Swift classes can inherit from traits

Builds on #2196, #2204 and #2297. Closes #2169.

fixtures/coverall/tests/bindings/test_coverall.swift

@@ -464,6 +464,27 @@ do {
     traits[0].setParent(parent: nil)
 }
 
+// A struct which implements the node trait.
+do {
+    let n = Node(name: "node")
+    assert(String(describing: n).starts(with: "Node { name: Some(\"node\"), parent: Mutex { "))
+    assert(n.getParent()?.name() == "via node")
+
+    n.setParent(parent: n.getParent())
+    // doubly-wrapped :(
+    // Get: "Some(UniFFICallbackHandlerNodeTrait { handle: 19 })"
+    // Want: Like the Rust node above.
+    // debugPrint("parent \(n.describeParent())")
+
+    let rustParent = Node(name: "parent")
+    n.setParent(parent: rustParent)
+    assert(n.getParent()?.name() == "parent")
+
+    let swiftParent = SwiftNode()
+    rustParent.setParent(parent: swiftParent)
+    assert(ancestorNames(node: n) == ["parent", "node-swift"])
+}
+
 // Test round tripping
 do {
     let rustGetters = makeRustGetters()

uniffi_bindgen/src/bindings/swift/gen_swift/mod.rs

@@ -272,6 +272,27 @@ impl Config {
     }
 }
 
+// Given a trait, work out what the protocol name we generate for it.
+// This differs based on whether the trait supports foreign impls (ie,
+// whether is has a "callback interface".
+fn trait_protocol_name(ci: &ComponentInterface, name: &str) -> Result<String> {
+    let (obj_name, has_callback_interface) = match ci.get_object_definition(name) {
+        Some(obj) => (obj.name(), obj.has_callback_interface()),
+        None => (
+            ci.get_callback_interface_definition(name)
+                .ok_or_else(|| anyhow::anyhow!("no interface {}", name))?
+                .name(),
+            true,
+        ),
+    };
+    let class_name = SwiftCodeOracle.class_name(obj_name);
+    if has_callback_interface {
+        Ok(class_name)
+    } else {
+        Ok(format!("{class_name}Protocol"))
+    }
+}
+
 /// Generate UniFFI component bindings for Swift, as strings in memory.
 pub fn generate_bindings(config: &Config, ci: &ComponentInterface) -> Result<Bindings> {
     let header = BridgingHeader::new(config, ci)

uniffi_bindgen/src/bindings/swift/templates/ObjectTemplate.swift

@@ -25,7 +25,11 @@ open class {{ impl_class_name }}:
     {%- if is_error %}
     Swift.Error,
     {% endif %}
-    {{ protocol_name }} {
+    {%- for t in obj.trait_impls() %}
+    {{ self::trait_protocol_name(ci, t.trait_name)? }},
+    {% endfor %}
+    {{ protocol_name }}
+    {
     fileprivate let pointer: UnsafeMutableRawPointer!
 
     /// Used to instantiate a [FFIObject] without an actual pointer, for fakes in tests, mostly.