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Comment by kevincox | Hacker News Reader

This pattern is fairly good but doesn't support required arguments.

I haven't filed an RFC yet, but I have a plan that I'm optimistic can land sometime that would let you write:

  struct User {
    id: Id,
    email: EMail,
    name: String = String::new(),
    age: u16 = 0,
  }
  
  let user = User { id, email, .. }; // valid
  let user = User { id, .. }; // complain about email missing

It would leverage `const` expressions, so it would desugar effectively to the same as if you had written:

  const DEFAULT_USER_NAME: String = String::new();
  const DEFAULT_USER_AGE: u16 = 0;

  struct User {
    id: Id,
    email: EMail,
    name: String,
    age: u16,
  }
  
  let user = User { id, email, name: DEFAULT_USER_NAME, age: DEFAULT_USER_AGE };

The above assumes that String::new will be const at some point (it can be).

If this ever lands, then there will be less need to write Builder traits by hand.

Also, at some point the free-standing `default()` function that calls `Default::default()` will land, making it that much shorter to write (without any new features like I'm proposing).

> assumes that String::new will be const at some point

It has been const since Rust 1.39 (https://doc.rust-lang.org/std/string/struct.String.html#meth...)

This old RFC came up elsewhere in the thread and sounds like a similar idea:

https://github.com/rust-lang/rfcs/pull/1806

I assume it would clone the defaults? Otherwise non-`Copy` types would have to be moved out of their const, which you can't do unless it's a refcell (et al).

OT, but how can a value that lives on the heap be const? And how could ownership of a (non-Copy) const be passed somewhere else?

The trick is that an empty string doesn’t actually own any data on the heap. It’s basically a null pointer, and all string methods check for that in some way. This is a typical optimization trick that many languages implement (empty lists and strings are very common). Defining new to be const just makes this optimization a requirement.

But still- at a language semantics level, values can't have multiple owners, and String couldn't implement Copy just for this one case. So how does this reconcile with the borrow-checker?

Or can consts have multiple owners since they're immutable and have a static lifetime? Is this just the first case of a non-Copy being const, so the question has never come up before?

Ah, I get your question now, I thought you were talking about const functions. Well, const values in Rust are funny things, they're not linked into the executable, they don't have an address or a lifetime. That confused me too for a while. They're purely a shortcut for a value expression, not that different from a #define in C.

Statics, in contrast, have a storage location and a static lifetime.

Compare:

  const CONST_STRING: String = String::new();
  static STATIC_STRING: String = String::new();

  fn main() {
      let x: String = CONST_STRING;  // fine
      let y: String = STATIC_STRING; // error: cannot move out of static item
  }

Oh. I think I didn't realize both existed; I'd only used statics before (and knew about const functions), but didn't know about const expressions.

You can mix-and-match, e.g.

  new(required_arg_1, required_arg_2, Options { foo: "bar", ..Default::default() })

  Options { foo: "bar", ..Default::default() }).build(required_arg_1, required_arg_2)

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