Manually implementing Deserialize for a struct

Only when derive is not getting the job done.

The Deserialize impl below corresponds to the following struct:

struct Duration {
    secs: u64,
    nanos: u32,
}

Deserializing a struct is somewhat more complicated than deserializing a map in order to avoid allocating a String to hold the field names. Instead there is a Field enum which is deserialized from a &str.

The implementation supports two possible ways that a struct may be represented by a data format: as a seq like in Bincode, and as a map like in JSON.

use std::fmt;

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

impl<'de> Deserialize<'de> for Duration {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where D: Deserializer<'de>
    {
        enum Field { Secs, Nanos };

        // This part could also be generated independently by:
        //
        //    #[derive(Deserialize)]
        //    #[serde(field_identifier, rename_all = "lowercase")]
        //    enum Field { Secs, Nanos }
        impl<'de> Deserialize<'de> for Field {
            fn deserialize<D>(deserializer: D) -> Result<Field, D::Error>
                where D: Deserializer<'de>
            {
                struct FieldVisitor;

                impl<'de> Visitor<'de> for FieldVisitor {
                    type Value = Field;

                    fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                        formatter.write_str("`secs` or `nanos`")
                    }

                    fn visit_str<E>(self, value: &str) -> Result<Field, E>
                        where E: de::Error
                    {
                        match value {
                            "secs" => Ok(Field::Secs),
                            "nanos" => Ok(Field::Nanos),
                            _ => Err(de::Error::unknown_field(value, FIELDS)),
                        }
                    }
                }

                deserializer.deserialize_identifier(FieldVisitor)
            }
        }

        struct DurationVisitor;

        impl<'de> Visitor<'de> for DurationVisitor {
            type Value = Duration;

            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
                formatter.write_str("struct Duration")
            }

            fn visit_seq<V>(self, mut seq: V) -> Result<Duration, V::Error>
                where V: SeqAccess<'de>
            {
                let secs = seq.next_element()?
                              .ok_or_else(|| de::Error::invalid_length(0, &self))?;
                let nanos = seq.next_element()?
                               .ok_or_else(|| de::Error::invalid_length(1, &self))?;
                Ok(Duration::new(secs, nanos))
            }

            fn visit_map<V>(self, mut map: V) -> Result<Duration, V::Error>
                where V: MapAccess<'de>
            {
                let mut secs = None;
                let mut nanos = None;
                while let Some(key) = map.next_key()? {
                    match key {
                        Field::Secs => {
                            if secs.is_some() {
                                return Err(de::Error::duplicate_field("secs"));
                            }
                            secs = Some(map.next_value()?);
                        }
                        Field::Nanos => {
                            if nanos.is_some() {
                                return Err(de::Error::duplicate_field("nanos"));
                            }
                            nanos = Some(map.next_value()?);
                        }
                    }
                }
                let secs = secs.ok_or_else(|| de::Error::missing_field("secs"))?;
                let nanos = nanos.ok_or_else(|| de::Error::missing_field("nanos"))?;
                Ok(Duration::new(secs, nanos))
            }
        }

        const FIELDS: &'static [&'static str] = &["secs", "nanos"];
        deserializer.deserialize_struct("Duration", FIELDS, DurationVisitor)
    }
}