Diagnostics: Suggest Unit Variant Enum in match

[jamesmunns]

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=5a6b1a3497842e4ddc883b2345febd8f

#[derive(PartialEq, Debug)]
enum FarmAnimal {
    Worm,
    Cow,
    Bull,
    Chicken { num_eggs: usize },
    Dog { name: String },
}

fn what_does_the_animal_say(animal: &FarmAnimal) {

    /* TODO: fill in the match statement below to make this code compile */
    
    let noise = match animal {
        FarmAnimal::Cow(_) => "moo".to_string(),
        // /* Chicken      */ => "cluck, cluck!".to_string(),
        // /* Dog          */  => format!("woof, woof! I am {}!", name),
        // /* Worm– or all silent animals?*/ => "-- (silence)".to_string(),
        _ => todo!()
    };
    
    /* Bonus task: Give Dogs named Lassie a different output */
    
    println!("{:?} says: {:?}", animal, noise);
}

fn main() {
    what_does_the_animal_say(
        &FarmAnimal::Dog {
            name: "Lassie".to_string()
    });
    what_does_the_animal_say(&FarmAnimal::Cow);
    what_does_the_animal_say(&FarmAnimal::Bull);
    what_does_the_animal_say(&FarmAnimal::Chicken{num_eggs: 3});
    what_does_the_animal_say(&FarmAnimal::Worm);

    /*
    Output should be:
    
    Dog { name: "Lassie" } says: "woof, woof! I am Lassie!"
    Cow says: "moo"
    Bull says: "moo"
    Chicken { num_eggs: 3 } says: "cluck, cluck!"
    Worm says: "-- (silence)"
    
    */
}

The current output is:

error[E0532]: expected tuple struct or tuple variant, found unit variant `FarmAnimal::Cow`
  --> src/main.rs:15:9
   |
15 |         FarmAnimal::Cow(_) => "moo".to_string(),
   |         ^^^^^^^^^^^^^^^ not a tuple struct or tuple variant

error: aborting due to previous error

For more information about this error, try `rustc --explain E0532`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

Ideally the output should look like:

I'd like it to say "try to use a tuple unit variant like "FarmAnimal::Cow" instead".

For example, here's what happens if you use Tuple syntax instead of Struct syntax:

error[E0532]: expected tuple struct or tuple variant, found struct variant `FarmAnimal::Chicken`
  --> src/main.rs:16:9
   |
6  |     Chicken { num_eggs: usize },
   |     --------------------------- `FarmAnimal::Chicken` defined here
...
16 |         FarmAnimal::Chicken(x)  => "cluck, cluck!".to_string(),
   |         ^^^^^^^^^^^^^^^^^^^^^^ help: use struct pattern syntax instead: `FarmAnimal::Chicken { num_eggs }`

error: aborting due to 2 previous errors

So I'd like the new diagnostic to look something like:

error[Exxxx]: expected tuple struct or tuple variant, found unit variant `FarmAnimal::Cow`
  --> src/main.rs:16:9
   |
6  |     Cow,
   |     --- `FarmAnimal::Cow` defined here
...
16 |         FarmAnimal::Cow(_)  => "moo".to_string(),
   |         ^^^^^^^^^^^^^^^^^^ help: use unit pattern syntax instead: `FarmAnimal::Cow`

[jamesmunns]

This is an example Ferrous Systems uses in our trainings, and some of our participants have tripped over this lack of suggestion here.

The compiler DOES provide diagnostics when you use a struct syntax instead of a tuple syntax.

[leonardo-m]

Alternatively the help could suggest a more direct: "Remove the parenthesis here".

[ABouttefeux]

@rustbot claim

[jamesmunns]

I just wanted to mention @ABouttefeux, we wrote a little bit of context and details from Ferrous' perspective on this fix, and I wanted to thank you personally for implementing this diagnostics improvement so quickly! Let me know if you have a blog or twitter you'd like us to link to (rather than your GH profile).

[ABouttefeux]

Hello, linking to my GH profile is fine. I did not know I would contribute to such an interesting learning experience.
Anyway thank you for mentioning me in your blog post 😄.