Make calling throw no longer UB by using "C-unwind"

objc2/Cargo.toml

@@ -40,6 +40,9 @@ malloc = ["malloc_buf"]
 # Uses nightly features to make AutoreleasePool zero-cost even in debug mode
 unstable_autoreleasesafe = []
 
+# Uses the c_unwind feature to make throwing safe. Nightly only
+unstable_c_unwind = []
+
 [dependencies]
 malloc_buf = { version = "1.0", optional = true }
 objc-sys = { path = "../objc-sys", version = "=0.2.0-alpha.0" }

objc2/src/exception.rs

@@ -25,11 +25,19 @@ use crate::rc::{Id, Shared};
 use crate::runtime::Object;
 
 extern "C" {
+    #[cfg(not(feature = "unstable_c_unwind"))]
     fn rust_objc_try_catch_exception(
         f: extern "C" fn(*mut c_void),
         context: *mut c_void,
         error: *mut *mut ffi::objc_object,
     ) -> c_uchar;
+
+    #[cfg(feature = "unstable_c_unwind")]
+    fn rust_objc_try_catch_exception(
+        f: extern "C-unwind" fn(*mut c_void),
+        context: *mut c_void,
+        error: *mut *mut objc_object,
+    ) -> c_uchar;
 }
 
 /// Throws an Objective-C exception.
@@ -57,14 +65,38 @@ pub unsafe fn throw(exception: Option<&Id<Object, Shared>>) -> ! {
 }
 
 unsafe fn try_no_ret<F: FnOnce()>(closure: F) -> Result<(), Option<Id<Object, Shared>>> {
-    extern "C" fn try_objc_execute_closure<F: FnOnce()>(closure: &mut Option<F>) {
-        // This is always passed Some, so it's safe to unwrap
-        let closure = closure.take().unwrap();
-        closure();
-    }
+    #[cfg(not(feature = "unstable_c_unwind"))]
+    let f = {
+        extern "C" fn try_objc_execute_closure<F>(closure: &mut Option<F>)
+        where
+            F: FnOnce(),
+        {
+            // This is always passed Some, so it's safe to unwrap
+            let closure = closure.take().unwrap();
+            closure();
+        }
+
+        let f: extern "C" fn(&mut Option<F>) = try_objc_execute_closure;
+        let f: extern "C" fn(*mut c_void) = unsafe { mem::transmute(f) };
+        f
+    };
+
+    #[cfg(feature = "unstable_c_unwind")]
+    let f = {
+        extern "C-unwind" fn try_objc_execute_closure<F>(closure: &mut Option<F>)
+        where
+            F: FnOnce(),
+        {
+            // This is always passed Some, so it's safe to unwrap
+            let closure = closure.take().unwrap();
+            closure();
+        }
+
+        let f: extern "C-unwind" fn(&mut Option<F>) = try_objc_execute_closure;
+        let f: extern "C-unwind" fn(*mut c_void) = unsafe { mem::transmute(f) };
+        f
+    };
 
-    let f: extern "C" fn(&mut Option<F>) = try_objc_execute_closure;
-    let f: extern "C" fn(*mut c_void) = unsafe { mem::transmute(f) };
     // Wrap the closure in an Option so it can be taken
     let mut closure = Some(closure);
     let context = &mut closure as *mut _ as *mut c_void;

objc2/src/lib.rs

@@ -125,6 +125,7 @@
     feature = "unstable_autoreleasesafe",
     feature(negative_impls, auto_traits)
 )]
+#![cfg_attr(feature = "unstable_c_unwind", feature(c_unwind))]
 #![warn(elided_lifetimes_in_paths)]
 #![warn(missing_docs)]
 #![deny(non_ascii_idents)]