Make sure empty struct always derives Default

[sergey-melnychuk]

I was wondering if requiring an explicit derive(Default) for empty struct makes sense. It feels right that empty (for whatever reason) struct should derive Default, well, by default :)

This is my first issue in rust-lang, so any feedback is welcome. If such suggestion makes sense, I'd like to implement it with very little guidance from more experienced Rustaceans.

//#[derive(Default)]
struct X;

fn f<T: Default>(_: T) {
    ()
}

fn main() {
    f(X);
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `X: std::default::Default` is not satisfied
 --> src/main.rs:9:7
  |
4 | fn f<T: Default>(_: T) {
  |    -    ------- required by this bound in `f`
...
9 |     f(X);
  |       ^ the trait `std::default::Default` is not implemented for `X`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[jonas-schievink]

Hi! Additions or changes to the language need to go through our RFC process. Before an RFC is written, the feature can also be discussed in our internals forum to find other people interested in it.

Closing in favor of an RFC.