cache_diff

Generate clean, human readable diffs between two cache structs

What

The CacheDiff trait provides a way to compare two structs and generate a list of differences between them. When it returns an empty list, the two structs are identical.

You can manually implement the trait, or you can use the #[derive(CacheDiff)] macro to automatically generate the implementation.

Attributes are:

cache_diff(rename = "<new name>") Specify custom name for the field
cache_diff(ignore) Ignores the given field
cache_diff(display = <function>) Specify a function to call to display the field

Why

Cloud Native Buildpacks (CNBs) written in Rust using libcnb.rs use a serializable struct to represent the state of a cache. When that data changes, we need to invalidate the cache, but also report back to the user what changed.

Due to the CNB layer implementation, this struct is often called "metadata".

Install

$ cargo add cache_diff

For ANSI colored output, add the bullet_stream feature:

$ cargo add cache_diff --features bullet_stream

Derive usage

By default a #[derive(CacheDiff)] will generate a diff function that compares each field in the struct. You can disable this dependency by specifying features = [].

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,
}
let diff = Metadata { version: "3.4.0".to_string() }
    .diff(&Metadata { version: "3.3.0".to_string() });

assert_eq!(diff.join(" "), "version (`3.3.0` to `3.4.0`)");

Struct fields must implement PartialEq and Display. Also note that PartialEq on the top level cache struct is not used or required. If you want to customize equality logic, you can implement the CacheDiff trait manually.

Ordering

The order of output will match the struct field definition from top to bottom:

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,
    distro: String,
}
let now = Metadata { version: "3.4.0".to_string(), distro: "Ubuntu".to_string() };
let diff = now.diff(&Metadata { version: "3.3.0".to_string(), distro: "Alpine".to_string() });

assert_eq!(diff.join(", "), "version (`3.3.0` to `3.4.0`), distro (`Alpine` to `Ubuntu`)");

Rename attributes

If your field name is not descriptive enough, you can rename it:

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    #[cache_diff(rename="Ruby version")]
    version: String,
}
let now = Metadata { version: "3.4.0".to_string() };
let diff = now.diff(&Metadata { version: "3.3.0".to_string() });

assert_eq!(diff.join(" "), "Ruby version (`3.3.0` to `3.4.0`)");

Ignore attributes

If the struct contains fields that should not be included in the diff comparison, you can ignore them:

use cache_diff::CacheDiff;

#[derive(CacheDiff)]
struct Metadata {
    version: String,

    #[cache_diff(ignore)]
    changed_by: String
}
let now = Metadata { version: "3.4.0".to_string(), changed_by: "Alice".to_string() };
let diff = now.diff(&Metadata { version: now.version.clone(), changed_by: "Bob".to_string() });

assert!(diff.is_empty());

Handle structs missing display

Not all structs implement the Display trait, for example std::path::PathBuf requires that you call display() on it. The #[derive(CacheDiff)] macro will automatically handle std::path::PathBuf for you, however if you have a custom struct that does not implement Display, you can specify a function to call instead:

use cache_diff::CacheDiff;


#[derive(CacheDiff)]
struct Metadata {
    #[cache_diff(display = my_function)]
    version: NoDisplay,
}

#[derive(PartialEq)]
struct NoDisplay(String);
fn my_function(s: &NoDisplay) -> String {
    format!("custom {}", s.0)
}

let now = Metadata { version: NoDisplay("3.4.0".to_string())};
let diff = now.diff(&Metadata { version: NoDisplay("3.3.0".to_string())});

assert_eq!(diff.join(" "), "version (`custom 3.3.0` to `custom 3.4.0`)");

cache_diff 0.1.0