Add localizedError support to swift errors.

Also some extra error string semantics tests for kotlin.

Fixes #2110

fixtures/error-types/tests/bindings/test.kts

@@ -13,6 +13,12 @@ try {
     assert(e.chain().size == 2)
     assert(e.link(0U) == "because uniffi told me so")
 }
+try {
+    oops()
+    throw RuntimeException("Should have failed")
+} catch (e: kotlin.Exception) {
+    assert(e.toString() == "because uniffi told me so\n\nCaused by:\n    oops")
+}
 
 try {
     oopsNowrap()

fixtures/error-types/tests/bindings/test.swift

@@ -3,8 +3,38 @@ do {
     try oops()
     fatalError("Should have thrown")
 } catch let e as ErrorInterface {
-    assert(String(describing: e) == "because uniffi told me so\n\nCaused by:\n    oops")
+    let msg = "because uniffi told me so\n\nCaused by:\n    oops"
+    assert(String(describing: e) == msg)
+    assert(String(reflecting: e) == "ErrorInterface { e: \(msg) }")
+}
+
+do {
+    try oops()
+    fatalError("Should have thrown")
+} catch {
+    let msg = "because uniffi told me so\n\nCaused by:\n    oops"
+    assert(String(describing: error) == msg)
+    assert(String(reflecting: error) == "ErrorInterface { e: \(msg) }")
+    assert(error.localizedDescription == "ErrorInterface { e: \(msg) }")
+}
+
+do {
+    try oopsEnum()
+    fatalError("Should have thrown")
+} catch let e as Error {
+    assert(e == Error.Oops)
+    assert(String(describing: e) == "Oops")
+    assert(String(reflecting: e) == "error_types.Error.Oops")
 }
+do {
+    try oopsEnum()
+    fatalError("Should have thrown")
+} catch {
+    assert(String(describing: error) == "Oops")
+    assert(String(reflecting: error) == "error_types.Error.Oops")
+    assert(error.localizedDescription == "error_types.Error.Oops")
+}
+
 do {
     try oopsNowrap()
     fatalError("Should have thrown")
@@ -22,5 +52,5 @@ do {
 let e = getError(message: "the error")
 assert(String(describing: e) == "the error")
 assert(String(reflecting: e) == "ErrorInterface { e: the error }")
-assert(Error.self is Swift.Error.Type)
+// assert(Error.self is Swift.Error.Type) -- always true!
 assert(Error.self != Swift.Error.self)

uniffi_bindgen/src/bindings/swift/templates/ErrorTemplate.swift

@@ -83,4 +83,8 @@ public struct {{ ffi_converter_name }}: FfiConverterRustBuffer {
 {% if !contains_object_references %}
 extension {{ type_name }}: Equatable, Hashable {}
 {% endif %}
-extension {{ type_name }}: Swift.Error { }
+extension {{ type_name }}: Foundation.LocalizedError {
+    public var errorDescription: String? {
+        String(reflecting: self)
+    }
+}

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

@@ -161,6 +161,13 @@ public struct {{ ffi_converter_name }}: FfiConverter {
 
 {# Objects as error #}
 {%- if is_error %}
+
+extension {{ type_name }}: Foundation.LocalizedError {
+    public var errorDescription: String? {
+        String(reflecting: self)
+    }
+}
+
 {# Due to some mismatches in the ffi converter mechanisms, errors are a RustBuffer holding a pointer #}
 public struct {{ ffi_converter_name }}__as_error: FfiConverterRustBuffer {
     public static func lift(_ buf: RustBuffer) throws -> {{ type_name }} {