Add Linit: missing_trait_impls

[DaAlbrecht]

Objective

This new lint checks if a unit structures that implement Component also implements the following traits:

• Clone
• Copy
• Default

Note

With how this is set up, if a Component does not implement any of those there will be a lint output for each trait on its own, this also applies to the fix.

Example

use bevy::prelude::*;

#[derive(Component)]
pub struct ImplCopy;

[package.metadata.bevy_lint]
missing_default = "warn"
missing_copy = "warn"
missing_clone = "warn"

lint output

❯ bevy_lint
    Checking test_project v0.1.0 (/tmp/test_project)
warning: defined a unit struct without a `Copy` implementation
 --> src/main.rs:4:1
  |
4 | pub struct ImplCopy;
  | ^^^^^^^^^^^^^^^^^^^^
  |
note: `Component` implemented here
 --> src/main.rs:3:10
  |
3 | #[derive(Component)]
  |          ^^^^^^^^^
  = note: requested on the command line with `-W bevy::missing-copy`
  = note: this warning originates in the derive macro `Component` (in Nightly builds, run with -Z macro-backtrace for more info)
help: `Copy` can be automatically derived
  |
4 + #[derive(Copy)]
5 | pub struct ImplCopy;
  |

warning: defined a unit struct without a `Clone` implementation
 --> src/main.rs:4:1
  |
4 | pub struct ImplCopy;
  | ^^^^^^^^^^^^^^^^^^^^
  |
note: `Component` implemented here
 --> src/main.rs:3:10
  |
3 | #[derive(Component)]
  |          ^^^^^^^^^
  = note: requested on the command line with `-W bevy::missing-clone`
  = note: this warning originates in the derive macro `Component` (in Nightly builds, run with -Z macro-backtrace for more info)
help: `Clone` can be automatically derived
  |
4 + #[derive(Clone)]
5 | pub struct ImplCopy;
  |

warning: defined a unit struct without a `Default` implementation
 --> src/main.rs:4:1
  |
4 | pub struct ImplCopy;
  | ^^^^^^^^^^^^^^^^^^^^
  |
note: `Component` implemented here
 --> src/main.rs:3:10
  |
3 | #[derive(Component)]
  |          ^^^^^^^^^
  = note: requested on the command line with `-W bevy::missing-default`
  = note: this warning originates in the derive macro `Component` (in Nightly builds, run with -Z macro-backtrace for more info)
help: `Default` can be automatically derived
  |
4 + #[derive(Default)]
5 | pub struct ImplCopy;
  |

warning: `test_project` (bin "test_project") generated 3 warnings (run `cargo fix --bin "test_project"` to apply 3 suggestions)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.30s

bevy_lint --fix will result in the following:

use bevy::prelude::*;

#[derive(Component)]
#[derive(Default)]
#[derive(Clone)]
#[derive(Copy)]
pub struct ImplCopy;

fn main() {}

and after cargo fmg:

use bevy::prelude::*;

#[derive(Component, Default, Clone, Copy)]
pub struct ImplCopy;

fn main() {}

Closes #211, #344

[DaAlbrecht]

I did not want to create our own proc macro for this / use an extra crate but it's rather sad that there is no built-in way to do this.