Add default!() macro to builtins

[icorderi]

I run into this across several files in my project, I have fields now and then on my structs that I don't care about much about and I want to initialize them to default... initialize them to default... to default.

You get the point.

I would like to suggest adding this macro to the builtins that makes using defaults slightly less painful.

macro_rules! default(
    () => (::std::default::Default::default());
)

[ftxqxd]

Is there any reason this can’t just be a function in the prelude? That would save a character at every invocation and would have the added advantage of being able to specify the type (e.g., default::<int>()). #18934 did something similar to this, although did not add the function to the prelude. (That was closed because it provided little use.) I think the best solution here would be add Default to the prelude, but I still don’t really see what the problem is. What’s wrong with simply calling Default::default()?

[icorderi]

@P1start having it in the prelude is a perfectly reasonable thing to do. It might be even better than having it as a macro.
The goal is not having to say default..default...default..

What’s wrong with simply calling Default::default()?

That doesn't work unless you add the use which means you went from saying default 3 times to saying default 4 times...

use std::default::Default;
// ...
// somewhere later in the code
let foo = Bar { bar: Default::default() };

On a side note, if you define it as a function would this work? I haven't checked what should formally go on the other side of the ..

Bar { foo: 8, ..default() }

[ftxqxd]

Any expression can go after the .., so that would work.

It would be nice if trait methods could be used, which removes some repetition if default() is called multiple times:

use std::default::Default::default;

let foo: Foo = default();
let bar: Bar = default();
let baz: Baz = default();
let qux: Qux = default();

[kmcallister]

Default is addressed in the std reforms and any further change would require an RFC.