(value: {{ type_name }}):
         else:
             return {{ ffi_converter_name }}._slab.insert(value)
         {%- else %}
-        if not isinstance(value, {{ impl_name }}):
-            raise TypeError("Expected {{ impl_name }} instance, {} found".format(type(value).__name__))
         return value._uniffi_clone_handle()
         {%- endif %}
 
diff --git a/uniffi_bindgen/src/bindings/python/templates/OptionalTemplate.py b/uniffi_bindgen/src/bindings/python/templates/OptionalTemplate.py
index e406c51d49..14b7a0f3af 100644
--- a/uniffi_bindgen/src/bindings/python/templates/OptionalTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/OptionalTemplate.py
@@ -1,6 +1,11 @@
 {%- let inner_ffi_converter = inner_type|ffi_converter_name %}
 
 class {{ ffi_converter_name }}(_UniffiConverterRustBuffer):
+    @classmethod
+    def check(cls, value):
+        if value is not None:
+            {{ inner_ffi_converter }}.check(value)
+
     @classmethod
     def write(cls, value, buf):
         if value is None:
diff --git a/uniffi_bindgen/src/bindings/python/templates/RecordTemplate.py b/uniffi_bindgen/src/bindings/python/templates/RecordTemplate.py
index 99a30e120f..b2420b6671 100644
--- a/uniffi_bindgen/src/bindings/python/templates/RecordTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/RecordTemplate.py
@@ -42,6 +42,16 @@ def read(buf):
             {%- endfor %}
         )
 
+    @staticmethod
+    def check(value):
+        {%- if rec.fields().is_empty() %}
+        pass
+        {%- else %}
+        {%- for field in rec.fields() %}
+        {{ field|check_fn }}(value.{{ field.name()|var_name }})
+        {%- endfor %}
+        {%- endif %}
+
     @staticmethod
     def write(value, buf):
         {%- for field in rec.fields() %}
diff --git a/uniffi_bindgen/src/bindings/python/templates/RustBufferHelper.py b/uniffi_bindgen/src/bindings/python/templates/RustBufferHelper.py
index daabd5b4b9..88cc96b6fb 100644
--- a/uniffi_bindgen/src/bindings/python/templates/RustBufferHelper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/RustBufferHelper.py
@@ -1,25 +1,13 @@
 # Types conforming to `_UniffiConverterPrimitive` pass themselves directly over the FFI.
 class _UniffiConverterPrimitive:
-    @classmethod
-    def check(cls, value):
-        return value
-
     @classmethod
     def lift(cls, value):
         return value
-
+ 
     @classmethod
     def lower(cls, value):
-        return cls.lowerUnchecked(cls.check(value))
-
-    @classmethod
-    def lowerUnchecked(cls, value):
         return value
 
-    @classmethod
-    def write(cls, value, buf):
-        cls.write_unchecked(cls.check(value), buf)
-
 class _UniffiConverterPrimitiveInt(_UniffiConverterPrimitive):
     @classmethod
     def check(cls, value):
@@ -31,7 +19,6 @@ def check(cls, value):
             raise TypeError("__index__ returned non-int (type {})".format(type(value).__name__))
         if not cls.VALUE_MIN <= value < cls.VALUE_MAX:
             raise ValueError("{} requires {} <= value < {}".format(cls.CLASS_NAME, cls.VALUE_MIN, cls.VALUE_MAX))
-        return super().check(value)
 
 class _UniffiConverterPrimitiveFloat(_UniffiConverterPrimitive):
     @classmethod
@@ -42,7 +29,6 @@ def check(cls, value):
             raise TypeError("must be real number, not {}".format(type(value).__name__))
         if not isinstance(value, float):
             raise TypeError("__float__ returned non-float (type {})".format(type(value).__name__))
-        return super().check(value)
 
