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I can't wait to see how people use default field values in #Rust!
It felt like an eternity to land this (specially if we count the years of discussion before the, I believe, 3rd RFC was accepted), but as of next nightly you will be able to write
struct Foo {
bar: Type = Type::const_method(),
}
Foo { .. } // implicit `bar: Type::const_method()` call
The main difference between https://github.com/rust-lang/rust/pull/129514 and derive(Default) is that the latter doesn't support having mandatory fields.
#RustLang
![Rust Compiler output:
error[E0658]: default values on fields are experimental
--> def.rs:2:11
|
2 | x: i32 = 101,
| ^^^^^^
|
= note: see issue #132162 <https://github.com/rust-lang/rust/issues/132162> for more information
= help: add `#![feature(default_field_values)]` to the crate attributes to enable
= note: this compiler was built on 2024-12-10; consider upgrading it if it is out of date
error[E0797]: base expression required after `..`
--> def.rs:6:19
|
6 | let s = S { .. };
| ^
|
= help: add `#![feature(default_field_values)]` to the crate attributes to enable default values on `struct` fields
help: add a base expression here
|
6 | let s = S { ../* expr */ };
| ++++++++++](https://media.hachyderm.io/media_attachments/files/113/635/417/222/672/459/original/d9ec8030370c26b7.png "Rust Compiler output:
error[E0658]: default values on fields are experimental
--> def.rs:2:11
|
2 | x: i32 = 101,
| ^^^^^^
|
= note: see issue #132162 <https://github.com/rust-lang/rust/issues/132162> for more information
= help: add `#![feature(default_field_values)]` to the crate attributes to enable
= note: this compiler was built on 2024-12-10; consider upgrading it if it is out of date
error[E0797]: base expression required after `..`
--> def.rs:6:19
|
6 | let s = S { .. };
| ^
|
= help: add `#![feature(default_field_values)]` to the crate attributes to enable default values on `struct` fields
help: add a base expression here
|
6 | let s = S { ../* expr */ };
| ++++++++++")
![$ more def.rs
#![feature(default_field_values)]
struct S {
x: i32 = 101,
}
fn main() {
let s = S { .. };
println!("{}", s.x);
}
$ rustc +nightly def.rs && ./def
101](https://media.hachyderm.io/media_attachments/files/113/635/421/842/062/353/original/7bfe5ab368a9ac76.png "$ more def.rs
#![feature(default_field_values)]
struct S {
x: i32 = 101,
}
fn main() {
let s = S { .. };
println!(\"{}\", s.x);
}
$ rustc +nightly def.rs && ./def
101")
@piggo @ekuber I believe it's more of a language feature than an extension for `Default`. Check out the RFC: https://rust-lang.github.io/rfcs/3681-default-field-values.html
@piggo You can't use traits to encode "some of these fields are mandatory". Default is all are optional. The normal state is all are mandatory. This feature sits in the middle.