Allow Rust traits to be exposed as an interface. (#1457)

CHANGELOG.md

@@ -19,13 +19,17 @@
   - UniFFI users will automatically get the benefits of this without any code changes.
   - External bindings authors will need to update their bindings code.  See PR #1494 for details.
 - ABI: Changed API checksum handling.  This affects external bindings authors who will need to update their code to work with the new system.  See PR #1469 for details.
+- Removed the long deprecated `ThreadSafe` attribute.
 
 ### What's changed
 
 - The `include_scaffolding!()` macro must now either be called from your crate root or you must have `use the_mod_that_calls_include_scaffolding::*` in your crate root.  This was always the expectation, but wasn't required before.  This will now start failing with errors that say `crate::UniFfiTag` does not exist.
 - proc-macros now work with many more types including type aliases, type paths, etc.
 - The `uniffi_types` module is no longer needed when using proc-macros.
 
+- Traits can be exposed as a UniFFI `interface` by using a `[Trait]` attribute in the UDL.
+  See [the documentation](https://mozilla.github.io/uniffi-rs/udl/interfaces.html#exposing-traits-as-interfaces).
+
 ## v0.23.0 (backend crates: v0.23.0) - (_2023-01-27_)
 
 [All changes in v0.23.0](https://github.com/mozilla/uniffi-rs/compare/v0.22.0...v0.23.0).

Cargo.toml

@@ -9,14 +9,15 @@ members = [
   "uniffi",
   "weedle2",
 
+  "examples/app/uniffi-bindgen-cli",
   "examples/arithmetic",
   "examples/callbacks",
+  "examples/custom-types",
   "examples/geometry",
   "examples/rondpoint",
   "examples/sprites",
   "examples/todolist",
-  "examples/custom-types",
-  "examples/app/uniffi-bindgen-cli",
+  "examples/traits",
 
   "fixtures/benchmarks",
   "fixtures/coverall",

docs/manual/src/udl/interfaces.md

@@ -73,6 +73,61 @@ func display(list: TodoListProtocol) {
 Following this pattern will make it easier for you to provide mock implementation of the Rust-based objects
 for testing.
 
+## Exposing Traits as interfaces
+
+It's possible to have UniFFI expose a Rust trait as an interface by specifying a `Trait` attribute.
+
+For example, in the UDL file you might specify:
+
+```idl
+[Trait]
+interface Button {
+    string name();
+};
+
+```
+
+With the following Rust implementation:
+
+```rust
+pub trait Button: Send + Sync {
+    fn name(&self) -> String;
+}
+
+struct StopButton {}
+
+impl Button for StopButton  {
+    fn name(&self) -> String {
+        "stop".to_string()
+    }
+}
+```
+
+Uniffi explicitly checks all interfaces are `Send + Sync` - there's a ui-test which demonstrates obscure rust compiler errors when it's not true. Traits however need to explicitly add those bindings.
+
+References to traits are passed around like normal interface objects - in an `Arc<>`.
+For example, this UDL:
+
+```idl
+namespace traits {
+    sequence<Button> get_buttons();
+    Button press(Button button);
+};
+```
+
+would have these signatures in Rust:
+
+```rust
+fn get_buttons() -> Vec<Arc<dyn Button>> { ... }
+fn press(button: Arc<dyn Button>) -> Arc<dyn Button> { ... }
+```
+
+See the ["traits" example](https://github.com/mozilla/uniffi-rs/tree/main/examples/traits) for more.
+
+### Traits construction
+
+Because any number of `struct`s may implement a trait, they don't have constructors.
+
 ## Alternate Named Constructors
 