 # Helper class for wrapper types that will always go through a _UniffiRustBuffer.
 # Classes should inherit from this and implement the `read` and `write` static methods.
diff --git a/uniffi_bindgen/src/bindings/python/templates/SequenceTemplate.py b/uniffi_bindgen/src/bindings/python/templates/SequenceTemplate.py
index 3c9f5a4596..c69a36f178 100644
--- a/uniffi_bindgen/src/bindings/python/templates/SequenceTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/SequenceTemplate.py
@@ -1,6 +1,11 @@
 {%- let inner_ffi_converter = inner_type|ffi_converter_name %}
 
 class {{ ffi_converter_name}}(_UniffiConverterRustBuffer):
+    @classmethod
+    def check(cls, value):
+        for item in value:
+            {{ inner_ffi_converter }}.check(item)
+
     @classmethod
     def write(cls, value, buf):
         items = len(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/StringHelper.py b/uniffi_bindgen/src/bindings/python/templates/StringHelper.py
index 40890b6abc..80a014c888 100644
--- a/uniffi_bindgen/src/bindings/python/templates/StringHelper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/StringHelper.py
@@ -15,7 +15,6 @@ def read(buf):
 
     @staticmethod
     def write(value, buf):
-        value = _UniffiConverterString.check(value)
         utf8_bytes = value.encode("utf-8")
         buf.write_i32(len(utf8_bytes))
         buf.write(utf8_bytes)
@@ -27,7 +26,6 @@ def lift(buf):
 
     @staticmethod
     def lower(value):
-        value = _UniffiConverterString.check(value)
         with _UniffiRustBuffer.alloc_with_builder() as builder:
             builder.write(value.encode("utf-8"))
             return builder.finalize()
diff --git a/uniffi_bindgen/src/bindings/python/templates/TimestampHelper.py b/uniffi_bindgen/src/bindings/python/templates/TimestampHelper.py
index 8402f6095d..9acbb2fee9 100644
--- a/uniffi_bindgen/src/bindings/python/templates/TimestampHelper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/TimestampHelper.py
@@ -17,6 +17,10 @@ def read(buf):
         else:
             return datetime.datetime.fromtimestamp(0, tz=datetime.timezone.utc) - datetime.timedelta(seconds=-seconds, microseconds=microseconds)
 
+    @staticmethod
+    def check(value):
+        pass
+
     @staticmethod
     def write(value, buf):
         if value >= datetime.datetime.fromtimestamp(0, datetime.timezone.utc):
diff --git a/uniffi_bindgen/src/bindings/python/templates/TopLevelFunctionTemplate.py b/uniffi_bindgen/src/bindings/python/templates/TopLevelFunctionTemplate.py
index f258b60a1c..8faa11bedf 100644
--- a/uniffi_bindgen/src/bindings/python/templates/TopLevelFunctionTemplate.py
+++ b/uniffi_bindgen/src/bindings/python/templates/TopLevelFunctionTemplate.py
@@ -1,6 +1,7 @@
 {%- if func.is_async() %}
 
 def {{ func.name()|fn_name }}({%- call py::arg_list_decl(func) -%}):
+    {%- call py::setup_args(func) %}
     return _uniffi_rust_call_async(
         _UniffiLib.{{ func.ffi_func().name() }}({% call py::arg_list_lowered(func) %}),
         _UniffiLib.{{func.ffi_rust_future_poll(ci) }},
diff --git a/uniffi_bindgen/src/bindings/python/templates/UInt16Helper.py b/uniffi_bindgen/src/bindings/python/templates/UInt16Helper.py
index 081c6731ce..039bf76162 100644
--- a/uniffi_bindgen/src/bindings/python/templates/UInt16Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/UInt16Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_u16()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_u16(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/UInt32Helper.py b/uniffi_bindgen/src/bindings/python/templates/UInt32Helper.py
index b80e75177d..1650bf9b60 100644
--- a/uniffi_bindgen/src/bindings/python/templates/UInt32Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/UInt32Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_u32()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_u32(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/UInt64Helper.py b/uniffi_bindgen/src/bindings/python/templates/UInt64Helper.py
index 4b87e58547..f354545e26 100644
--- a/uniffi_bindgen/src/bindings/python/templates/UInt64Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/UInt64Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_u64()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_u64(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/UInt8Helper.py b/uniffi_bindgen/src/bindings/python/templates/UInt8Helper.py
index 33026706f2..cee130b4d9 100644
--- a/uniffi_bindgen/src/bindings/python/templates/UInt8Helper.py
+++ b/uniffi_bindgen/src/bindings/python/templates/UInt8Helper.py
@@ -8,5 +8,5 @@ def read(buf):
         return buf.read_u8()
 
