Unable to de-serialize optional fields

[mnpw]

A missing field in a .toml file is treated as Option::None when de-serialized using toml crate. When using serde-regex, I'm not able to have this behaviour.

Minimal reproducible example:

# Cargo.toml

[dependencies]
serde = {version = "1", features = ["derive"] }
regex = "1.9.3"
serde_regex = "1.1.0"
toml = "0.7

// main.rs

use regex::Regex;
use serde::{Deserialize, Serialize};

#[derive(Clone, Debug, Serialize, Deserialize)]
struct TimestampString {
    pattern_1: String,
    pattern_2: Option<String>,
}

#[derive(Clone, Debug, Serialize, Deserialize)]
struct TimestampRegex {
    #[serde(with = "serde_regex")]
    pattern_1: Regex,
    #[serde(with = "serde_regex")]
    pattern_2: Option<Regex>,
}

fn main() {
    let timestamps_toml = r#"
pattern_1 = "^\\d{4}-\\d{2}-\\d{2}$"
"#;

    // this works fine
    let t_string: TimestampString = toml::from_str(timestamps_toml).expect("Should not panic!");
    println!("{t_string:?}");

    // this does not work fine
    let t_regex: TimestampRegex = toml::from_str(timestamps_toml).expect("Should not panic!");
    println!("{t_regex:?}");
}

[blind-oracle]

Just FYI for whomever stumbles upon it. Yes, it doesn't work as is, but adding default helps e.g.

#[derive(Clone, Debug, Serialize, Deserialize)]
struct TimestampRegex {
    #[serde(with = "serde_regex")]
    pattern_1: Regex,
    #[serde(default, with = "serde_regex")]
    pattern_2: Option<Regex>,
}

[artemudovyk]

Just FYI for whomever stumbles upon it. Yes, it doesn't work as is, but adding default helps e.g.

#[derive(Clone, Debug, Serialize, Deserialize)]
struct TimestampRegex {
    #[serde(with = "serde_regex")]
    pattern_1: Regex,
    #[serde(default, with = "serde_regex")]
    pattern_2: Option<Regex>,
}

Doesn't work for Option<Vec<Regex>>, unfortunately. But empty Vec will do as well.

#[derive(Serialize, Deserialize, Debug)]
struct WatchPolicy {
    dir: PathBuf,
    #[serde(default, with = "serde_regex")]
    exclude: Option<Vec<Regex>>,
}

error[E0277]: the trait bound `for<'a> Serde<&'a std::option::Option<Vec<regex::Regex>>>: Serialize` is not satisfied
   --> src/watch.rs:11:10
    |
 11 | #[derive(Serialize, Deserialize, Debug)]
    |          ^^^^^^^^^ the trait `for<'a> Serialize` is not implemented for `Serde<&'a std::option::Option<Vec<regex::Regex>>>`
    |
    = note: for local types consider adding `#[derive(serde::Serialize)]` to your `Serde<&'a std::option::Option<Vec<regex::Regex>>>` type
    = note: for types from other crates check whether the crate offers a `serde` feature flag
    = help: the following other types implement trait `Serialize`:
              Serde<&Vec<regex::Regex>>
              Serde<&Vec<regex::bytes::Regex>>
              Serde<&regex::Regex>
              Serde<&regex::bytes::Regex>
              Serde<&std::option::Option<regex::Regex>>
              Serde<&std::option::Option<regex::bytes::Regex>>
              Serde<Vec<regex::Regex>>
              Serde<Vec<regex::bytes::Regex>>
            and 6 others
note: required by a bound in `serde_regex::serialize`
   --> /home/***/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/serde_regex-1.1.0/src/lib.rs:223:27
    |
220 | pub fn serialize<T, S>(value: &T, serializer: S) -> Result<S::Ok, S::Error>
    |        --------- required by a bound in this function
...
223 |     for<'a> Serde<&'a T>: Serialize,
    |                           ^^^^^^^^^ required by this bound in `serialize`
    = note: this error originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)

[blind-oracle]

@artemudovyk You can try dumping serde-regex in favor of https://docs.rs/serde_with/latest/serde_with/ crate. It can (de)serialize using Display and FromStr traits that Regex implements, see https://docs.rs/serde_with/3.14.1/serde_with/struct.DisplayFromStr.html

Maybe that would work better, e.g. in your case:

use serde_with::{DisplayFromStr, serde_as};

#[derive(Serialize, Deserialize, Debug)]
struct WatchPolicy {
    dir: PathBuf,
    #[serde_as(as = "Option<Vec<DisplayFromStr>>")]
    exclude: Option<Vec<Regex>>,
}