Attribute to specify that type representation is the same as its only field

[dtolnay]

By analogy with #[repr(transparent)].

// This should serialize and deserialize directly as String, rather than as newtype.
#[derive(Serialize, Deserialize)]
#[serde(transparent)]
struct Transparent(String);

// Generated code.
impl Serialize for Transparent {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
        where S: Serializer
    {
        self.0.serialize(serializer)
    }
}

impl<'de> Deserialize<'de> for Transparent {
    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
        where D: Deserializer<'de>
    {
        Deserialize::deserialize(deserializer).map(Transparent)
    }
}

Should also work for structs with one field, and structs and tuple structs with all but one skipped field.

@bluss

[Boscop]

Can this attribute also be used to inline the fields of a struct member type into the parent struct for the purpose of (de)serialization?

Like this:

#[derive(Serialize, Deserialize)]
struct Common {
    a: i32,
}

#[derive(Serialize, Deserialize)]
struct Foo {
    #[serde(transparent)]
    common: Common,
    foo: u8,
}

#[derive(Serialize, Deserialize)]
struct Bar {
    #[serde(transparent)]
    common: Common,
    bar: u64,
}

[Boscop]

Ah, nvm, I found out that it's #[serde(flatten)]..