     @staticmethod
-    def write_unchecked(value, buf):
+    def write(value, buf):
         buf.write_u8(value)
diff --git a/uniffi_bindgen/src/bindings/python/templates/macros.py b/uniffi_bindgen/src/bindings/python/templates/macros.py
index 99d48d343b..3359e759e6 100644
--- a/uniffi_bindgen/src/bindings/python/templates/macros.py
+++ b/uniffi_bindgen/src/bindings/python/templates/macros.py
@@ -70,6 +70,7 @@
  #}
 {%- macro setup_args(func) %}
     {%- for arg in func.arguments() %}
+    {{ arg|check_fn }}({{ arg.name()|var_name }})
     {%- match arg.default_value() %}
     {%- when None %}
     {%- when Some with(literal) %}
@@ -85,6 +86,7 @@
  #}
 {%- macro setup_args_extra_indent(func) %}
         {%- for arg in func.arguments() %}
+        {{ arg|check_fn }}({{ arg.name()|var_name }})
         {%- match arg.default_value() %}
         {%- when None %}
         {%- when Some with(literal) %}
diff --git a/uniffi_bindgen/src/bindings/ruby/gen_ruby/mod.rs b/uniffi_bindgen/src/bindings/ruby/gen_ruby/mod.rs
index 2471c6aafc..d8bb65df47 100644
--- a/uniffi_bindgen/src/bindings/ruby/gen_ruby/mod.rs
+++ b/uniffi_bindgen/src/bindings/ruby/gen_ruby/mod.rs
@@ -265,6 +265,22 @@ mod filters {
         })
     }
 
+    pub fn check_rb(nm: &str, type_: &Type) -> Result<String, askama::Error> {
+        Ok(match type_ {
+            Type::Object { name, .. } => format!("({}._uniffi_check {nm})", class_name_rb(name)?),
+            Type::Enum { .. }
+            | Type::Record { .. }
+            | Type::Optional { .. }
+            | Type::Sequence { .. }
+            | Type::Map { .. } => format!(
+                "RustBuffer.check_{}({})",
+                class_name_rb(&canonical_name(type_))?,
+                nm
+            ),
+            _ => "".to_owned(),
+        })
+    }
+
     pub fn lower_rb(nm: &str, type_: &Type) -> Result<String, askama::Error> {
         Ok(match type_ {
             Type::Int8
diff --git a/uniffi_bindgen/src/bindings/ruby/templates/ObjectTemplate.rb b/uniffi_bindgen/src/bindings/ruby/templates/ObjectTemplate.rb
index 4482ccfe88..e221a7c427 100644
--- a/uniffi_bindgen/src/bindings/ruby/templates/ObjectTemplate.rb
+++ b/uniffi_bindgen/src/bindings/ruby/templates/ObjectTemplate.rb
@@ -29,20 +29,23 @@ def _uniffi_clone_handle()
     return @handle
   end
 
-  # A private helper for lowering instances into a handle.
+  # Private helpers for lowering instances into a handle.
   # This does an explicit typecheck, because accidentally lowering a different type of
   # object in a place where this type is expected, could lead to memory unsafety.
-  def self._uniffi_lower(inst)
+  def self._uniffi_check(inst)
     if not inst.is_a? self
       raise TypeError.new "Expected a {{ obj.name()|class_name_rb }} instance, got #{inst}"
     end
+  end
+
+  def self._uniffi_lower(inst)
     return inst._uniffi_clone_handle()
   end
 
   {%- match obj.primary_constructor() %}
   {%- when Some with (cons) %}
   def initialize({% call rb::arg_list_decl(cons) -%})
-    {%- call rb::coerce_args_extra_indent(cons) %}
+    {%- call rb::setup_args_extra_indent(cons) %}
     handle = {% call rb::to_ffi_call(cons) %}
     @handle = handle
     ObjectSpace.define_finalizer(self, self.class._uniffi_define_finalizer_by_handle(handle, self.object_id))
@@ -52,7 +55,7 @@ def initialize({% call rb::arg_list_decl(cons) -%})
 
