Remove warnings for zero-sized structs #89.

This follows the servo pattern mentioned in
https://github.com/rust-lang/rust/issues/27303 to replace the zero-sized
structs with an `enum`.

src/base.rs

@@ -28,7 +28,9 @@ pub type CGFloat = libc::c_float;
 
 pub type CGError = libc::int32_t;
 
-pub type CGAffineTransform = ();
+// This is an enum due to zero-sized types warnings.
+// For more details see https://github.com/rust-lang/rust/issues/27303
+pub enum CGAffineTransform {}
 
 pub const kCGImageAlphaNone: u32 = 0;
 pub const kCGImageAlphaPremultipliedLast: u32 = 1;

src/context.rs

@@ -28,8 +28,9 @@ pub enum CGTextDrawingMode {
     CGTextClip
 }
 
-#[repr(C)]
-pub struct __CGContext;
+// This is an enum due to zero-sized types warnings.
+// For more details see https://github.com/rust-lang/rust/issues/27303
+pub enum __CGContext {}
 
 pub type CGContextRef = *const __CGContext;
 

src/font.rs

@@ -17,8 +17,9 @@ use std::ptr;
 
 pub type CGGlyph = libc::c_ushort;
 
-#[repr(C)]
-pub struct __CGFont;
+// This is an enum due to zero-sized types warnings.
+// For more details see https://github.com/rust-lang/rust/issues/27303
+pub enum __CGFont {}
 
 pub type CGFontRef = *const __CGFont;