#[optional_struct]
#[derive(Bar)]
struct Foo {
	meow: u32,
	woof: String,
}

// Generated:
#[derive(Clone, Debug, PartialEq, Bar)]
struct OptionalFoo {
	meow: Option<u32>,
	woof: Option<String>,
}

impl OptionalFoo {
	pub fn apply_to(self, foo: &mut OptionalFoo) {
        if let Some(meow) = self.meow {
            foo.meow = meow;
        }

        if let Some(woof) = self.woof {
            foo.woof = woof;
        }
	}
}

fn main() {
    let mut f = Foo {
        meow: 4,
        woof: "I am hungry rn".to_owned(),
    };

    let opt_f = OptionalFoo {
        meow: Some(10),
        woof: None,
    };

    opt_f.apply_to(&mut f);

    assert_eq!(f.meow, 10);
    assert_eq!(&f.woof, "I am hungry rn");
}

Since rust does not have default arguments, and some tools are strict when deserializing data (e.g. serde), missing configuration values can be quite frustrating to deal with. For example:

#[derive(Deserialize)]
struct Config {
    log_file: PathBuf,
}

If we read the configuration from a file, and the log_file is not specified, serde will e.g. fail to create the struct. While serde offers ways to set the default value for a field with e.g.

#[derive(Deserialize)]
struct Config {
    #[serde(default = "get_next_log_filename")]
    log_file: PathBuf,
}

there are obvious limitations. This crate aims to fill this gap by allowing optional values, and providing an easy way to apply values obtained from different sources to construct our configuration.

With optional_struct, one can define the required configuration as it shall be used and only use the generated struct to handle configuration/missing values/default values.

The macro optional_struct generates a structure containing the same fields as the one it was tagged on, but wrapped by an Option. A function on the new structure allows applying its values to the original one (if the Options are not None). This can be called multiple times, to apply configuration from different source, while giving the caller complete control over how to set the values, since the generated struct can be easily manipulated and passed around before constructing the final configuration.

1. Rename the generated struct:

#[optional_struct(HeyU)]
struct Config();

let me = HeyU();

2. Handle recursive types:

#[optional_struct]
struct Foo {
    // Replaces Option<Bar> with Option<OptionalBar>
    #[optional_rename(OptionalBar)]
    bar: Bar,
}

#[optional_struct]
struct bar ();

3. Handle Options in the original struct (by ignoring them):

#[optional_struct]
struct Foo {
    bar: Option<u8>,
}

let opt_f = OptionalFoo { bar: Some(1) }

4. Force wrapping (or not) of fields:

#[optional_struct]
struct Foo {
    #[optional_skip_wrap]
    bar: char,

    // Useless here since we wrap by default
    #[optional_wrap]
    baz: bool,
}

let opt_f = OptionalFoo { bar: Some(1) }

5. Change the default wrapping behavior:

#[optional_struct(OptionalFoo, false)]
struct Foo {
    bar: u8,
    #[optional_wrap]
    baz: i8,
}

let opt_f = OptionalFoo { bar: 1, baz: None };

The simplest way to understand how this function is generated is to look at the Applyable helper trait we define in this crate:

pub trait Applyable<T> {
    fn apply_to(self, t: &mut T);
}

impl<T> Applyable<T> for Option<T> {
    fn apply_to(self, t: &mut T) {
        if let Some(s) = self {
            *t = s;
        }
    }
}

impl<T> Applyable<T> for T {
    fn apply_to(self, t: &mut T) {
        *t = self;
    }
}

Each structure generated by optional_struct implements Applyable<Foo> for OptionalFoo. This implementation itself calls apply_to recursively on all fields of the structure. Besides those implementations, the ones defined just above tell you how this function behaves: values are applied when they exist, options are simply overridden.

1. Allow not overridding Option<T> -> Option<T> if the original Option is set but not the applied one.

2. Allow wrapping behavior to be set without manually writing the struct name.

3. Automatically find out which fields have an associated optional_struct structure (and avoid the optional_rename). This might require better support for associated types? Could not get it to work with only constant generics :(

4. Make generated fields public by default? For now the fields have the same visibility as the original struct, and this is generated locally, so this seems to be a minor detail?

5. Control which derive attributes are tagged onto the generated struct. Right now, they are copied verbatim, and then some are added (PartialEq / Default / Clone / Debug). Problem: conflicting implementation, missing impl. (e.g. serde's Ser/Deser), invalid impl (e.g. if Default is missing on some skip_wrap field) etc.