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 <shestak.irina@gmail.com>
Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>
Co-authored-by: Yoshua Wuyts <yoshuawuyts+github@gmail.com>
tree: c460fe4d703963cc82b888704532ea2022ec96cb
  1. .cargo/
  2. .github/
  3. .vscode/
  4. assets/
  5. bench_data/
  6. crates/
  7. docs/
  8. editors/
  9. lib/
  10. xtask/
  11. .gitattributes
  12. .gitignore
  13. bors.toml
  14. Cargo.lock
  15. Cargo.toml
  16. LICENSE-APACHE
  17. LICENSE-MIT
  18. PRIVACY.md
  19. README.md
  20. rustfmt.toml
README.md

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