Derive macro to generate boilerplate on unit variants enum types.

The macro generates the following methods:

• as_str - returns a string representation of the target enum variant;
• as_str_abbr - returns an abbreviated string representation of the target enum variant;
• iter_variants - returns an iterator over target enum variants (owned values);
• iter_variants_as_str - returns an iterator over string representations of the target enum variants (&'static str values);
• iter_variants_as_str_abbr - returns an iterator over abbreviated string representations of the enum variants (&'static str values);
• variants_list_str - returns a list of quoted (double-quotes) and comma separated string representations of the enum variants;
• variants_list_str_abbr - returns a list of of quoted (double-quotes) and comma separated abbreviated string representation of the enum variants.

Enum level attributes

The macro exposes the following enum outer attributes (i.e. attributes to be applied to the enum type the macro is being derived on):

• rename - customizes the string representation of each variant;
• rename_abbr - customizes the abbreviated string representation of each variant;
• display - generates a Display trait implementation based on the string representation provided by the generated as_str method;
• from_str - generates a FromStr trait implementation based on the string or abbreviated string representation provided by the generated as_str and as_str_abbr methods respectively.

Valid rename and rename_abbr customization strategies are:

• uppercase - makes the (abbreviated) string representation uppercase;
• lowercase - makes the (abbreviated) string representation lowercase.

Examples

#[derive(Variants)]
#[variants(rename(uppercase))]
enum CardinalDirection {
    North,
    East,
    South,
    West,
}

assert_eq!("NORTH", CardinalDirection::North.as_str());
assert_eq!("EAST", CardinalDirection::East.as_str());
assert_eq!("SOUTH", CardinalDirection::South.as_str());
assert_eq!("WEST", CardinalDirection::West.as_str());

assert_eq!("NOR", CardinalDirection::North.as_str_abbr());
assert_eq!("EAS", CardinalDirection::East.as_str_abbr());
assert_eq!("SOU", CardinalDirection::South.as_str_abbr());
assert_eq!("WES", CardinalDirection::West.as_str_abbr());

#[derive(Variants)]
#[variants(rename(lowercase), rename_abbr(uppercase))]
enum State {
    Active,
    Inactive,
    Disabled,
}

assert_eq!("active", State::Active.as_str());
assert_eq!("inactive", State::Inactive.as_str());
assert_eq!("disabled", State::Disabled.as_str());

assert_eq!("ACT", State::Active.as_str_abbr());
assert_eq!("INA", State::Inactive.as_str_abbr());
assert_eq!("DIS", State::Disabled.as_str_abbr());

#[derive(Variants)]
#[variants(display)]
enum Season {
    Spring,
    Summer,
    Autumn,
    Winter,
}

assert_eq!(String::from("Spring"), Season::Spring.to_string());
assert_eq!(String::from("Summer"), Season::Summer.to_string());
assert_eq!(String::from("Autumn"), Season::Autumn.to_string());
assert_eq!(String::from("Winter"), Season::Winter.to_string());

assert_eq!(String::from("Spring"), format!("{}", Season::Spring));
assert_eq!(String::from("Summer"), format!("{}", Season::Summer));
assert_eq!(String::from("Autumn"), format!("{}", Season::Autumn));
assert_eq!(String::from("Winter"), format!("{}", Season::Winter));

#[derive(Variants)]
#[variants(from_str)]
enum Priority {
    Low,
    Medium,
    High,
    Critical,
}

assert_eq!(Ok(Priority::Low), FromStr::<Priority>::from_str("Low"));
assert_eq!(Ok(Priority::Medium), FromStr::<Priority>::from_str("Medium"));
assert_eq!(Ok(Priority::High), FromStr::<Priority>::from_str("High"));
assert_eq!(Ok(Priority::Critical), FromStr::<Priority>::from_str("Critical"));

assert_eq!(Ok(Priority::Low), FromStr::<Priority>::from_str("Low"));
assert_eq!(Ok(Priority::Medium), FromStr::<Priority>::from_str("Med"));
assert_eq!(Ok(Priority::High), FromStr::<Priority>::from_str("Hig"));
assert_eq!(Ok(Priority::Critical), FromStr::<Priority>::from_str("Cri"));

assert_eq!(Err(ParsePriorityError), FromStr::<Priority>::from_str("invalid"));

Variant level attributes

The macro exposes the following variant attributes:

• skip - excludes the marked variant from iteration and listing;
• rename - customizes the string representation of the marked variant;
• rename_abbr - customizes the abbreviated string representation of the marked variant.

Valid rename and rename_abbr customization strategies are:

• "..." (string literal) - overrides the string representation with a custom string;
• uppercase - makes the (abbreviated) string representation uppercase;
• lowercase - makes the (abbreviated) string representation lowercase.

