dyn-inventory

dyn-inventory

proc macro for building runtime plugin registries using dyn-compatible traits and the inventory crate.

this crate generates code to:

• register plugins that implement a trait object (dyn trait)
• carry typed metadata alongside each plugin
• collect and instantiate all registered plugins at runtime

use dyn_inventory::dyn_inventory;

pub trait MyPlugin {
    fn handle(&self);
}

dyn_inventory! {
    Plugin<Handle: MyPlugin> {
        pub name: &'static str,
        desc: &'static str,
        handle: Handle
    }
}

mod my_plugin {
    use crate::{MyPlugin, Plugin, PluginInit};

    dyn_inventory::emit! {
        Handle MyPlugin as Plugin {
            name = "my plugin for abc-framework",
            desc = "implements my plugin by doing xyz"
        }
    }

    impl MyPlugin for Handle {
        fn handle(&self) {
            println!("MyPlugin was used");
        }
    }
}


fn main() {
    let collected = PluginCollector::new();
    for plugin in &collected.plugins {
        plugin.handle.handle();
        // >> "MyPlugin was used"
    }
}

Why dyn-compatible traits

the plugins produced by this crate are stored and used as Box<dyn Trait>. when used with inventory, this allows for new plugin registries to be developed for decentralized libraries and frameworks.

Quick Start

1. add dependencies:

[dependencies]
inventory = "0.3"
dyn-inventory = "0.2"

2. define a trait that is dyn-compatible:

pub trait Greeter {
    fn greet(&self) -> String;
}

3. declare your inventory using the dyn_inventory! proc macro:

pub trait Greeter {
    fn greet(&self) -> String;
}

dyn_inventory::dyn_inventory!(
    GreeterPlugin<T: Greeter> {
        name: &'static str,
        version: u32,
        t: T,
    };
);

[!TIP] what this generates:

• a struct GreeterPlugin with the fields you declared, and a Box<dyn Greeter>
• an inventory registration type inventory::collect!(GreeterPluginInit)
• a collector GreeterPluginCollector that has plugin of type Vec<GreeterPlugin>

4. register a plugin somewhere in your code (could be another crate that depends on your trait crate):

use crate::{Greeter, GreeterPlugin, GreeterPluginInit};
use dyn_inventory::emit;

// this expands to a unit struct named `MyGreeter` and registers it into the inventory
emit! {
    MyGreeter Greeter for GreeterPlugin {
        name = "hello",
        version = 1,
    }
}

// you implement the trait for the generated unit struct
impl Greeter for MyGreeter {
    fn greet(&self) -> String { "hi".to_string() }
}

5. collect your plugins at runtime:

let collected = GreeterPluginCollector::new();
for plugin in &collected.plugins {
    // `plugin.t` is now a `Box<dyn Greeter>`; other fields are your metadata
    println!("{} -> {}", plugin.name, plugin.t.greet());
}

Macro Syntax

use dyn_inventory::dyn_inventory;

dyn_inventory!(
    // StructName = the name of the struct that holds the Box<dyn TraitName>
    // TraitName - the trait which needs a dyn-inventory
    StructName<Handle: TraitName> {
        // exactly one field must have type `Handle`.
        // the field whose type equals the generic parameter (`Generic`) is treated as the plugin “handle”.
        // internally during registration this field is filled with a function pointer `fn() -> Box<dyn TraitName>`, and the collector converts it to `Box<dyn TraitName>` by calling it.
        handle: Handle,

        // optional visibity specifier
        // any number of metadata fields are preserved
        pub|pub(crate) field_name: &'static str,
        pub other_field: usize,
    };
    // optional, comma-separated extra params
    init_name = InitStructName,
);

Extra Parameters

two extra params are currently accepted:

• init_name = ident
  • sets the name of the generated initialization struct. by default it is the snake_case of StructName (for example, GreeterPlugin -> greeter_plugin).

Advanced: customizing collection

the collector type is named by appending Collector to your struct name. it exposes:

• new() -> builds the collection without modification
• new_with(|item: &mut StructName| {...}) -> allows you to mutate the raw entries after they are instantiated into Box<dyn TraitName>

Constraints

• your trait must be object-safe (dyn-compatible)
• the inventory crate must be linked into the final binary; ensure your plugin crates depend on inventory and your main binary pulls in the crates that perform registrations
• plugins must not carry state. instead, pass state as trait function parameters.