Pre-pre-RFC: syntactic sugar for `Default::default()`

language design
75

Thanks @spacekookie!

I know is a long thread so it is a good time to summarize what the RFC I'll be writing will propose, particularly because it is removed from what I originally wrote:

  • Language support for default values on structs through const expressions:

    struct Car {
    wheel_count: usize = 4;
}

  • Language support for partially defaulted structs:

    struct Person {
    name: String,
    age: i128 = 0;
}

  • Teach #[derive(Default)] to use the default values

    #[derive(Default)]
struct Pet {
    name: Option<String>, // impl Default for Pet will use Default::default() for name
    age: i128 = 0; // impl Default for Pet will use the literal 0 for age
}

  • Expand the "spread" .. operator to make the RHS expression optional, if it is not present it will use the default const expressions set in the struct definition:

    let person = Person { name: "Jane Austen".to_string(), .. };
let person = Person { name: "Margaret Atwood".to_string(), ..Default::default() }; // Compilation error, Person doesn't impl Default
let pet = Pet { name: None, .. };
let pet = Pet { .. }; // Compilation error, name doesn't have a `const` default value
let pet = Pet { ..Default::default() }; // Ok

  • Considerations around letting non-const expressions in the struct default are to be punted initially

  • Considerations around making partial defaults a user definable feature are to be left for a future RFC (explicitly not adding a new PartialDefault trait, at least for now)

  • Further conversation around "should a struct that has all default values set automagically impl Default?" and "should a struct that impls Default automagically work with ..?" is needed

  • fn default<T: Default>(t: T) -> T { T::default() } is tangential and independent of this RFC

  • I will be basing the RFC on prior work that has been closed in the past due to lack of bandwidth.

Please everyone, let me know if there's any item that I have forgotten in this quick summary.

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