Require users of msg_send! to specify the return type

This is a breaking change, to prevent misuses such as SSheldon#62
SimonSapin · Oct 15, 2019 · bdb7202 · bdb7202
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -1,3 +1,22 @@
+## 0.3.0
+
+### Changed
+
+* Require users of `msg_send!` to specify the return type.
+
+  Previously, the return type was inferred based on context, and
+  `let () = msg_send![…];` was documented as the correct use when there is no return value.
+  However `msg_send![…];` compiled fine and was equivalent as of Rust 1.38,
+  even though it happened to rely on an unspecified part of the language
+  (type inference fallback for diverging expressions)
+
+  When Rust first tried to change fallback from `()` (the unit type) to `!` (the never type),
+  this cause such uses of `msg_send!` to have Undefined Behavior
+  beacause a function was returning `Result<!, SomeError>::Ok`, which should be impossible:
+  <https://github.com/SSheldon/rust-objc/issues/62>
+
+  With this change, both `let () = msg_send![…];` and `msg_send![…];` become `msg_send![… => ()];`.
+
 ## 0.2.6
 
 ### Fixed

diff --git a/Cargo.toml b/Cargo.toml
@@ -1,6 +1,6 @@
 [package]
 name = "objc"
-version = "0.2.6"
+version = "0.3.0"
 authors = ["Steven Sheldon"]
 edition = "2018"
 

diff --git a/README.md b/README.md
@@ -9,11 +9,11 @@ Objective-C objects can be messaged using the `msg_send!` macro:
 
 ``` rust
 let cls = class!(NSObject);
-let obj: *mut Object = msg_send![cls, new];
-let hash: usize = msg_send![obj, hash];
-let is_kind: BOOL = msg_send![obj, isKindOfClass:cls];
+let obj = msg_send![cls, new => *mut Object];
+let hash = msg_send![obj, hash => usize];
+let is_kind = msg_send![obj, isKindOfClass:cls => BOOL];
 // Even void methods must have their return type annotated
-let _: () = msg_send![obj, release];
+msg_send![obj, release => ()];
 ```
 
 ## Reference counting
@@ -27,7 +27,7 @@ and safely fails if the object has been deallocated.
 ``` rust
 // StrongPtr will release the object when dropped
 let obj = unsafe {
-    StrongPtr::new(msg_send![class!(NSObject), new])
+    StrongPtr::new(msg_send![class!(NSObject), new => *mut Object])
 };
 
 // Cloning retains the object an additional time

diff --git a/examples/example.rs b/examples/example.rs
@@ -18,8 +18,8 @@ fn main() {
 
     // Allocate an instance
     let obj = unsafe {
-        let obj: *mut Object = msg_send![cls, alloc];
-        let obj: *mut Object = msg_send![obj, init];
+        let obj = msg_send![cls, alloc => *mut Object];
+        let obj = msg_send![obj, init => *mut Object];
         StrongPtr::new(obj)
     };
     println!("NSObject address: {:p}", obj);
@@ -38,8 +38,8 @@ fn main() {
     assert!(*hash_return == usize::ENCODING);
 
     // Invoke a method on the object
-    let hash: usize = unsafe {
-        msg_send![*obj, hash]
+    let hash = unsafe {
+        msg_send![*obj, hash => usize]
     };
     println!("NSObject hash: {}", hash);
 }
diff --git a/src/declare.rs b/src/declare.rs
@@ -322,18 +322,16 @@ mod tests {
         // Registering the custom class is in test_utils
         let obj = test_utils::custom_object();
         unsafe {
-            let _: () = msg_send![obj, setFoo:13u32];
-            let result: u32 = msg_send![obj, foo];
-            assert!(result == 13);
+            msg_send![obj, setFoo:13u32 => ()];
+            assert!(msg_send![obj, foo => u32] == 13);
         }
     }
 
     #[test]
     fn test_class_method() {
         let cls = test_utils::custom_class();
         unsafe {
-            let result: u32 = msg_send![cls, classFoo];
-            assert!(result == 7);
+            assert!(msg_send![cls, classFoo => u32] == 7);
         }
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
@@ -11,11 +11,11 @@ Objective-C objects can be messaged using the [`msg_send!`](macro.msg_send!.html
 # fn main() {
 # unsafe {
 let cls = class!(NSObject);
-let obj: *mut Object = msg_send![cls, new];
-let hash: usize = msg_send![obj, hash];
-let is_kind: BOOL = msg_send![obj, isKindOfClass:cls];
+let obj = msg_send![cls, new => *mut Object];
+let hash = msg_send![obj, hash => usize];
+let is_kind = msg_send![obj, isKindOfClass:cls => BOOL];
 // Even void methods must have their return type annotated
-let _: () = msg_send![obj, release];
+msg_send![obj, release => ()];
 # }
 # }
 ```