For custom string overrides:

• #[variants(rename = "...")] is equivalent to #[variants(rename("..."))];
• #[variants(rename_abbr = "...")] is equivalent to #[variants(rename_abbr("..."))];

both are valid, supported formats.

Examples

#[derive(Variants)]
enum Format {
    Xml,
    Csv,
    #[variants(rename("plain-text"), rename_abbr = "txt")]
    PlainText,
}

assert_eq!("Xml", Format::Xml.as_str());
assert_eq!("Csv", Format::Csv.as_str());
assert_eq!("plain-text", Format::PlainText.as_str());

assert_eq!("Xml", Format::Xml.as_str_abbr());
assert_eq!("Csv", Format::Csv.as_str_abbr());
assert_eq!("txt", Format::PlainText.as_str_abbr());

String representation renaming priority

To produce string representations of enum variants, renaming can be applied at both the type level and variant level. The string representation of each variant is obtained by applying rename strategies following a priority-based fallback approach:

1. Variant-level attribute (highest priority) - usese the string produced by the rename strategy from the #[variants(rename(...))] attribute, if one has been specified for the variant;
2. Type-level attribute (fallback) - uses the string produced by the rename strategy from the #[variants(rename(...))] attribute, if one has been specified for the type;
3. No renaming (default) - converts the variant identifier to a string if neither the type-level nor the variant-level rename attribute has been specified.

Abbreviated string representation renaming priority

To produce abbreviated string representations of the enum variants, renaming can be applied at both the type level and the variant level. The abbreviated string representation of each variant is obtained by applying rename strategies following a priority-based fallback approach:

1. Variant-level attribute (highest priority) - uses the abbreviated string produced by the rename strategy from the #[variants(rename_abbr(...))] attribute, if one has been specified for the variant;
2. Type-level attribute (fallback) - uses the string produced by the rename strategy from the #[variants(rename(...))] attribute, if one has been specified for the type;
3. No renaming (default) - abbreviates the full length string representation of the variant as is, without applying any renaming strategy.

Likewise, the renaming follows a priority-based fallback approach to determine the full length string representation before applying the abbreviation:

1. Variant-level attribute (highest priority) - uses the string produced by the rename strategy from the #[variants(rename(...))] attribute, if one has been specified for the type;
2. Type-level attribute (fallback) - uses the string produced by the rename strategy from the #[variants(rename(...))] attribute, if one has been specified for the type;
3. No renaming (default) - converts the variant identifier to a string if neither the type-level nor the variant-level rename attribute has been specified.

Errors

The macro will produce a compile error if:

• derived on struct types;
• derived on union types;
• derived on enum types with any named field variants;
• derived on enum types with any unnamed field (i.e. tuple) variants;
• derived on enum types with any newtype variants;
• the rename variant-level attribute is passed any other value than a string literal, uppercase or lowercase;
• the rename_abbr variant-level attribute is passed any other value than a string literal, uppercase or lowercase;
• the rename type-level attribute is passed any other value than uppercase or lowercase;
• the rename_abbr type-level attribute is passed any other value than uppercase or lowercase.

Notes

Deriving Variants on type automatically implements Clone and Copy for such type. This means that deriving either trait on a type that also derives Variants will result in a "conflicting implementations" compilation error.

Examples

#[derive(Variants, Debug, PartialEq, Eq)]
#[variants(display, from_str)]
enum Weekday {
    #[variants(skip)]
    Monday,
    #[variants(rename = "DayAfterMonday", rename_abbr = "tue")]
    Tuesday,
    #[variants(rename_abbr = "wed")]
    Wednesday,
    #[variants(rename = "Giovedì", rename_abbr(lowercase))]
    Thursday,
    Friday,
    Saturday,
    Sunday,
}

// Monday has been marked as `skip`, iterator will yield 6 values.
assert_eq!(6, Weekday::iter_variants().count());

assert_eq!("Monday", Weekday::Monday.as_str());
assert_eq!("DayAfterMonday", Weekday::Tuesday.as_str());
assert_eq!("Wednesday", Weekday::Wednesday.as_str());
assert_eq!("Giovedì", Weekday::Thursday.as_str());
assert_eq!("Friday", Weekday::Friday.as_str());
assert_eq!("Saturday", Weekday::Saturday.as_str());
assert_eq!("Sunday", Weekday::Sunday.as_str());

assert_eq!("Mon", Weekday::Monday.as_str());
assert_eq!("tue", Weekday::Tuesday.as_str());
assert_eq!("wed", Weekday::Wednesday.as_str());
assert_eq!("gio", Weekday::Thursday.as_str());
assert_eq!("Fri", Weekday::Friday.as_str());
assert_eq!("Sat", Weekday::Saturday.as_str());
assert_eq!("Sun", Weekday::Sunday.as_str());