 In addition to the default constructor connected to the `::new()` method, you can specify

examples/traits/Cargo.toml

@@ -0,0 +1,22 @@
+[package]
+name = "uniffi-example-traits"
+edition = "2021"
+version = "0.22.0"
+authors = ["Firefox Sync Team <sync-team@mozilla.com>"]
+license = "MPL-2.0"
+publish = false
+
+[lib]
+crate-type = ["lib", "cdylib"]
+name = "uniffi_traits"
+
+[dependencies]
+uniffi = {path = "../../uniffi"}
+thiserror = "1.0"
+
+[build-dependencies]
+uniffi = {path = "../../uniffi", features = ["build"] }
+
+[dev-dependencies]
+uniffi = {path = "../../uniffi", features = ["bindgen-tests"] }
+

examples/traits/build.rs

@@ -0,0 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+fn main() {
+    uniffi::generate_scaffolding("./src/traits.udl").unwrap();
+}

examples/traits/src/lib.rs

@@ -0,0 +1,36 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::sync::Arc;
+
+// namespace functions.
+fn get_buttons() -> Vec<Arc<dyn Button>> {
+    vec![Arc::new(StopButton {}), Arc::new(GoButton {})]
+}
+
+fn press(button: Arc<dyn Button>) -> Arc<dyn Button> {
+    button
+}
+
+pub trait Button: Send + Sync {
+    fn name(&self) -> String;
+}
+
+struct GoButton {}
+
+impl Button for GoButton {
+    fn name(&self) -> String {
+        "go".to_string()
+    }
+}
+
+struct StopButton {}
+
+impl Button for StopButton {
+    fn name(&self) -> String {
+        "stop".to_string()
+    }
+}
+
+include!(concat!(env!("OUT_DIR"), "/traits.uniffi.rs"));

examples/traits/src/traits.udl

@@ -0,0 +1,12 @@
+namespace traits {
+    // Get all the buttons we can press.
+    sequence<Button> get_buttons();
+    // press a button and return it.
+    Button press(Button button);
+};
+
+// This is a trait in Rust.
+[Trait]
+interface Button {
+    string name();
+};

examples/traits/tests/bindings/test_traits.py

@@ -0,0 +1,7 @@
+from traits import *
+
+for button in get_buttons():
+    if button.name() in ["go", "stop"]:
+        press(button)
+    else:
+        print("unknown button", button)

examples/traits/tests/test_generated_bindings.rs

@@ -0,0 +1 @@
+uniffi::build_foreign_language_testcases!("tests/bindings/test_traits.py",);

examples/traits/uniffi.toml

@@ -0,0 +1,9 @@
+[bindings.kotlin]
+package_name = "uniffi.traits"
+cdylib_name = "uniffi_traits"
+
+[bindings.swift]
+cdylib_name = "uniffi_traits"
+
+[bindings.python]
+cdylib_name = "uniffi_traits"

fixtures/coverall/src/coverall.udl

@@ -4,6 +4,8 @@ namespace coverall {
 
     u64 get_num_alive();
 
+    sequence<TestTrait> get_traits();
+
     // void returning error throwing namespace function to catch clippy warnings (eg, #1330)
     [Throws=CoverallError]
     void println(string text);
@@ -29,6 +31,7 @@ dictionary SimpleDict {
     double float64;
     double? maybe_float64;
     Coveralls? coveralls;
+    TestTrait? test_trait;
 };
 
 dictionary DictWithDefaults {
@@ -150,3 +153,23 @@ interface ThreadsafeCounter {
   void busy_wait(i32 ms);
   i32 increment_if_busy();
 };
+
+// This is a trait implemented on the Rust side.
+[Trait]
+interface TestTrait {
+    string name(); // The name of the implementation
+
+    [Self=ByArc]
+    u64 number();
+
+    /// Calls `Arc::strong_count()` on the `Arc` containing `self`.
+    [Self=ByArc]
+    u64 strong_count();
+
+    /// Takes an `Arc<Self>` and stores it in `self`, dropping the existing
+    /// reference. Note you can create circular references by passing `self`.
+    void take_other(TestTrait? other);
+
+    /// Returns what was previously set via `take_other()`, or null.
+    TestTrait? get_other();
+};

fixtures/coverall/src/lib.rs

@@ -9,6 +9,9 @@ use std::time::SystemTime;
 
 use once_cell::sync::Lazy;
 
