serde-untagged

This crate provides a Serde Visitor implementation that is useful for deserializing untagged enums.

[dependencies]
serde-untagged = "0.1"

Untagged enum Deserialize impls look like this:

use serde::de::{Deserialize, Deserializer};
use serde_untagged::UntaggedEnumVisitor;

impl<'de> Deserialize<'de> for $MyType {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        UntaggedEnumVisitor::new()
            /*
             *
             */
            .deserialize(deserializer)
    }
}

Inside the /* ... */, we list each type that the untagged enum needs to support deserializing from, giving a closure that turns the input into $MyType. The following types are supported:

• bool
• i8, i16, i32, i64, i128, u8, u16, u32, u64, u128
• f32
• f64
• char
• string
• borrowed_str
• bytes
• borrowed_bytes
• byte_buf
• unit
• seq
• map

Example: string or struct

Cargo's http.ssl-version configuration supports deserialization from the following two representations:

[http]
ssl-version = "tlsv1.3"

[http]
ssl-version.min = "tlsv1.2"
ssl-version.max = "tlsv1.3"

use serde::de::{Deserialize, Deserializer};
use serde_derive::Deserialize;
use serde_untagged::UntaggedEnumVisitor;

pub enum SslVersionConfig {
    Single(String),
    Range(SslVersionConfigRange),
}

impl<'de> Deserialize<'de> for SslVersionConfig {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        UntaggedEnumVisitor::new()
            .string(|single| Ok(SslVersionConfig::Single(single.to_owned())))
            .map(|map| map.deserialize().map(SslVersionConfig::Range))
            .deserialize(deserializer)
    }
}

#[derive(Deserialize)]
pub struct SslVersionConfigRange {
    pub min: Option<String>,
    pub max: Option<String>,
}

Example: unit variant or bool

Cargo's LTO setting in profiles supports the 5 values false, true, "fat", "thin", and "off".

[profile.release]
lto = "thin"

use serde::de::{Deserialize, Deserializer, IntoDeserializer};
use serde_derive::Deserialize;
use serde_untagged::UntaggedEnumVisitor;

pub enum LinkTimeOptimization {
    Enabled(bool),
    Enum(LinkTimeOptimizationString),
}

impl<'de> Deserialize<'de> for LinkTimeOptimization {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        UntaggedEnumVisitor::new()
            .bool(|b| Ok(LinkTimeOptimization::Enabled(b)))
            .string(|string| {
                let de = string.into_deserializer();
                LinkTimeOptimizationString::deserialize(de).map(LinkTimeOptimization::Enum)
            })
            .deserialize(deserializer)
    }
}

#[derive(Deserialize)]
#[serde(rename = "lowercase")]
pub enum LinkTimeOptimizationString {
    Fat,
    Thin,
    Off,
}

Since lto = true means the same thing as lto = "fat" to Cargo, there are really only 4 distinct options. This type could be implemented alternatively as:

use serde::de::{Deserialize, Deserializer, Unexpected};
use serde_untagged::UntaggedEnumVisitor;

pub enum LinkTimeOptimization {
    ThinLocal,  // false
    Fat,        // true or "fat"
    Thin,       // "thin"
    Off,        // "off"
}

impl<'de> Deserialize<'de> for LinkTimeOptimization {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
    where
        D: Deserializer<'de>,
    {
        UntaggedEnumVisitor::new()
            .bool(|b| match b {
                false => Ok(LinkTimeOptimization::ThinLocal),
                true => Ok(LinkTimeOptimization::Fat),
            })
            .string(|string| match string {
                "fat" => Ok(LinkTimeOptimization::Fat),
                "thin" => Ok(LinkTimeOptimization::Thin),
                "off" => Ok(LinkTimeOptimization::Off),
                _ => Err(serde::de::Error::invalid_value(
                    Unexpected::Str(string),
                    &r#""fat" or "thin" or "off""#,
                )),
            })
            .deserialize(deserializer)
    }
}

License