That's pretty neat. I still don't completely understand why #[non_exhaustive] is so desirable in the first place though.

Let's say I am using a crate called zoo-bar. Let's say this crate is not using non-exhaustive.

In my code where I use this crate I do:

let my_workplace = zoo_bar::ZooBar::new();

let mut animal_pens_iter = my_workplace.hungry_animals.iter();

while let Some(ap) = animal_pens_iter.next() {
match ap {
zoo_bar::AnimalPen::Tigers => {
me.go_feed_tigers(&mut raw_meat_that_tigers_like_stock).await?;
}
zoo_bar::AnimalPen::Elephants => {
me.go_feed_elephants(&mut peanut_stock).await?;
}
}
}

I update or upgrade the zoo-bar dependency and there's a new enum variant of AnimalPens called Monkeys.

Great! I get a compile error and I update my code to feed the monkeys.

diff --git a/src/main.rs b/src/main.rs
index 202c10c..425d649 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -10,5 +10,8 @@
zoo_bar::AnimalPen::Elephants => {
me.go_feed_elephants(&mut peanut_stock).await?;
}
+ zoo_bar::AnimalPen::Monkeys => {
+ me.go_feed_monkeys(&mut banana_stock).await?;
+ }
}
}

Now let's say instead that the AnimalPen enum was marked non-exhaustive.

So I'm forced to have a default match arm. In this alternate universe I start off with:

let my_workplace = zoo_bar::ZooBar::new();

let mut animal_pens_iter = my_workplace.hungry_animals.iter();

while let Some(ap) = animal_pens_iter.next() {
match ap {
zoo_bar::AnimalPen::Tigers => {
me.go_feed_tigers(&mut raw_meat_that_tigers_like_stock).await?;
}
zoo_bar::AnimalPen::Elephants => {
me.go_feed_elephants(&mut peanut_stock).await?;
}
_ => {
eprintln!("Whoops! I sure hope someone notices this default match in the logs and goes and updates the code.");
}
}
}

When the monkeys are added, and I update or upgrade the dependency on zoo-bar, I don't notice the warning in the logs right away after we deploy to prod. Because the logs contain too many things no one can go and read everything.

One week passes and then we have a monkey starving incident at work.

After careful review we realize that it was due to the default match arm and we forgot to update our program.

So we learn from the terrible catastrophe with the monkeys and I update my code using the attributes from your link.

diff --git a/src/main.rs b/src/main.rs
index e01fcd1..aab0112 100644
--- a/wp/src/main.rs
+++ b/wp/src/main.rs
@@ -1,3 +1,5 @@
+#![feature(non_exhaustive_omitted_patterns_lint)]
+
use std::error::Error;

#[tokio::main]
@@ -11,6 +13,7 @@ async fn main() -> anyhow::Result<()> {
let mut animal_pens_iter = my_workplace.hungry_animals.iter();

while let Some(ap) = animal_pens_iter.next() {
+ #[warn(non_exhaustive_omitted_patterns)]
match ap {
zoo_bar::AnimalPen::Tigers => {
me.go_feed_tigers(&mut raw_meat_that_tigers_like_stock).await?;
@@ -18,8 +21,12 @@ async fn main() -> anyhow::Result<()> {
zoo_bar::AnimalPen::Elephants => {
me.go_feed_elephants(&mut peanut_stock).await?;
}
+ zoo_bar::AnimalPen::Monkeys => {
+ // Our monkeys died before we started using proper attributes. If they are hungry it means they have turned into zombies :O
+ me.alert_authorities_about_potential_outbreak_of_zombie_monkeys().await?;
+ }
_ => {
- eprintln!("Whoops! I sure hope someone notices this default match in the logs and goes and updates the code.");
+ unreachable!("We have an attribute that is supposed to tell us if there were any unmatched new variants.");
}
}
}

And next time we update or upgrade the crate version to latest, another new variant exists, but thanks to your tip we get a lint warning and we happily update our code so that we won't have more starving animals.

diff --git a/wp/src/main.rs b/wp/src/main.rs
index aab0112..4fc4041 100644
--- a/wp/src/main.rs
+++ b/wp/src/main.rs
@@ -25,6 +25,9 @@ async fn main() -> anyhow::Result<()> {
// Our monkeys died before we started using proper attributes. If they are hungry it means they have turned into zombies :O
me.alert_authorities_about_potential_outbreak_of_zombie_monkeys().await?;
}
+ zoo_bar::AnimalPen::Capybaras => {
+ me.go_feed_capybaras(&mut whatever_the_heck_capybaras_eat_stock).await?;
+ }
_ => {
unreachable!("We have an attribute that is supposed to tell us if there were any unmatched new variants.");
}

But what was the advantage of marking the enum as #[non_exhaustive] in the first place?