diff --git a/src/macros.rs b/src/macros.rs
@@ -69,39 +69,39 @@ Variadic arguments are not currently supported.
 # fn main() {
 # unsafe {
 let obj: *mut Object;
-# let obj: *mut Object = 0 as *mut Object;
-let description: *const Object = msg_send![obj, description];
-let _: () = msg_send![obj, setArg1:1 arg2:2];
+# let obj = 0 as *mut Object;
+let description = msg_send![obj, description => *mut Object];
+msg_send![obj, setArg1:1 arg2:2 => ()];
 # }
 # }
 ```
 */
 #[macro_export]
 macro_rules! msg_send {
-    (super($obj:expr, $superclass:expr), $name:ident) => ({
+    (super($obj:expr, $superclass:expr), $name:ident => $ret:ty) => ({
         let sel = $crate::sel!($name);
-        match $crate::__send_super_message(&*$obj, $superclass, sel, ()) {
+        match $crate::__send_super_message::<_, _, $ret>(&*$obj, $superclass, sel, ()) {
             Err(s) => panic!("{}", s),
             Ok(r) => r,
         }
     });
-    (super($obj:expr, $superclass:expr), $($name:ident : $arg:expr)+) => ({
+    (super($obj:expr, $superclass:expr), $($name:ident : $arg:expr)+ => $ret:ty) => ({
         let sel = $crate::sel!($($name:)+);
-        match $crate::__send_super_message(&*$obj, $superclass, sel, ($($arg,)*)) {
+        match $crate::__send_super_message::<_, _, $ret>(&*$obj, $superclass, sel, ($($arg,)*)) {
             Err(s) => panic!("{}", s),
             Ok(r) => r,
         }
     });
-    ($obj:expr, $name:ident) => ({
+    ($obj:expr, $name:ident => $ret:ty) => ({
         let sel = $crate::sel!($name);
-        match $crate::__send_message(&*$obj, sel, ()) {
+        match $crate::__send_message::<_, _, $ret>(&*$obj, sel, ()) {
             Err(s) => panic!("{}", s),
             Ok(r) => r,
         }
     });
-    ($obj:expr, $($name:ident : $arg:expr)+) => ({
+    ($obj:expr, $($name:ident : $arg:expr)+ => $ret:ty) => ({
         let sel = $crate::sel!($($name:)+);
-        match $crate::__send_message(&*$obj, sel, ($($arg,)*)) {
+        match $crate::__send_message::<_, _, $ret>(&*$obj, sel, ($($arg,)*)) {
             Err(s) => panic!("{}", s),
             Ok(r) => r,
         }

diff --git a/src/message/mod.rs b/src/message/mod.rs
@@ -88,7 +88,7 @@ pub unsafe trait Message {
     # use objc::Message;
     # fn main() {
     let obj: &Object;
-    # obj = unsafe { msg_send![class!(NSObject), new] };
+    # obj = unsafe { msg_send![class!(NSObject), new => &Object] };
     let sel = sel!(isKindOfClass:);
     // Verify isKindOfClass: takes one Class and returns a BOOL
     let result = obj.verify_message::<(&Class,), BOOL>(sel);
@@ -230,18 +230,18 @@ mod tests {
     #[test]
     fn test_send_message() {
         let obj = test_utils::custom_object();
-        let result: u32 = unsafe {
-            let _: () = msg_send![obj, setFoo:4u32];
-            msg_send![obj, foo]
+        let result = unsafe {
+            msg_send![obj, setFoo:4u32 => ()];
+            msg_send![obj, foo => u32]
         };
         assert!(result == 4);
     }
 
     #[test]
     fn test_send_message_stret() {
         let obj = test_utils::custom_object();
-        let result: test_utils::CustomStruct = unsafe {
-            msg_send![obj, customStruct]
+        let result  = unsafe {
+            msg_send![obj, customStruct => test_utils::CustomStruct]
         };
         let expected = test_utils::CustomStruct { a: 1, b:2, c: 3, d: 4 };
         assert!(result == expected);
@@ -251,18 +251,18 @@ mod tests {
     #[test]
     fn test_send_message_nil() {
         let nil: *mut Object = ::std::ptr::null_mut();
-        let result: usize = unsafe {
-            msg_send![nil, hash]
+        let result = unsafe {
+            msg_send![nil, hash => usize]
         };
         assert!(result == 0);
 
-        let result: *mut Object = unsafe {
-            msg_send![nil, description]
+        let result = unsafe {
+            msg_send![nil, description => *mut Object]
         };
         assert!(result.is_null());
 
-        let result: f64 = unsafe {
-            msg_send![nil, doubleValue]
+        let result = unsafe {
+            msg_send![nil, doubleValue => f64]
         };
         assert!(result == 0.0);
     }
@@ -272,13 +272,12 @@ mod tests {
         let obj = test_utils::custom_subclass_object();
         let superclass = test_utils::custom_class();
         unsafe {
-            let _: () = msg_send![obj, setFoo:4u32];
-            let foo: u32 = msg_send![super(obj, superclass), foo];
+            msg_send![obj, setFoo:4u32 => ()];
+            let foo = msg_send![super(obj, superclass), foo => u32];
             assert!(foo == 4);
 
             // The subclass is overriden to return foo + 2
-            let foo: u32 = msg_send![obj, foo];
-            assert!(foo == 6);
+            assert!(msg_send![obj, foo => u32] == 6);
         }
     }
 

diff --git a/src/rc/mod.rs b/src/rc/mod.rs
@@ -18,10 +18,11 @@ For more information on Objective-C's reference counting, see Apple's documentat
 ``` no_run
 # #[macro_use] extern crate objc;
 # use objc::rc::{autoreleasepool, StrongPtr};
+# use objc::runtime::Object;
 # fn main() {
 // StrongPtr will release the object when dropped
 let obj = unsafe {
-    StrongPtr::new(msg_send![class!(NSObject), new])
+    StrongPtr::new(msg_send![class!(NSObject), new => *mut Object])
 };
 