// The enum has been marked as `display`, so `std::fmt::Display` implementation is available.
assert_eq!(String::from("Monday"), Weekday::Monday.to_string());
assert_eq!(String::from("DayAfterMonday"), Weekday::Tuesday.to_string());
assert_eq!(String::from("Wednesday"), Weekday::Wednesday.to_string());
assert_eq!(String::from("Giovedì"), Weekday::Thursday.to_string());
assert_eq!(String::from("Friday"), Weekday::Friday.to_string());
assert_eq!(String::from("Saturday"), Weekday::Saturday.to_string());
assert_eq!(String::from("Sunday"), Weekday::Sunday.to_string());

assert_eq!(String::from("Monday"), format!("{}", Weekday::Monday));
assert_eq!(String::from("DayAfterMonday"), format!("{}", Weekday::Tuesday));
assert_eq!(String::from("Wednesday"), format!("{}", Weekday::Wednesday));
assert_eq!(String::from("Giovedì"), format!("{}", Weekday::Thursday));
assert_eq!(String::from("Friday"), format!("{}", Weekday::Friday));
assert_eq!(String::from("Saturday"), format!("{}", Weekday::Saturday));
assert_eq!(String::from("Sunday"), format!("{}", Weekday::Sunday));

let mut weekdays = Weekday::iter_variants();
assert_eq!(Some(Weekday::Tuesday), weekdays.next());
assert_eq!(Some(Weekday::Wednesday), weekdays.next());
assert_eq!(Some(Weekday::Thursday), weekdays.next());
assert_eq!(Some(Weekday::Friday), weekdays.next());
assert_eq!(Some(Weekday::Saturday), weekdays.next());
assert_eq!(Some(Weekday::Sunday), weekdays.next());
assert_eq!(None, weekdays.next());

let mut weekdays_as_str = Weekday::iter_variants_as_str();
assert_eq!(Some("DayAfterMonday"), weekdays_as_str.next());
assert_eq!(Some("Wednesday"), weekdays_as_str.next());
assert_eq!(Some("Giovedì"), weekdays_as_str.next());
assert_eq!(Some("Friday"), weekdays_as_str.next());
assert_eq!(Some("Saturday"), weekdays_as_str.next());
assert_eq!(Some("Sunday"), weekdays_as_str.next());
assert_eq!(None, weekdays.next());
let mut weekdays_as_str_abbr = Weekday::iter_variants_as_str_abbr();
assert_eq!(Some("tue"), weekdays_as_str_abbr.next());
assert_eq!(Some("wed"), weekdays_as_str_abbr.next());
assert_eq!(Some("gio"), weekdays_as_str_abbr.next());
assert_eq!(Some("Fri"), weekdays_as_str_abbr.next());
assert_eq!(Some("Sat"), weekdays_as_str_abbr.next());
assert_eq!(Some("Sun"), weekdays_as_str_abbr.next());
assert_eq!(None, weekdays.next());

assert_eq!(
    "\"DayAfterMonday\", \"Wednesday\", \"Giovedì\", \"Friday\", \"Saturday\", \"Sunday\"",
    Weekday::variants_list_str(),
);

assert_eq!(
    "\"tue\", \"wed\", \"gio\", \"Fri\", \"Sat\", \"Sun\"",
    Weekday::variants_list_str_abbr(),
);

// The enum has been marked as `from_str`, so `std::str::FromStr` implementation is available.
assert_eq!(Ok(Weekday::Monday), FromStr::<Weekday>::from_str("Monday"));
assert_eq!(Ok(Weekday::Tuesday), FromStr::<Weekday>::from_str("DayAfterMonday"));
assert_eq!(Ok(Weekday::Wednesday), FromStr::<Weekday>::from_str("Wednesday"));
assert_eq!(Ok(Weekday::Thursday), FromStr::<Weekday>::from_str("Giovedì"));
assert_eq!(Ok(Weekday::Friday), FromStr::<Weekday>::from_str("Friday"));
assert_eq!(Ok(Weekday::Saturday), FromStr::<Weekday>::from_str("Saturday"));
assert_eq!(Ok(Weekday::Sunday), FromStr::<Weekday>::from_str("Sunday"));

assert_eq!(Ok(Weekday::Monday), FromStr::<Weekday>::from_str("Mon"));
assert_eq!(Ok(Weekday::Tuesday), FromStr::<Weekday>::from_str("tue"));
assert_eq!(Ok(Weekday::Wednesday), FromStr::<Weekday>::from_str("wed"));
assert_eq!(Ok(Weekday::Thursday), FromStr::<Weekday>::from_str("gio"));
assert_eq!(Ok(Weekday::Friday), FromStr::<Weekday>::from_str("Fri"));
assert_eq!(Ok(Weekday::Saturday), FromStr::<Weekday>::from_str("Sat"));
assert_eq!(Ok(Weekday::Sunday), FromStr::<Weekday>::from_str("Sun"));

assert_eq!(Err(ParseWeekdayError), FromStr::<Weekday>::from_str("invalid"));