clarify how to implement C-CUSTOM-TYPE

[nickolay]

https://rust-lang-nursery.github.io/api-guidelines/type-safety.html#arguments-convey-meaning-through-types-not-bool-or-option-c-custom-type says

Prefer

let w = Widget::new(Small, Round)

How do you define the types to achieve this though? The guideline says "newtypes", but that doesn't seem applicable here - what do you put in a struct Foo(Bar) to make Small available?

This seems what enums are for, but they require much less pleasant syntax: Widget::new(MyEnum::Small, MyOtherEnum::Round).

While writing this I found that you can use Enum::Variant, like so:

enum MySize { Small, Large }
enum MyShape { Round, Square }
fn foo(size: MySize, shape: MyShape) { unimplemented!() }

use MySize::*, MyShape::*;
foo(Small, Round);

- does the example rely on this?

Would it make sense to hint at it in the guideline:

Using custom types makes it easier to expand the options later on, for example by adding an ExtraLarge variant:

enum WidgetSize { Small, Large, ExtraLarge }
use WidgetSize::*;

or is obvious to everyone else?