 // Cloning retains the object an additional time
@@ -58,11 +59,11 @@ mod tests {
     #[test]
     fn test_strong_clone() {
         fn retain_count(obj: *mut Object) -> usize {
-            unsafe { msg_send![obj, retainCount] }
+            unsafe { msg_send![obj, retainCount => usize] }
         }
 
         let obj = unsafe {
-            StrongPtr::new(msg_send![class!(NSObject), new])
+            StrongPtr::new(msg_send![class!(NSObject), new => *mut Object])
         };
         assert!(retain_count(*obj) == 1);
 
@@ -77,7 +78,7 @@ mod tests {
     #[test]
     fn test_weak() {
         let obj = unsafe {
-            StrongPtr::new(msg_send![class!(NSObject), new])
+            StrongPtr::new(msg_send![class!(NSObject), new => *mut Object])
         };
         let weak = obj.weak();
 
@@ -92,7 +93,7 @@ mod tests {
     #[test]
     fn test_weak_copy() {
         let obj = unsafe {
-            StrongPtr::new(msg_send![class!(NSObject), new])
+            StrongPtr::new(msg_send![class!(NSObject), new => *mut Object])
         };
         let weak = obj.weak();
 
@@ -104,11 +105,11 @@ mod tests {
     #[test]
     fn test_autorelease() {
         let obj = unsafe {
-            StrongPtr::new(msg_send![class!(NSObject), new])
+            StrongPtr::new(msg_send![class!(NSObject), new => *mut Object])
         };
 
         fn retain_count(obj: *mut Object) -> usize {
-            unsafe { msg_send![obj, retainCount] }
+            unsafe { msg_send![obj, retainCount => usize] }
         }
         let cloned = obj.clone();
 

diff --git a/src/runtime.rs b/src/runtime.rs
@@ -593,8 +593,8 @@ mod tests {
     #[test]
     fn test_protocol_method() {
         let class = test_utils::custom_class();
-        let result: i32 = unsafe {
-            msg_send![class, addNumber:1 toNumber:2]
+        let result = unsafe {
+            msg_send![class, addNumber:1 toNumber:2 => i32]
         };
         assert_eq!(result, 3);
     }

diff --git a/src/test_utils.rs b/src/test_utils.rs
@@ -157,8 +157,8 @@ pub fn custom_subclass() -> &'static Class {
         let mut decl = ClassDecl::new("CustomSubclassObject", superclass).unwrap();
 
         extern fn custom_subclass_get_foo(this: &Object, _cmd: Sel) -> u32 {
-            let foo: u32 = unsafe {
-                msg_send![super(this, custom_class()), foo]
+            let foo = unsafe {
+                msg_send![super(this, custom_class()), foo => u32]
             };
             foo + 2
         }

diff --git a/tests/use_macros.rs b/tests/use_macros.rs
@@ -9,9 +9,9 @@ use objc::runtime::Object;
 fn use_class_and_msg_send() {
     unsafe {
         let cls = class!(NSObject);
-        let obj: *mut Object = msg_send![cls, new];
-        let _hash: usize = msg_send![obj, hash];
-        let _: () = msg_send![obj, release];
+        let obj = msg_send![cls, new => *mut Object];
+        let _hash = msg_send![obj, hash => usize];
+        msg_send![obj, release => ()];
     }
 }