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//! This example demonstrates:
//!
//! - The behavior of a derived `FromMeta` implementation for heterogeneous enums
//! (i.e. enums that include a mix of unit, newtype and struct variants).
//! - Using `#[darling(word)]` to specify a unit variant to use when a receiver field
//! is specified without a value (i.e. a unit variant to use for deriving the
//! `FromMeta::from_word` method).
//! - Using `#[darling(default)]` on a receiver field to fall back to `Default::default()`
//! for the enum's value when the receiver field is not specified by the caller.
use darling::{Error, FromDeriveInput, FromMeta};
use syn::parse_quote;
/// A playback volume.
#[derive(Debug, FromMeta, PartialEq, Eq)]
enum Volume {
Normal,
#[darling(word)]
Low,
High,
#[darling(rename = "dB")]
Decibels(u8),
}
impl Default for Volume {
fn default() -> Self {
Volume::Normal
}
}
#[derive(Debug, FromDeriveInput)]
#[darling(attributes(play))]
struct PlayReceiver {
#[darling(default)]
volume: Volume,
}
fn main() {
// `Default::default()` is used when `volume` is not specified.
let missing_volume = PlayReceiver::from_derive_input(&parse_quote! {
#[play]
struct Player;
})
.unwrap();
assert_eq!(Volume::Normal, missing_volume.volume);
// `#[darling(word)]` unit variant is used when `volume` is specified as a word with no value.
let empty_volume = PlayReceiver::from_derive_input(&parse_quote! {
#[play(volume)]
struct Player;
})
.unwrap();
assert_eq!(Volume::Low, empty_volume.volume);
// Specified `volume` value is used when provided.
let unit_variant_volume = PlayReceiver::from_derive_input(&parse_quote! {
#[play(volume(high))]
struct Player;
})
.unwrap();
assert_eq!(Volume::High, unit_variant_volume.volume);
let newtype_volume = PlayReceiver::from_derive_input(&parse_quote! {
#[play(volume(dB = 100))]
struct Player;
})
.unwrap();
assert_eq!(Volume::Decibels(100), newtype_volume.volume);
// Multiple `volume` values result in an error.
let err = PlayReceiver::from_derive_input(&parse_quote! {
#[play(volume(low, dB = 20))]
struct Player;
})
.unwrap_err();
assert_eq!(
err.to_string(),
Error::too_many_items(1).at("volume").to_string()
);
}