Implement `default_overrides_default_fields` lint

Detect when a manual `Default` implementation isn't using the existing default field values and suggest using `..` instead:

```
error: `Default` impl doesn't use the declared default field values
  --> $DIR/manual-default-impl-could-be-derived.rs:14:1
   |
LL | / impl Default for A {
LL | |     fn default() -> Self {
LL | |         A {
LL | |             y: 0,
   | |                - this field has a default value
...  |
LL | | }
   | |_^
   |
   = help: use the default values in the `impl` with `Struct { mandatory_field, .. }` to avoid them diverging over time
note: the lint level is defined here
  --> $DIR/manual-default-impl-could-be-derived.rs:5:9
   |
LL | #![deny(default_overrides_default_fields)]
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

0 files changed, 0 insertions, 0 deletions