Add into methods for Value

[retep998]

Specifically into_object into_array and into_string. These methods would significantly improve ergonomics in certain cases by reducing large match expressions into simple method calls.

match json {
    Value::Object(x) => x,
    _ => return Err(Error::None),
}

vs

json.into_object()?

[dtolnay]

Seems reasonable. Would these return Option and just destroy your value if it is the wrong type?

[retep998]

Alternatively they could return Result and return the Value inside the Err variant.

[dtolnay]

I think I would slightly prefer the Result way.

There are at least 3 reasonable error types:

• Result<_, Value>—just give them back the value. I would prefer not to do this because it does not play well with ? and c-good-err.
• Result<_, SomeConcreteError>—with an accessor for the original value. This could work but requires a map_err(de::Error::custom) if you want to call it from a deserialize impl.
• <E> Result<_, E> where E: some trait—the way that Visitor methods are structured. We could make this easily usable in a deserialize impl without map_err but may require a type annotation elsewhere.

I would like to see an example of a deserialize impl using these into methods and see what we can do to make that ergonomic.

[kangalio]

I think I prefer the Option way, for several reasons

• It seems intuitive and semantically correct
  • The absence of a specific data type does not necessarily indicate an "error"
• It is consistent with the existing as_* methods, which all return Option
• I've encountered the need for into_* methods on Value a few times before, and every time an Option-like implementation would have been more useful than a Result-like implementation
• If the need arises to get back the value on type mismatch, the user can easily write a match statement
• It avoids the bikeshedding about the Result::Err type from the previous comment