How would this work, given that the WeakRef holds an array that is uniquely referenced in the WeakMap? Wouldn't each individual array item need to be a WeakRef—or alternatively replace the array with a WeakSet (that probably won't work as I assume iteration is necessary)—to avoid collecting the array entirely since the WeakMap is the sole owner of the array otherwise?

Using a WeakRef for each individual item would also work, however it seems redundant.

From my understanding is that the leak was being because whilst the WeakMap doesn’t hold keys as strong reference, the value are, setting the containing array in a WeakRef seems to solve this, in fact in the profiler the leak was no longer observed.

Are you thinking that the value WeakRef might be collected before the WeakMap key?

My reasoning is that the array that is stored in the WeakRef is the only place it's stored, so the only retainer is the WeakRef itself which is a weak retainer... so I'd presume that the array is always eligible to be GC'd. That's my mental model though, my understanding might certainly be inaccurate here.

That is what I though at first too but it does not appear to be the case.

const wm = new WeakMap();
const key = { a: 1 };
wm.set(key, new WeakRef([2]));

globalThis.gc();

console.log(wm.get(key)?.deref()); // [ 2 ]

Not that you run into a #54051 situation :)

https://caniuse.com/mdn-javascript_builtins_array_at
https://caniuse.com/mdn-javascript_builtins_weakref

Of course it's totally in the realm of https://angular.dev/tools/cli/build#configuring-browser-compatibility; but a mention in the changelog or migration guide would be very appreciated for folks that like to include a polyfill for a while to not have that harsh of a cutoff (and not be surprised by error reporting).

It's only used if the browser/environment supports it. You can read the rest of the PR, but basically this:

    // Only configure the injector profiler when running in the browser and WeakRef is defined.
    // In some G3 tests WeakRef is undefined as they user older (unsupported) browsers to test.
    if (typeof window !== 'undefined' && typeof WeakRef !== 'undefined') {