Struct Patch

A lib help you patch Rust instance, and easy to partial update configures.

This crate provides the Patch trait and an accompanying derive macro.

Deriving Patch on a struct will generate a struct similar to the original one, but with all fields wrapped in an Option.
An instance of such a patch struct can be applied onto the original struct, replacing values only if they are set to Some, leaving them unchanged otherwise.

use struct_patch::Patch;
use serde::{Deserialize, Serialize};

#[derive(Default, Debug, PartialEq, Patch)]
#[patch(attribute(derive(Debug, Default, Deserialize, Serialize)))]
struct Item {
    field_bool: bool,
    field_int: usize,
    field_string: String,
}

fn patch_json() {
    let mut item = Item {
        field_bool: true,
        field_int: 42,
        field_string: String::from("hello"),
    };

    let data = r#"{
        "field_int": 7
    }"#;

    let patch: ItemPatch = serde_json::from_str(data).unwrap();

    item.apply(patch);
    // You can do 
    // `let new_item = item << patch;`

    // For multiple patches,
    // you can do this
    // `let new_item = item << patch_1 << patch_2;`
    // or make an aggregated one, but please make sure the patch fields do not conflict, else will panic
    // ```
    // let overall_patch = patch_1 + patch_2 + patch_3;
    // let new_item = item << overall_patch;
    // ```

    assert_eq!(
        item,
        Item {
            field_bool: true,
            field_int: 7,
            field_string: String::from("hello")
        }
    );
}

Also, you can modify the patch structure by defining #[patch(...)] attributes on the original struct or fields.

Struct attributes:

• #[patch(name = "...")]: change the name of the generated patch struct.
• #[patch(attribute(...))]: add attributes to the generated patch struct.
• #[patch(attribute(derive(...)))]: add derives to the generated patch struct.

Field attributes:

• #[patch(skip)]: skip the field in the generated patch struct.
• #[patch(name = "...")]: change the type of the field in the generated patch struct.
• #[patch(attribute(...))]: add attributes to the field in the generated patch struct.
• #[patch(attribute(derive(...)))]: add derives to the field in the generated patch struct.

Please check the traits of document to learn more.

The examples demo following scenarios.

• diff two instance for a patch
• create a patch from json string
• rename the patch structure
• check a patch is empty or not
• add attribute to patch struct
• show option field behavior
• show operators about patches

This crate also includes the following optional features:

• status(default): implements the PatchStatus trait for the patch struct, which provides the is_empty method.
• op (default): provide operators << between instance and patch, and + for patches
  • default: when there is a field conflict between patches, + will add together if the #[patch(addable)] or #[patch(add=fn)] is provided, else it will panic.
  • merge (optional): implements the Merge trait for the patch struct, which provides the merge method, and << between patches.
• std(optional):
  • box: implements the Patch<Box<P>> trait for T where T implements Patch<P>. This let you patch a boxed (or not) struct with a boxed patch.
  • option: implements the Patch<Option<P>> trait for Option<T> where T implements Patch<P>, please take a look at the example to learn more.
    • default: T needs to implement From<P>. When patching on None, it will based on from<P> to cast T, and this let you patch structs containing fields with optional values.
    • none_as_default: T needs to implement Default. When patching on None, it will patch on a default instance, and this also let you patch structs containing fields with optional values.
    • keep_none: When patching on None, it is still None.