Make objc_sys::IMP nullable

Probably all of the underlying APIs can handle NULL values, so not having this possibility is restricting.

And most of them can sometimes _return_ a NULL value, so not handling that is unsound!

objc/src/declare.rs

@@ -182,8 +182,12 @@ impl ClassDecl {
         );
 
         let types = method_type_encoding(&F::Ret::ENCODING, encs);
-        let success =
-            runtime::class_addMethod(self.cls as _, sel.as_ptr() as _, func.imp(), types.as_ptr());
+        let success = runtime::class_addMethod(
+            self.cls as _,
+            sel.as_ptr() as _,
+            Some(func.imp()),
+            types.as_ptr(),
+        );
         assert!(success != NO, "Failed to add method {:?}", sel);
     }
 
@@ -213,8 +217,12 @@ impl ClassDecl {
 
         let types = method_type_encoding(&F::Ret::ENCODING, encs);
         let metaclass = (*self.cls).metaclass() as *const _ as *mut _;
-        let success =
-            runtime::class_addMethod(metaclass, sel.as_ptr() as _, func.imp(), types.as_ptr());
+        let success = runtime::class_addMethod(
+            metaclass,
+            sel.as_ptr() as _,
+            Some(func.imp()),
+            types.as_ptr(),
+        );
         assert!(success != NO, "Failed to add class method {:?}", sel);
     }
 

objc/src/message/gnustep.rs

@@ -16,7 +16,7 @@ where
 
     let receiver = obj as *mut T as *mut Object;
     let msg_send_fn = objc_msg_lookup(receiver as *mut _, sel.as_ptr() as *const _);
-    objc_try!({ A::invoke(msg_send_fn, receiver, sel, args) })
+    objc_try!({ A::invoke(msg_send_fn.expect("Null IMP"), receiver, sel, args) })
 }
 
 pub unsafe fn send_super_unverified<T, A, R>(
@@ -36,5 +36,5 @@ where
         super_class: superclass as *const Class as *const _,
     };
     let msg_send_fn = objc_msg_lookup_super(&sup, sel.as_ptr() as *const _);
-    objc_try!({ A::invoke(msg_send_fn, receiver, sel, args) })
+    objc_try!({ A::invoke(msg_send_fn.expect("Null IMP"), receiver, sel, args) })
 }

objc/src/runtime.rs

@@ -60,7 +60,7 @@ pub struct Protocol(objc_sys::Protocol);
 pub struct Object(objc_sys::objc_object);
 
 /// A pointer to the start of a method implementation.
-pub type Imp = objc_sys::IMP;
+pub type Imp = unsafe extern "C" fn();
 
 impl Sel {
     /// Registers a method with the Objective-C runtime system,
@@ -189,7 +189,7 @@ impl Method {
 
     /// Returns the implementation of self.
     pub fn implementation(&self) -> Imp {
-        unsafe { method_getImplementation(self.as_ptr()) }
+        unsafe { method_getImplementation(self.as_ptr()).expect("Null IMP") }
     }
 }
 

objc_sys/src/class.rs

@@ -113,7 +113,7 @@ extern "C" {
         name: *const objc_selector,
         imp: IMP,
         types: *const c_char,
-    ) -> Option<IMP>;
+    ) -> IMP;
     pub fn class_replaceProperty(
         cls: *mut objc_class,
         name: *const c_char,

objc_sys/src/various.rs

@@ -10,10 +10,10 @@ pub struct objc_ivar {
     _p: OpaqueData,
 }
 
-/// A pointer to the start of a method implementation.
+/// A nullable pointer to the start of a method implementation.
 ///
-/// This must be non-null. Use `Option<IMP>` when nullability is desired.
-pub type IMP = unsafe extern "C" fn();
+/// Not all APIs are guaranteed to take NULL values; read the docs!
+pub type IMP = Option<unsafe extern "C" fn()>;
 
 /// Not available on macOS x86.
 ///