serde Add new field attributes: from/try_from/from

Addresses #1550.

The new field attributes from, try_from, from_str and into act as a shorthand for the deserialize_with and serialize_with attributes. They mirror the functionality found in the container attributes from, try_from and into with the exception of from_str, which is a new kind of attribute this PR adds. A field with the from_str attribute first deserializes the field as a String and then attempts to convert it into the correct type by calling FromStr::from_str.

Example

#[derive(Debug, Deserialize)]
struct Foo {
    #[serde(from = "i32")]
    sign: Sign,
    #[serde(from_str)]
    number: u32,
}

#[derive(Debug, PartialEq)]
enum Sign {
    Negative,
    Zero,
    Positive,
}

impl From<i32> for Sign {
    fn from(v: i32) -> Sign {
        match v {
            _ if v < 0 => Sign::Negative,
            _ if v == 0 => Sign::Zero,
            _ => Sign::Positive
        }
    }
}

fn main() {
    let json = r#"{ "sign": -1, "number": "14" }"#;

    let foo: Foo = serde_json::from_str(json).unwrap();

    assert_eq!(foo.sign, Sign::Negative);
    assert_eq!(foo.number, 14);
}

Sep 07 '19 11:09 nolanderc

@dtolnay , David, this is an interesting PR. This "from" / "into" approach can make for less boilerplate than with using variant type enums. I'm happy to spend some to finish what @nolanderc started, if you think it has legs.

Oct 15 '20 10:10 rimutaka

@dtolnay what is the current status of this PR. The feature offered by it is interesting for me in a current project, so I would also be willing to put some time into this if that means it has a good chance of getting it merged.