Add support for restricting trait implementation for local typesa

compiler traits working group
1

As the orphan rule suggest I could implement a local trait to a foreign type,so sometimes people could implement it accidentally,like I want to create a newtype trait


trait newtype{
type output;
fn get(&self)->self::output
fn new(input:output)->Self;
}

struct kg(u32)
impl newtype for kg{
fn get(&self){
    self.0
}

fn new(input:output){
    Self(input);
    }
}

than I relized there is no restriction here,I could implement to every foreign type like u32,u16 which I don't want. Also I wanted to impl Add trait to every newtype which I can't do because of the orphan rule,I want to know the compiler that newtype will be only implemented on local types,so as newtype is also a local trait I could implement Add trait to every newtype

2

This and related matters have been previously discussed. Most recently at the thread Sealed traits. Also, you should try to format you code using backticks. Write

```rust
trait newtype{
  type output;
  fn get(&self)->self::output
  fn new(input:output)->Self;
}
```

to display

trait newtype{
  type output;
  fn get(&self)->self::output
  fn new(input:output)->Self;
}
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