Struct IgnoredAny

pub 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)
    }
}

// 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)?;

Trait Implementations

impl Clone for IgnoredAny

fn clone(&self) -> IgnoredAny

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for IgnoredAny

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

Formats the value using the given formatter.

impl Default for IgnoredAny

fn default() -> IgnoredAny

Returns the “default value” for a type.

impl<'de> Deserialize<'de> for IgnoredAny

fn deserialize<D>(deserializer: D) -> Result<IgnoredAny, D::Error>

where D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl PartialEq for IgnoredAny

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

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl<'de> Visitor<'de> for IgnoredAny

type Value = IgnoredAny

The value produced by this visitor.

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

Format a message stating what data this Visitor expects to receive.

fn visit_bool<E>(self, x: bool) -> Result<Self::Value, E>

The input contains a boolean.

fn visit_i64<E>(self, x: i64) -> Result<Self::Value, E>

The input contains an i64.

fn visit_i128<E>(self, x: i128) -> Result<Self::Value, E>

The input contains a i128.

fn visit_u64<E>(self, x: u64) -> Result<Self::Value, E>

The input contains a u64.

fn visit_u128<E>(self, x: u128) -> Result<Self::Value, E>

The input contains a u128.

fn visit_f64<E>(self, x: f64) -> Result<Self::Value, E>

The input contains an f64.

fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>

where E: Error,

The input contains a string. The lifetime of the string is ephemeral and it may be destroyed after this method returns.

fn visit_none<E>(self) -> Result<Self::Value, E>

The input contains an optional that is absent.

fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>

where D: Deserializer<'de>,

The input contains an optional that is present.

fn visit_newtype_struct<D>( self, deserializer: D, ) -> Result<Self::Value, D::Error>

where D: Deserializer<'de>,

The input contains a newtype struct.

fn visit_unit<E>(self) -> Result<Self::Value, E>

The input contains a unit ().

fn visit_seq<A>(self, seq: A) -> Result<Self::Value, A::Error>

where A: SeqAccess<'de>,

The input contains a sequence of elements.

fn visit_map<A>(self, map: A) -> Result<Self::Value, A::Error>

where A: MapAccess<'de>,

The input contains a key-value map.

fn visit_bytes<E>(self, bytes: &[u8]) -> Result<Self::Value, E>

where E: Error,

The input contains a byte array. The lifetime of the byte array is ephemeral and it may be destroyed after this method returns.

fn visit_enum<A>(self, data: A) -> Result<Self::Value, A::Error>

where A: EnumAccess<'de>,

The input contains an enum.

fn visit_i8<E>(self, v: i8) -> Result<Self::Value, E>

where E: Error,

The input contains an i8.

fn visit_i16<E>(self, v: i16) -> Result<Self::Value, E>

where E: Error,

The input contains an i16.

fn visit_i32<E>(self, v: i32) -> Result<Self::Value, E>

where E: Error,

The input contains an i32.

fn visit_u8<E>(self, v: u8) -> Result<Self::Value, E>

where E: Error,

The input contains a u8.

fn visit_u16<E>(self, v: u16) -> Result<Self::Value, E>

where E: Error,

The input contains a u16.

fn visit_u32<E>(self, v: u32) -> Result<Self::Value, E>

where E: Error,

The input contains a u32.

fn visit_f32<E>(self, v: f32) -> Result<Self::Value, E>

where E: Error,

The input contains an f32.

fn visit_char<E>(self, v: char) -> Result<Self::Value, E>

where E: Error,

The input contains a char.

fn visit_borrowed_str<E>(self, v: &'de str) -> Result<Self::Value, E>

where E: Error,

The input contains a string that lives at least as long as the Deserializer.

fn visit_borrowed_bytes<E>(self, v: &'de [u8]) -> Result<Self::Value, E>

where E: Error,

The input contains a byte array that lives at least as long as the Deserializer.

impl Copy for IgnoredAny

impl StructuralPartialEq for IgnoredAny

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference's “Type Layout” chapter for details on type layout guarantees.

Size: 0 bytes