firefox/third_party/rust/derive_more-impl/doc/unwrap.md

What #[derive(Unwrap)] generates

When an enum is decorated with #[derive(Unwrap)], for each variant foo in the enum, with fields (a, b, c, ...) a public instance method unwrap_foo(self) -> (a, b, c, ...) is generated. If you don't want the unwrap_foo method generated for a variant, you can put the #[unwrap(ignore)] attribute on that variant. If you want to treat a reference, you can put the #[unwrap(ref)] attribute on the enum declaration or that variant, then unwrap_foo_ref(self) -> (&a, &b, &c, ...) will be generated. You can also use mutable references by putting #[unwrap(ref_mut)].

Example usage

# use derive_more::Unwrap;
# 
# #[derive(Debug, PartialEq)]
#[derive(Unwrap)]
#[unwrap(ref)]
enum Maybe<T> {
    Just(T),
    Nothing,
}

fn main() {
    assert_eq!(Maybe::Just(1).unwrap_just(), 1);

    // Panics if variants are different
    // assert_eq!(Maybe::<()>::Nothing.unwrap_just(), /* panic */);
    // assert_eq!(Maybe::Just(2).unwrap_nothing(), /* panic */);

    assert_eq!((&Maybe::Just(42)).unwrap_just_ref(), &42);
}

What is generated?

The derive in the above example code generates the following code:

# enum Maybe<T> {
#     Just(T),
#     Nothing,
# }
#
impl<T> Maybe<T> {
    pub fn unwrap_nothing(self) -> () {
        match self {
            Maybe::Nothing => (),
            _ => panic!(),
        }
    }
    pub fn unwrap_nothing_ref(&self) -> () {
        match self {
            Maybe::Nothing => (),
            _ => panic!(),
        }
    }
    pub fn unwrap_just(self) -> T {
        match self {
            Maybe::Just(field_0) => field_0,
            _ => panic!(),
        }
    }
    pub fn unwrap_just_ref(&self) -> &T {
        match self {
            Maybe::Just(field_0) => field_0,
            _ => panic!(),
        }
    }
}