Kotlin classes can inherit from traits.

Builds on mozilla#2204 which landed the metadata and Python support.

CHANGELOG.md

@@ -17,6 +17,11 @@
 
 - Fixed bug in metadata extraction with large ELF files.
 
+### What's new?
+
+- Kotlin: Proc-macros exporting an `impl Trait for Struct` block now has a class inheritance
+  hierarcy to reflect that. [#2297](https://github.com/mozilla/uniffi-rs/pull/2297)
+
 [All changes in [[UnreleasedUniFFIVersion]]](https://github.com/mozilla/uniffi-rs/compare/v0.28.2...HEAD).
 
 ### What's changed?

fixtures/coverall/tests/bindings/test_coverall.kts

@@ -415,6 +415,17 @@ getTraits().let { traits ->
     // not possible through the `NodeTrait` interface (see #1787).
 }
 
+// Structs which implement traits.
+Node("node").let { n ->
+    assert(n.describeParent() == "Some(Node { name: Some(\"via node\"), parent: Mutex { data: None, poisoned: false, .. } })")
+    // NOTE same re-wrap problem described in the Python tests.
+    n.setParent(n.getParent())
+    // Expect: as above
+    // Get: `Some(UniFFICallbackHandlerNodeTrait { handle: 18 })`
+    // assert(n.describeParent() == "Some(Node { name: Some(\"via node\"), parent: Mutex { data: None, poisoned: false, .. } })")
+    n.setParent(Node("parent"))
+}
+
 makeRustGetters().let { rustGetters ->
     // Check that these don't cause use-after-free bugs
     testRoundTripThroughRust(rustGetters)

uniffi_bindgen/src/bindings/kotlin/gen_kotlin/mod.rs

@@ -303,6 +303,34 @@ impl<'a> KotlinWrapper<'a> {
     }
 }
 
+/// Get the name of the interface and class name for a trait.
+///
+/// For a regular `struct Foo` or `trait Foo`, there's `FooInterface` with `Foo` as
+/// the name of the (Rust implemented) object. But if it's a foreign trait:
+/// * The name `Foo` is the name of the interface used by a the Kotlin implementation of the trait.
+/// * The Rust implemented object is `FooImpl`.
+///
+/// This all impacts what types `FfiConverter.lower()` inputs.  If it's a "foreign trait"
+/// `lower` must lower anything that implements the interface (ie, a kotlin implementation).
+/// If not, then lower only lowers the concrete class (ie, our simple instance with the pointer).
+fn object_interface_name(ci: &ComponentInterface, obj: &Object) -> String {
+    let class_name = KotlinCodeOracle.class_name(ci, obj.name());
+    if obj.has_callback_interface() {
+        class_name
+    } else {
+        format!("{class_name}Interface")
+    }
+}
+
+fn object_impl_name(ci: &ComponentInterface, obj: &Object) -> String {
+    let class_name = KotlinCodeOracle.class_name(ci, obj.name());
+    if obj.has_callback_interface() {
+        format!("{class_name}Impl")
+    } else {
+        class_name
+    }
+}
+
 #[derive(Clone)]
 pub struct KotlinCodeOracle;
 
@@ -443,24 +471,6 @@ impl KotlinCodeOracle {
             FfiType::VoidPointer => "Pointer".to_string(),
         }
     }
-
-    /// Get the name of the interface and class name for an object.
-    ///
-    /// If we support callback interfaces, the interface name is the object name, and the class name is derived from that.
-    /// Otherwise, the class name is the object name and the interface name is derived from that.
-    ///
-    /// This split determines what types `FfiConverter.lower()` inputs.  If we support callback
-    /// interfaces, `lower` must lower anything that implements the interface.  If not, then lower
-    /// only lowers the concrete class.
-    fn object_names(&self, ci: &ComponentInterface, obj: &Object) -> (String, String) {
-        let class_name = self.class_name(ci, obj.name());
-        if obj.has_callback_interface() {
-            let impl_name = format!("{class_name}Impl");
-            (class_name, impl_name)
-        } else {
-            (format!("{class_name}Interface"), class_name)
-        }
-    }
 }
 
 trait AsCodeType {
@@ -659,13 +669,6 @@ mod filters {
         Ok(KotlinCodeOracle.ffi_struct_name(nm.as_ref()))
     }
 
-    pub fn object_names(
-        obj: &Object,
-        ci: &ComponentInterface,
-    ) -> Result<(String, String), rinja::Error> {
-        Ok(KotlinCodeOracle.object_names(ci, obj))
-    }
-
     pub fn async_poll(
         callable: impl Callable,
         ci: &ComponentInterface,

uniffi_bindgen/src/bindings/kotlin/templates/ObjectTemplate.kt

@@ -101,7 +101,8 @@
 {%- endif %}
 
 {%- let obj = ci|get_object_definition(name) %}
-{%- let (interface_name, impl_class_name) = obj|object_names(ci) %}
+{%- let interface_name = self::object_interface_name(ci, obj) %}
+{%- let impl_class_name = self::object_impl_name(ci, obj) %}
 {%- let methods = obj.methods() %}
 {%- let interface_docstring = obj.docstring() %}
 {%- let is_error = ci.is_name_used_as_error(name) %}
@@ -113,7 +114,12 @@
 {% if (is_error) %}
 open class {{ impl_class_name }} : kotlin.Exception, Disposable, AutoCloseable, {{ interface_name }} {
 {% else -%}
-open class {{ impl_class_name }}: Disposable, AutoCloseable, {{ interface_name }} {
+open class {{ impl_class_name }}: Disposable, AutoCloseable, {{ interface_name }}
+{%- for t in obj.trait_impls() %}
+{%- let trait = ci|get_object_definition(t.trait_name) %}
+, {{ self::object_interface_name(ci, trait) }}
+{% endfor %}
+{
 {%- endif %}
 
     constructor(pointer: Pointer) {