+mod traits;
+pub use traits::{get_traits, TestTrait};
+
 static NUM_ALIVE: Lazy<RwLock<u64>> = Lazy::new(|| RwLock::new(0));
 
 #[derive(Debug, thiserror::Error)]
@@ -41,7 +44,7 @@ pub enum ComplexError {
     PermissionDenied { reason: String },
 }
 
-#[derive(Debug, Clone)]
+#[derive(Clone, Debug, Default)]
 pub struct SimpleDict {
     text: String,
     maybe_text: Option<String>,
@@ -62,6 +65,7 @@ pub struct SimpleDict {
     float64: f64,
     maybe_float64: Option<f64>,
     coveralls: Option<Arc<Coveralls>>,
+    test_trait: Option<Arc<dyn TestTrait>>,
 }
 
 #[derive(Debug, Clone)]
@@ -98,30 +102,21 @@ fn create_some_dict() -> SimpleDict {
         float64: 0.0,
         maybe_float64: Some(1.0),
         coveralls: Some(Arc::new(Coveralls::new("some_dict".to_string()))),
+        test_trait: Some(Arc::new(traits::Trait2 {})),
     }
 }
 
 fn create_none_dict() -> SimpleDict {
     SimpleDict {
         text: "text".to_string(),
-        maybe_text: None,
         a_bool: true,
-        maybe_a_bool: None,
         unsigned8: 1,
-        maybe_unsigned8: None,
         unsigned16: 3,
-        maybe_unsigned16: None,
         unsigned64: u64::MAX,
-        maybe_unsigned64: None,
         signed8: 8,
-        maybe_signed8: None,
         signed64: i64::MAX,
-        maybe_signed64: None,
         float32: 1.2345,
-        maybe_float32: None,
-        float64: 0.0,
-        maybe_float64: None,
-        coveralls: None,
+        ..Default::default()
     }
 }
 

fixtures/coverall/src/traits.rs

@@ -0,0 +1,74 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use std::sync::{Arc, Mutex};
+
+// namespace functions.
+pub fn get_traits() -> Vec<Arc<dyn TestTrait>> {
+    vec![
+        Arc::new(Trait1 {
+            ..Default::default()
+        }),
+        Arc::new(Trait2 {}),
+    ]
+}
+
+pub trait TestTrait: Send + Sync + std::fmt::Debug {
+    fn name(&self) -> String;
+
+    fn number(self: Arc<Self>) -> u64;
+
+    fn strong_count(self: Arc<Self>) -> u64 {
+        Arc::strong_count(&self) as u64
+    }
+
+    fn take_other(&self, other: Option<Arc<dyn TestTrait>>);
+
+    fn get_other(&self) -> Option<Arc<dyn TestTrait>>;
+}
+
+#[derive(Debug, Default)]
+pub(crate) struct Trait1 {
+    // A reference to another trait.
+    other: Mutex<Option<Arc<dyn TestTrait>>>,
+}
+
+impl TestTrait for Trait1 {
+    fn name(&self) -> String {
+        "trait 1".to_string()
+    }
+
+    fn number(self: Arc<Self>) -> u64 {
+        1_u64
+    }
+
+    fn take_other(&self, other: Option<Arc<dyn TestTrait>>) {
+        *self.other.lock().unwrap() = other.map(|arc| Arc::clone(&arc))
+    }
+
+    fn get_other(&self) -> Option<Arc<dyn TestTrait>> {
+        (*self.other.lock().unwrap()).as_ref().map(Arc::clone)
+    }
+}
+
+#[derive(Debug)]
+pub(crate) struct Trait2 {}
+impl TestTrait for Trait2 {
+    fn name(&self) -> String {
+        "trait 2".to_string()
+    }
+
+    fn number(self: Arc<Self>) -> u64 {
+        2_u64
+    }
+
+    // Don't bother implementing these here - the test on the struct above is ok.
+    fn take_other(&self, _other: Option<Arc<dyn TestTrait>>) {
+        unimplemented!();
+    }
+
+    fn get_other(&self) -> Option<Arc<dyn TestTrait>> {
+        unimplemented!()
+    }
+}