feat: add LazyDirect and WeakWatcher

[Frando]

Description

This adds two new structs that allow to create watchers without having to store the full current value inline.

• A LazyDirect that works like Direct only that it doesn't store the current value inline, only the epoch. It requires T: Default and returns T::default() when the underlying watchable has been dropped.
• A WeakWatcher that contains only a weak pointer to the shared state and thus is only a single pointer size long. It has upgrade() -> Option<Direct> and lazy_upgrade() -> LazyDirect functions to upgrade the WeakWatcher to a watcher.

With these two additions, you can use watchers for big values without increasing the size of the structs where the watcher is stored on.

Breaking Changes

Notes & open questions

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.
• All breaking changes documented.

[github-actions]

Documentation for this PR has been generated and is available at: https://n0-computer.github.io/n0-watcher/pr/31/docs/n0_watcher/

Last updated: 2025-11-10T12:18:07Z

[matheus23]

Unfortunately the latest changes to the Watcher trait require you to implement a fn peek(&self) -> &Self::Value, which makes implementing LazyWatcher impossible without storing a current: Self::Value in the struct.

The downstream use-case for this ended up using a Box<impl Watcher> instead of LazyWatcher to avoid ballooning struct sizes.

Closing this for these reasons ✌️