Language proposal: Return object construction in type

[efikay]

I'm not a good writer, so let's skip to examples right now.

Before:

mod rfc {
    type Size = (usize, usize);

    #[derive(Default)]
    struct Window {
        pub title: String,
        pub size: Size,
        pub close_when_requested: bool,
    }

    // ⚙️ Just regular Rust, nothing inconvenient
    fn make_diagnostic_window(size: Size) -> Window {
        Window {
            size,
            title: "Diagnostic window".into(),
            ..Default::default()
        }
    }
}

After:

mod rfc {
    type Size = (usize, usize);

    #[derive(Default)]
    struct Window {
        pub title: String,
        pub size: Size,
        pub close_when_requested: bool,
    }

    // ⚡️ Shorthand proposal
    fn make_diagnostic_window(size: Size) => Window {
        size,
        title: "Diagnostic window".into(),
        ..Default::default()
    }

    // Still an option
    fn old_fashioned_way(size: Size) -> Window {
        Window {
            size,
            title: "Diagnostic window".into(),
            ..Default::default()
        }
    }
}

As average language user, I'm not sure about all possible caveats but in my opinion this syntax looks appropriate enough. Simple, clean, fast to write & maintain.

[kennytm]

Duplicate of #3369 (and also using #3444 to avoid repeating the word "Window").

fn make_diagnostic_window(size: Size) -> Window = _ {
    size,
    title: "Diagnostic window".into(),
    ..Default::default()
};

Note that you're supposed to discuss ideas in https://internals.rust-lang.org/ rather than issues in this repo esp. for language changes. See #3756 for details.