push: Add push rule to ignore room server ACLs

[zecakeh]

According to MSC3786.

---

Preview: https://pr-1242--ruma-docs.surge.sh

[zecakeh]

It seems like the indexset! macro doesn't like to have feature flags inside its definition and tries to insert an empty value in the set.

I don't understand how that's possible… Shouldn't the feature flag leave or remove the line below, so it should be transparent for the macro?

[jplatte]

Attributes (cfg or otherwise) can't be transparent to function-like macros or attribute macros as there is no way for the compiler to know how much of the macro input the attribute would apply to.

[zecakeh]

Ah right. How can we solve this then? I don't think that creating a set for every feature flag combination is a good idea…

It seems like in recent versions we can create an IndexSet from an array.

[jplatte]

Using an array literal seems good, otherwise we can use mutation to first create the initial set, then add elements conditionally.

[ShadowJonathan]

LGTM