Struct IgnoredAny

struct IgnoredAny

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.

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(())
# }

Implementations

impl Clone for IgnoredAny

fn clone(self: &Self) -> IgnoredAny

impl Copy for IgnoredAny

impl Debug for IgnoredAny

fn fmt(self: &Self, f: &mut Formatter<'_>) -> Result

impl Default for IgnoredAny

fn default() -> IgnoredAny

impl Freeze for IgnoredAny

impl PartialEq for IgnoredAny

fn eq(self: &Self, other: &IgnoredAny) -> bool

impl RefUnwindSafe for IgnoredAny

impl Send for IgnoredAny

impl StructuralPartialEq for IgnoredAny

impl Sync for IgnoredAny

impl Unpin for IgnoredAny

impl UnsafeUnpin for IgnoredAny

impl UnwindSafe for IgnoredAny

impl<'de> Deserialize for IgnoredAny

fn deserialize<D>(deserializer: D) -> Result<IgnoredAny, <D as >::Error>
where
    D: Deserializer<'de>

impl<'de> Visitor for IgnoredAny

fn expecting(self: &Self, formatter: &mut Formatter<'_>) -> Result
fn visit_bool<E>(self: Self, x: bool) -> Result<<Self as >::Value, E>
fn visit_i64<E>(self: Self, x: i64) -> Result<<Self as >::Value, E>
fn visit_i128<E>(self: Self, x: i128) -> Result<<Self as >::Value, E>
fn visit_u64<E>(self: Self, x: u64) -> Result<<Self as >::Value, E>
fn visit_u128<E>(self: Self, x: u128) -> Result<<Self as >::Value, E>
fn visit_f64<E>(self: Self, x: f64) -> Result<<Self as >::Value, E>
fn visit_str<E>(self: Self, s: &str) -> Result<<Self as >::Value, E>
where
    E: Error
fn visit_none<E>(self: Self) -> Result<<Self as >::Value, E>
fn visit_some<D>(self: Self, deserializer: D) -> Result<<Self as >::Value, <D as >::Error>
where
    D: Deserializer<'de>
fn visit_newtype_struct<D>(self: Self, deserializer: D) -> Result<<Self as >::Value, <D as >::Error>
where
    D: Deserializer<'de>
fn visit_unit<E>(self: Self) -> Result<<Self as >::Value, E>
fn visit_seq<A>(self: Self, seq: A) -> Result<<Self as >::Value, <A as >::Error>
where
    A: SeqAccess<'de>
fn visit_map<A>(self: Self, map: A) -> Result<<Self as >::Value, <A as >::Error>
where
    A: MapAccess<'de>
fn visit_bytes<E>(self: Self, bytes: &[u8]) -> Result<<Self as >::Value, E>
where
    E: Error
fn visit_enum<A>(self: Self, data: A) -> Result<<Self as >::Value, <A as >::Error>
where
    A: EnumAccess<'de>

impl<'de, T> Expected for IgnoredAny

fn fmt(self: &Self, formatter: &mut Formatter<'_>) -> Result

impl<T> Any for IgnoredAny

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for IgnoredAny

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for IgnoredAny

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> CloneToUninit for IgnoredAny

unsafe fn clone_to_uninit(self: &Self, dest: *mut u8)

impl<T> DeserializeOwned for IgnoredAny

impl<T> From for IgnoredAny

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> ToOwned for IgnoredAny

fn to_owned(self: &Self) -> T
fn clone_into(self: &Self, target: &mut T)

impl<T, U> Into for IgnoredAny

fn into(self: Self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of [From]<T> for U chooses to do.

impl<T, U> TryFrom for IgnoredAny

fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

impl<T, U> TryInto for IgnoredAny

fn try_into(self: Self) -> Result<U, <U as TryFrom<T>>::Error>