diff --git a/src/doc/unstable-book/src/language-features/default-field-values.md b/src/doc/unstable-book/src/language-features/default-field-values.md new file mode 100644 index 0000000000000..3143b2d7cae3d --- /dev/null +++ b/src/doc/unstable-book/src/language-features/default-field-values.md @@ -0,0 +1,93 @@ +# `default_field_values` + +The tracking issue for this feature is: [#132162] + +[#132162]: https://github.com/rust-lang/rust/issues/132162 + +The RFC for this feature is: [#3681] + +[#3681]: https://github.com/rust-lang/rfcs/blob/master/text/3681-default-field-values.md + +------------------------ + +The `default_field_values` feature allows users to specify a const value for +individual fields in struct definitions, allowing those to be omitted from +initializers. + +## Examples + +```rust +#![feature(default_field_values)] + +#[derive(Default)] +struct Pet { + name: Option, // impl Default for Pet will use Default::default() for name + age: i128 = 42, // impl Default for Pet will use the literal 42 for age +} + +fn main() { + let a = Pet { name: Some(String::new()), .. }; // Pet { name: Some(""), age: 42 } + let b = Pet::default(); // Pet { name: None, age: 42 } + assert_eq!(a.age, b.age); + // The following would be a compilation error: `name` needs to be specified + // let _ = Pet { .. }; +} +``` + +## `#[derive(Default)]` + +When deriving Default, the provided values are then used. On enum variants, +the variant must still be marked with `#[default]` and have all its fields +with default values. + +```rust +#![feature(default_field_values)] + +#[derive(Default)] +enum A { + #[default] + B { + x: i32 = 0, + y: i32 = 0, + }, + C, +} +``` + +## Enum variants + +This feature also supports enum variants for both specifying default values +and `#[derive(Default)]`. + +## Interaction with `#[non_exhaustive]` + +A struct or enum variant marked with `#[non_exhaustive]` is not allowed to +have default field values. + +## Lints + +When manually implementing the `Default` trait for a type that has default +field values, if any of these are overriden in the impl the +`default_overrides_default_fields` lint will trigger. This lint is in place +to avoid surprising diverging behavior between `S { .. }` and +`S::default()`, where using the same type in both ways could result in +different values. The appropriate way to write a manual `Default` +implementation is to use the functional update syntax: + +```rust +#![feature(default_field_values)] + +struct Pet { + name: String, + age: i128 = 42, // impl Default for Pet will use the literal 42 for age +} + +impl Default for Pet { + fn default() -> Pet { + Pet { + name: "no-name".to_string(), + .. + } + } +} +```