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Serde Visitor for deserializing untagged enums

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serde-untagged
==============

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

```toml
[dependencies]
serde-untagged = "0.1"
```

Untagged enum `Deserialize` impls look like this:

```rust
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:

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

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

```rust
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"`.

```toml
[profile.release]
lto = "thin"
```

```rust
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:

```rust
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

Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
be dual licensed as above, without any additional terms or conditions.

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