2021-08-09

Merge #9814

9814: Generate default impl when converting `#[derive(Debug)]` to manual impl r=yoshuawuyts a=yoshuawuyts

This patch makes it so when you convert `#[derive(Debug)]` to a manual impl, a default body is provided that's equivalent to the original output of `#[derive(Debug)]`. This should make it drastically easier to write custom `Debug` impls, especially when all you want to do is quickly omit a single field which is `!Debug`.

This is implemented for enums, record structs, tuple structs, empty structs - and it sets us up to implement variations on this in the future for other traits (like `PartialEq` and `Hash`).

Thanks!

## Codegen diff
This is the difference in codegen for record structs with this patch:
```diff
struct Foo {
    bar: String,
}

impl fmt::Debug for Foo {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        todo!();
+        f.debug_struct("Foo").field("bar", &self.bar).finish()
    }
}
```

Co-authored-by: Irina Shestak <[email protected]>
Co-authored-by: Yoshua Wuyts <[email protected]>
Co-authored-by: Yoshua Wuyts <[email protected]>