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//! Module for [`DeserializeAs`][] implementations //! //! The module contains the [`DeserializeAs`][] trait and helper code. //! Additionally, it contains implementations of [`DeserializeAs`][] for types defined in the Rust Standard Library or this crate. //! //! You can find more details on how to implement this trait for your types in the documentation of the [`DeserializeAs`][] trait and details about the usage in the [user guide][]. //! //! [user guide]: crate::guide pub(crate) mod impls; use super::*; /// A **data structure** that can be deserialized from any data format supported by Serde, analogue to [`Deserialize`]. /// /// The trait is analogue to the [`serde::Deserialize`][`Deserialize`] trait, with the same meaning of input and output arguments. /// It can and should the implemented using the same code structure as the [`Deserialize`] trait. /// As such, the same advice for [implementing `Deserialize`][impl-deserialize] applies here. /// /// # Differences to [`Deserialize`] /// /// The trait is only required for container-like types or types implementing specific conversion functions. /// Container-like types are [`Vec`][], [`BTreeMap`][], but also [`Option`][] and [`Box`][]. /// Conversion types deserialize into a different Rust type. /// For example, [`DisplayFromStr`] uses the [`FromStr`] trait after deserializing a string and [`DurationSeconds`] creates a [`Duration`] from either String or integer values. /// /// This code shows how to implement [`Deserialize`] for [`Box`]: /// /// ```rust,ignore /// impl<'de, T: Deserialize<'de>> Deserialize<'de> for Box<T> { /// fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> /// where /// D: Deserializer<'de>, /// { /// Ok(Box::new(Deserialize::deserialize(deserializer)?)) /// } /// } /// ``` /// /// and this code shows how to do the same using [`DeserializeAs`][]: /// /// ```rust,ignore /// impl<'de, T, U> DeserializeAs<'de, Box<T>> for Box<U> /// where /// U: DeserializeAs<'de, T>, /// { /// fn deserialize_as<D>(deserializer: D) -> Result<Box<T>, D::Error> /// where /// D: Deserializer<'de>, /// { /// Ok(Box::new( /// DeserializeAsWrap::<T, U>::deserialize(deserializer)?.into_inner(), /// )) /// } /// } /// ``` /// /// It uses two type parameters, `T` and `U` instead of only one and performs the deserialization step using the `DeserializeAsWrap` type. /// The `T` type is the on the Rust side after deserialization, whereas the `U` type determines how the value will be deserialized. /// These two changes are usually enough to make a container type implement [`DeserializeAs`][]. /// /// [`BTreeMap`]: std::collections::BTreeMap /// [`Duration`]: std::time::Duration /// [`FromStr`]: std::str::FromStr /// [impl-deserialize]: https://serde.rs/impl-deserialize.html pub trait DeserializeAs<'de, T>: Sized { /// Deserialize this value from the given Serde deserializer. fn deserialize_as<D>(deserializer: D) -> Result<T, D::Error> where D: Deserializer<'de>; } /// Helper type to implement [`DeserializeAs`] for container-like types. #[derive(Debug)] pub struct DeserializeAsWrap<T, U> { value: T, marker: PhantomData<U>, } impl<'de, T, U> DeserializeAsWrap<T, U> { /// Return the inner value of type `T`. pub fn into_inner(self) -> T { self.value } } impl<'de, T, U> Deserialize<'de> for DeserializeAsWrap<T, U> where U: DeserializeAs<'de, T>, { fn deserialize<D>(deserializer: D) -> Result<Self, D::Error> where D: Deserializer<'de>, { U::deserialize_as(deserializer).map(|value| Self { value, marker: PhantomData, }) } }