Update sizes.md (#228)

Adding more details in how to control Rust enum size and discriminant values.

Author: igor-petruk

src/enums/sizes.md

@@ -17,8 +17,16 @@ enum Foo {
     B,
 }
 
+#[repr(u32)]
+enum Bar {
+    A,  // 0
+    B = 10000,
+    C,  // 10001
+}
+
 fn main() {
     dbg_size!(Foo);
+    dbg_size!(Bar);
     dbg_size!(bool);
     dbg_size!(Option<bool>);
     dbg_size!(&i32);
@@ -32,6 +40,7 @@ fn main() {
 
 Key Points: 
  * Internally Rust is using a field (discriminant) to keep track of the enum variant.
+ * `Bar` enum demonstrates that there is a way to control the discriminant value and type. If `repr` is removed, the discriminant type takes 2 bytes, becuase 10001 fits 2 bytes.
  * As a niche optimization an enum discriminant is merged with the pointer so that `Option<&Foo>` is the same size as `&Foo`.
  * `Option<bool>` is another example of tight packing.
  * For [some types](https://doc.rust-lang.org/std/option/#representation), Rust guarantees that `size_of::<T>()` equals `size_of::<Option<T>>()`.