Use mergeme to merge CLI config

[BD103]

Objective

mergeme is a little crate that I wrote¹ that makes it easy to merge structures together. For example:

use mergeme::Merge;

#[derive(Merge)]
#[partial(PartialPerson)]
struct Person {
    name: String,
    age: u16,
    #[strategy(merge)]
    friends: Vec<String>,
}

let person = Person {
    name: "Janette".to_string(),
    age: 19,
    friends: vec!["Lou".to_string()],
};

// Change Janette's age to be 25 and add a friend, but preserve her original name.
let partial = PartialPerson {
    name: None,
    age: Some(25),
    friends: Some(vec!["Kylie".to_string()]),
};

let merged = person.merge(partial);

assert_eq!(merged.name, "Janette");
assert_eq!(merged.age, 25);
assert_eq!(merged.friends, ["Lou", "Kylie"]);

This PR greatly simplifies the API surface of CliConfig by offloading the merging logic to mergeme's derive macro.

Major Changes

CliConfig::for_package() and CliConfig::merged_from_metadata() have been combined into a single CliConfig::from_metadata() function that takes a &serde_json::Value for the [package.metadata] table².

That should be the only visible change, and that's only API-related. The CLI should behave the same, but please be sure to test on actual projects with nested configuration to ensure it works!

Footnotes

1. You can tell because there are more docs and comments than actual code >:D ↩

2. Not [package.metadata.bevy_cli], but just [package.metadata]. This is why the unit tests were changed in the way they were. ↩

[DaAlbrecht]

Excited for this!! Didnt have time to do a review just yet but i would wait with this for 0.2.0 as we have quite a lot of testing with the old system and this is a pretty major internal change.

[BD103]

Excited for this!! Didnt have time to do a review just yet but i would wait with this for 0.2.0 as we have quite a lot of testing with the old system and this is a pretty major internal change.

Agreed, I'll mark this as blocked until the release