How to use `serde::format_description` with `format_description::well_known` ? about time

[jhpratt]

What feature flags do you have enabled?

[loganmzz]

I use: serde and serde-well-known

[jhpratt]

That can't be the case, as time::serde::format_description is enabled by the macros feature. Also note that the serde-well-known flag is deliberately undocumented, as it is deprecated in favor of using the documented flags directly (namely serde, formatting, and parsing).

Have you looked at the documentation for this macro? The final example shows how to use it with ISO 8601.

[loganmzz]

Ok, here is my test case.

• Cargo.toml:

[dependencies]
serde = { version = "1.0.198", features = ["derive"] }
time = { version = "0.3.36", features = ["formatting", "macros", "parsing", "serde"] }

[dev-dependencies]
serde_json = { version = "1.0.116" }

• lib.rs:

use time::{
    format_description::well_known::{iso8601, Iso8601},
    serde::format_description,
    OffsetDateTime,
};

const CONFIG: iso8601::EncodedConfig = iso8601::Config::DEFAULT
    .set_formatted_components(iso8601::FormattedComponents::DateTime)
    .encode();
const FORMAT: Iso8601<CONFIG> = Iso8601::<CONFIG>;
format_description!(serde_datetime, OffsetDateTime, FORMAT);

#[derive(serde::Deserialize, Debug, PartialEq)]
pub struct Container {
    #[serde(with="serde_datetime")]
    pub datetime: OffsetDateTime,
}

• test:

use time::{
    Date,
    Month,
    OffsetDateTime,
    Time,
};

#[test]
fn without_offset() {
    let json = serde_json::json!({
        "datetime": "2024-04-17T12:45:00.0"
    });

    let actual: Container = serde_json::from_value(json).unwrap();

    assert_eq!(
        actual,
        Container {
            datetime: OffsetDateTime::new_utc(
                Date::from_calendar_date(2024, Month::April, 17).unwrap(),
                Time::from_hms(12, 45, 0).unwrap(),
            ),
        },
    );
}

Then, I got:

---- tests::without_offset stdout ----
thread 'tests::without_offset' panicked at 'called `Result::unwrap()` on an `Err` value: Error("the `Parsed` struct did not include enough information to construct the type", line: 0, column: 0)', src/lib.rs:36:62
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Worth to mention that error isn't very explicit :/

[jhpratt]

The failure is as expected, as there is no UtcOffset present in the formatted string. As such, an OffsetDateTime cannot be constructed.

I will look into what can be done in terms of an error message, but I suspect there isn't much.