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)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
r? `@compiler-errors`
This is a simpler version of #134441, detecting the simpler case when a field with a default should have not been specified in the manual `Default::default()`, instead using `..` for it. It doesn't provide any suggestions, nor the checks for "equivalences" nor whether the value used in the imp being used would be suitable as a default field value.
3e3db73c9b