Struct serde::de::IgnoredAny

source ·
pub struct IgnoredAny;
Expand description

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 std::fmt;
use std::marker::PhantomData;

use serde::de::{
    self, Deserialize, DeserializeSeed, Deserializer, IgnoredAny, SeqAccess, Visitor,
};

/// 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

Returns a copy of the value. Read more
Performs copy-assignment from source. Read more
Formats the value using the given formatter. Read more
Returns the “default value” for a type. Read more
Deserialize this value from the given Serde deserializer. Read more
The value produced by this visitor.
Format a message stating what data this Visitor expects to receive. Read more
The input contains a boolean. Read more
The input contains an i64. Read more
The input contains a u64. Read more
The input contains an f64. Read more
The input contains a string. The lifetime of the string is ephemeral and it may be destroyed after this method returns. Read more
The input contains an optional that is absent. Read more
The input contains an optional that is present. Read more
The input contains a newtype struct. Read more
The input contains a unit (). Read more
The input contains a sequence of elements. Read more
The input contains a key-value map. Read more
The input contains a byte array. The lifetime of the byte array is ephemeral and it may be destroyed after this method returns. Read more
The input contains an enum. Read more
The input contains an i8. Read more
The input contains an i16. Read more
The input contains an i32. Read more
The input contains a i128. Read more
The input contains a u8. Read more
The input contains a u16. Read more
The input contains a u32. Read more
The input contains a u128. Read more
The input contains an f32. Read more
The input contains a char. Read more
The input contains a string that lives at least as long as the Deserializer. Read more
The input contains a string and ownership of the string is being given to the Visitor. Read more
The input contains a byte array that lives at least as long as the Deserializer. Read more
The input contains a byte array and ownership of the byte array is being given to the Visitor. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more
Immutably borrows from an owned value. Read more
Mutably borrows from an owned value. Read more
Format an explanation of what data was being expected. Same signature as the Display and Debug traits. Read more

Returns the argument unchanged.

Calls U::from(self).

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

The resulting type after obtaining ownership.
Creates owned data from borrowed data, usually by cloning. Read more
Uses borrowed data to replace owned data, usually by cloning. Read more
The type returned in the event of a conversion error.
Performs the conversion.
The type returned in the event of a conversion error.
Performs the conversion.