   {% for cons in obj.alternate_constructors() -%}
   def self.{{ cons.name()|fn_name_rb }}({% call rb::arg_list_decl(cons) %})
-    {%- call rb::coerce_args_extra_indent(cons) %}
+    {%- call rb::setup_args_extra_indent(cons) %}
     # Call the (fallible) function before creating any half-baked object instances.
     # Lightly yucky way to bypass the usual "initialize" logic
     # and just create a new instance with the required handle.
@@ -65,14 +68,14 @@ def self.{{ cons.name()|fn_name_rb }}({% call rb::arg_list_decl(cons) %})
 
   {%- when Some with (return_type) -%}
   def {{ meth.name()|fn_name_rb }}({% call rb::arg_list_decl(meth) %})
-    {%- call rb::coerce_args_extra_indent(meth) %}
+    {%- call rb::setup_args_extra_indent(meth) %}
     result = {% call rb::to_ffi_call_with_prefix("_uniffi_clone_handle()", meth) %}
     return {{ "result"|lift_rb(return_type) }}
   end
 
   {%- when None -%}
   def {{ meth.name()|fn_name_rb }}({% call rb::arg_list_decl(meth) %})
-      {%- call rb::coerce_args_extra_indent(meth) %}
+      {%- call rb::setup_args_extra_indent(meth) %}
       {% call rb::to_ffi_call_with_prefix("_uniffi_clone_handle()", meth) %}
   end
   {% endmatch %}
diff --git a/uniffi_bindgen/src/bindings/ruby/templates/RustBufferTemplate.rb b/uniffi_bindgen/src/bindings/ruby/templates/RustBufferTemplate.rb
index 0194c9666d..154867e7ee 100644
--- a/uniffi_bindgen/src/bindings/ruby/templates/RustBufferTemplate.rb
+++ b/uniffi_bindgen/src/bindings/ruby/templates/RustBufferTemplate.rb
@@ -128,6 +128,12 @@ def consumeInto{{ canonical_type_name }}
   {%- let rec = ci|get_record_definition(record_name) -%}
   # The Record type {{ record_name }}.
 
+  def self.check_{{ canonical_type_name }}(v)
+    {%- for field in rec.fields() %}
+    {{ "v.{}"|format(field.name()|var_name_rb)|check_rb(field.as_type().borrow()) }}
+    {%- endfor %}
+  end
+
   def self.alloc_from_{{ canonical_type_name }}(v)
     RustBuffer.allocWithBuilder do |builder|
       builder.write_{{ canonical_type_name }}(v)
@@ -146,6 +152,19 @@ def consumeInto{{ canonical_type_name }}
   {%- let e = ci|get_enum_definition(enum_name) -%}
   # The Enum type {{ enum_name }}.
 
+  def self.check_{{ canonical_type_name }}(v)
+    {%- if !e.is_flat() %}
+    {%- for variant in e.variants() %}
+    if v.{{ variant.name()|var_name_rb }}?
+      {%- for field in variant.fields() %}
+        {{ "v.{}"|format(field.name())|check_rb(field.as_type().borrow()) }}
+      {%- endfor %}
+      return
+    end
+    {%- endfor %}
+    {%- endif %}
+  end
+
   def self.alloc_from_{{ canonical_type_name }}(v)
     RustBuffer.allocWithBuilder do |builder|
       builder.write_{{ canonical_type_name }}(v)
@@ -162,6 +181,12 @@ def consumeInto{{ canonical_type_name }}
 
   {% when Type::Optional { inner_type } -%}
   # The Optional<T> type for {{ canonical_name(inner_type) }}.
+  
+  def self.check_{{ canonical_type_name }}(v)
+    if not v.nil?
+      {{ "v"|check_rb(inner_type.borrow()) }}
+    end
+  end
 
   def self.alloc_from_{{ canonical_type_name }}(v)
     RustBuffer.allocWithBuilder do |builder|
@@ -179,6 +204,12 @@ def consumeInto{{ canonical_type_name }}
   {% when Type::Sequence { inner_type } -%}
   # The Sequence<T> type for {{ canonical_name(inner_type) }}.
 
+  def self.check_{{ canonical_type_name }}(v)
+    v.each do |item|
+      {{ "item"|check_rb(inner_type.borrow()) }}
+    end
+  end
+
   def self.alloc_from_{{ canonical_type_name }}(v)
     RustBuffer.allocWithBuilder do |builder|
       builder.write_{{ canonical_type_name }}(v)
@@ -195,6 +226,13 @@ def consumeInto{{ canonical_type_name }}
   {% when Type::Map { key_type: k, value_type: inner_type } -%}
   # The Map<T> type for {{ canonical_name(inner_type) }}.
 
