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use crate::lib::*;
use crate::de::{
Deserialize, Deserializer, EnumAccess, Error, MapAccess, SeqAccess, VariantAccess, Visitor,
};
/// An efficient way of discarding data from a deserializer.
///
/// Think of this like `serde_json::Value` in that it can be deserialized from
/// any type, except that it does not store any information about the data that
/// gets deserialized.
///
/// ```edition2021
/// use serde::de::{
/// self, Deserialize, DeserializeSeed, Deserializer, IgnoredAny, SeqAccess, Visitor,
/// };
/// use std::fmt;
/// use std::marker::PhantomData;
///
/// /// A seed that can be used to deserialize only the `n`th element of a sequence
/// /// while efficiently discarding elements of any type before or after index `n`.
/// ///
/// /// For example to deserialize only the element at index 3:
/// ///
/// /// ```
/// /// NthElement::new(3).deserialize(deserializer)
/// /// ```
/// pub struct NthElement<T> {
/// n: usize,
/// marker: PhantomData<T>,
/// }
///
/// impl<T> NthElement<T> {
/// pub fn new(n: usize) -> Self {
/// NthElement {
/// n: n,
/// marker: PhantomData,
/// }
/// }
/// }
///
/// impl<'de, T> Visitor<'de> for NthElement<T>
/// where
/// T: Deserialize<'de>,
/// {
/// type Value = T;
///
/// fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
/// write!(
/// formatter,
/// "a sequence in which we care about element {}",
/// self.n
/// )
/// }
///
/// fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
/// where
/// A: SeqAccess<'de>,
/// {
/// // Skip over the first `n` elements.
/// for i in 0..self.n {
/// // It is an error if the sequence ends before we get to element `n`.
/// if seq.next_element::<IgnoredAny>()?.is_none() {
/// return Err(de::Error::invalid_length(i, &self));
/// }
/// }
///
/// // Deserialize the one we care about.
/// let nth = match seq.next_element()? {
/// Some(nth) => nth,
/// None => {
/// return Err(de::Error::invalid_length(self.n, &self));
/// }
/// };
///
/// // Skip over any remaining elements in the sequence after `n`.
/// while let Some(IgnoredAny) = seq.next_element()? {
/// // ignore
/// }
///
/// Ok(nth)
/// }
/// }
///
/// impl<'de, T> DeserializeSeed<'de> for NthElement<T>
/// where
/// T: Deserialize<'de>,
/// {
/// type Value = T;
///
/// fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
/// where
/// D: Deserializer<'de>,
/// {
/// deserializer.deserialize_seq(self)
/// }
/// }
///
/// # fn example<'de, D>(deserializer: D) -> Result<(), D::Error>
/// # where
/// # D: Deserializer<'de>,
/// # {
/// // Deserialize only the sequence element at index 3 from this deserializer.
/// // The element at index 3 is required to be a string. Elements before and
/// // after index 3 are allowed to be of any type.
/// let s: String = NthElement::new(3).deserialize(deserializer)?;
/// # Ok(())
/// # }
/// ```
#[derive(Copy, Clone, Debug, Default, PartialEq)]
pub struct IgnoredAny;
impl<'de> Visitor<'de> for IgnoredAny {
type Value = IgnoredAny;
fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
formatter.write_str("anything at all")
}
#[inline]
fn visit_bool<E>(self, x: bool) -> Result<Self::Value, E> {
let _ = x;
Ok(IgnoredAny)
}
#[inline]
fn visit_i64<E>(self, x: i64) -> Result<Self::Value, E> {
let _ = x;
Ok(IgnoredAny)
}
#[inline]
fn visit_i128<E>(self, x: i128) -> Result<Self::Value, E> {
let _ = x;
Ok(IgnoredAny)
}
#[inline]
fn visit_u64<E>(self, x: u64) -> Result<Self::Value, E> {
let _ = x;
Ok(IgnoredAny)
}
#[inline]
fn visit_u128<E>(self, x: u128) -> Result<Self::Value, E> {
let _ = x;
Ok(IgnoredAny)
}
#[inline]
fn visit_f64<E>(self, x: f64) -> Result<Self::Value, E> {
let _ = x;
Ok(IgnoredAny)
}
#[inline]
fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
where
E: Error,
{
let _ = s;
Ok(IgnoredAny)
}
#[inline]
fn visit_none<E>(self) -> Result<Self::Value, E> {
Ok(IgnoredAny)
}
#[inline]
fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
where
D: Deserializer<'de>,
{
IgnoredAny::deserialize(deserializer)
}
#[inline]
fn visit_newtype_struct<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
where
D: Deserializer<'de>,
{
IgnoredAny::deserialize(deserializer)
}
#[inline]
fn visit_unit<E>(self) -> Result<Self::Value, E> {
Ok(IgnoredAny)
}
#[inline]
fn visit_seq<A>(self, mut seq: A) -> Result<Self::Value, A::Error>
where
A: SeqAccess<'de>,
{
while let Some(IgnoredAny) = tri!(seq.next_element()) {
// Gobble
}
Ok(IgnoredAny)
}
#[inline]
fn visit_map<A>(self, mut map: A) -> Result<Self::Value, A::Error>
where
A: MapAccess<'de>,
{
while let Some((IgnoredAny, IgnoredAny)) = tri!(map.next_entry()) {
// Gobble
}
Ok(IgnoredAny)
}
#[inline]
fn visit_bytes<E>(self, bytes: &[u8]) -> Result<Self::Value, E>
where
E: Error,
{
let _ = bytes;
Ok(IgnoredAny)
}
fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>
where
A: EnumAccess<'de>,
{
tri!(data.variant::<IgnoredAny>()).1.newtype_variant()
}
}
impl<'de> Deserialize<'de> for IgnoredAny {
#[inline]
fn deserialize<D>(deserializer: D) -> Result<IgnoredAny, D::Error>
where
D: Deserializer<'de>,
{
deserializer.deserialize_ignored_any(IgnoredAny)
}
}