Auto merge of #45404 - giannicic:defaultimpl2, r=nikomatsakis

Auto merge of #45404 - giannicic:defaultimpl2, r=nikomatsakis

#37653 support `default impl` for specialization

this commit implements the second part of the `default impl` feature:

>  - a `default impl` need not include all items from the trait
>  - a `default impl` alone does not mean that a type implements the trait

The first point allows rustc to compile and run something like this:

```
trait Foo {
    fn foo_one(&self) -> &'static str;
    fn foo_two(&self) -> &'static str;
}

default impl<T> Foo for T {
    fn foo_one(&self) -> &'static str {
        "generic"
    }
}

struct MyStruct;

fn  main() {
    assert!(MyStruct.foo_one() == "generic");
}
```

but it shows a proper error if trying to call `MyStruct.foo_two()`

The second point allows a `default impl` to be considered as not implementing the `Trait` if it doesn't implement all the trait items.
The tests provided (in the compile-fail section) should cover all the possible trait resolutions.
Let me know if some tests is missed.

See [referenced ](https://github.com/rust-lang/rust/issues/37653) issue for further info

r? @nikomatsakis