Fix the definition of CFAllocatorContext

The function pointers can be null so should be wrapped in Option.

This struct was basically unusable before so I think we should
be fine making this change while only bumping the patch version.
I was unable to find any uses of it on github.

core-foundation-sys/Cargo.toml

@@ -3,7 +3,7 @@ name = "core-foundation-sys"
 description = "Bindings to Core Foundation for macOS"
 homepage = "https://github.com/servo/core-foundation-rs"
 repository = "https://github.com/servo/core-foundation-rs"
-version = "0.8.1"
+version = "0.8.2"
 authors = ["The Servo Project Developers"]
 license = "MIT / Apache-2.0"
 build = "build.rs"

core-foundation-sys/src/base.rs

@@ -71,13 +71,13 @@ pub type CFAllocatorPreferredSizeCallBack = extern "C" fn(size: CFIndex, hint: C
 pub struct CFAllocatorContext {
     pub version: CFIndex,
     pub info: *mut c_void,
-    pub retain: CFAllocatorRetainCallBack,
-    pub release: CFAllocatorReleaseCallBack,
-    pub copyDescription: CFAllocatorCopyDescriptionCallBack,
-    pub allocate: CFAllocatorAllocateCallBack,
-    pub reallocate: CFAllocatorReallocateCallBack,
-    pub deallocate: CFAllocatorDeallocateCallBack,
-    pub preferredSize: CFAllocatorPreferredSizeCallBack
+    pub retain: Option<CFAllocatorRetainCallBack>,
+    pub release: Option<CFAllocatorReleaseCallBack>,
+    pub copyDescription: Option<CFAllocatorCopyDescriptionCallBack>,
+    pub allocate: Option<CFAllocatorAllocateCallBack>,
+    pub reallocate: Option<CFAllocatorReallocateCallBack>,
+    pub deallocate: Option<CFAllocatorDeallocateCallBack>,
+    pub preferredSize: Option<CFAllocatorPreferredSizeCallBack>
 }
 
 /// Trait for all types which are Core Foundation reference types.