+  def self.check_{{ canonical_type_name }}(v)
+    v.each do |k, v|
+      {{ "k"|check_rb(k.borrow()) }}
+      {{ "v"|check_rb(inner_type.borrow()) }}
+    end
+  end
+
   def self.alloc_from_{{ canonical_type_name }}(v)
     RustBuffer.allocWithBuilder do |builder|
       builder.write_{{ canonical_type_name }}(v)
diff --git a/uniffi_bindgen/src/bindings/ruby/templates/TopLevelFunctionTemplate.rb b/uniffi_bindgen/src/bindings/ruby/templates/TopLevelFunctionTemplate.rb
index 13214cf31b..b6dce0effa 100644
--- a/uniffi_bindgen/src/bindings/ruby/templates/TopLevelFunctionTemplate.rb
+++ b/uniffi_bindgen/src/bindings/ruby/templates/TopLevelFunctionTemplate.rb
@@ -2,7 +2,7 @@
 {%- when Some with (return_type) %}
 
 def self.{{ func.name()|fn_name_rb }}({%- call rb::arg_list_decl(func) -%})
-  {%- call rb::coerce_args(func) %}
+  {%- call rb::setup_args(func) %}
   result = {% call rb::to_ffi_call(func) %}
   return {{ "result"|lift_rb(return_type) }}
 end
@@ -10,7 +10,7 @@ def self.{{ func.name()|fn_name_rb }}({%- call rb::arg_list_decl(func) -%})
 {% when None %}
 
 def self.{{ func.name()|fn_name_rb }}({%- call rb::arg_list_decl(func) -%})
-  {%- call rb::coerce_args(func) %}
+  {%- call rb::setup_args(func) %}
   {% call rb::to_ffi_call(func) %}
 end
 {% endmatch %}
diff --git a/uniffi_bindgen/src/bindings/ruby/templates/macros.rb b/uniffi_bindgen/src/bindings/ruby/templates/macros.rb
index 8dc3e5e613..e9962f5831 100644
--- a/uniffi_bindgen/src/bindings/ruby/templates/macros.rb
+++ b/uniffi_bindgen/src/bindings/ruby/templates/macros.rb
@@ -60,14 +60,16 @@
     [{%- for arg in func.arguments() -%}{{ arg.type_().borrow()|type_ffi }}, {% endfor -%} RustCallStatus.by_ref]
 {%- endmacro -%}
 
-{%- macro coerce_args(func) %}
+{%- macro setup_args(func) %}
     {%- for arg in func.arguments() %}
-    {{ arg.name() }} = {{ arg.name()|coerce_rb(ci.namespace()|class_name_rb, arg.as_type().borrow()) -}}
+    {{ arg.name() }} = {{ arg.name()|coerce_rb(ci.namespace()|class_name_rb, arg.as_type().borrow()) }}
+    {{ arg.name()|check_rb(arg.as_type().borrow()) }}
     {% endfor -%}
 {%- endmacro -%}
 
-{%- macro coerce_args_extra_indent(func) %}
-        {%- for arg in func.arguments() %}
+{%- macro setup_args_extra_indent(meth) %}
+        {%- for arg in meth.arguments() %}
         {{ arg.name() }} = {{ arg.name()|coerce_rb(ci.namespace()|class_name_rb, arg.as_type().borrow()) }}
+        {{ arg.name()|check_rb(arg.as_type().borrow()) }}
         {%- endfor %}
 {%- endmacro -%}
diff --git a/uniffi_bindgen/src/bindings/swift/templates/Slab.swift b/uniffi_bindgen/src/bindings/swift/templates/Slab.swift
index d96c1b5d31..5d1ac30601 100644
--- a/uniffi_bindgen/src/bindings/swift/templates/Slab.swift
+++ b/uniffi_bindgen/src/bindings/swift/templates/Slab.swift
@@ -1,4 +1,4 @@
-func uniffiHandleIsFromRust(_ handle: Int64) -> Bool {
+fileprivate func uniffiHandleIsFromRust(_ handle: Int64) -> Bool {
     return (handle & 0x0001_0000_0000) == 0
 }