json_typeof and jsonb_typeof function

[anna-ahmed19]

Added the json_typeof and jsonb_typeof functions under Issue#4216

[guissalustiano]

Thanks for opening the PR!

The changes look good, I only would like to see some tests about the null behavior, in past cases, Postgres returns NULL when you pass NULL to this kind of functions.
If this is case, the function signature is wrong, It should also cover the NULL input return.

To fix that we can use

fn json_typeof<E: JsonOrNullableJson + SingleValue + MaybeNullableValue<Text>>(e: E) -> E::Out;

The E: MaybeNulllableValue<Text> says that the output values has the same nullability as the input value. That means Nullable<Json<T>> evaluates to Nullable<Text> and Json<T> evaluates to Text via the E::Out return type

[guissalustiano]

Could you add a test with None input?
It is something like this:

    /// let result = diesel::select(json_typeof::<Nullable<Json>, _>(None::<Value>))
    ///     .get_result::<String>(connection)?;
    ///
    /// assert_eq!(<something>, result);

[guissalustiano]

Could you also add a None test here?

[anna-ahmed19]

Thank you @guissalustiano for the suggestions.
I have added MaybeNullableValue<Text> and the None tests.
Let me know any further changes.

[guissalustiano]

Thanks!

[weiznich]

Thanks for submitting this PR ❤️