The tracking issue for this feature is #13231

The optin_builtin_traits feature gate allows you to define auto traits.

Auto traits, like Send or Sync in the standard library, are marker traits that are automatically implemented for every type, unless the type, or a type it contains, has explicitly opted out via a negative impl.

impl !Type for Trait

Example:

#![feature(optin_builtin_traits)]

auto trait Valid {}

struct True;
struct False;

impl !Valid for False {}

struct MaybeValid<T>(T);

fn must_be_valid<T: Valid>(_t: T) { }

fn main() {
    // works
    must_be_valid( MaybeValid(True) );
                
    // compiler error - trait bound not satisfied
    // must_be_valid( MaybeValid(False) );
}