As far as only allowing in the constructor, I think property-initializers would need to be considered valid as well

class Foo {
    private readonly bar = inject(Bar);
}

Might be also worth checking that the class is also @Injectable in the first place.

Beyond specifics, it would also be nice to have similar rules for effect and toSignal which both require an injection context.

This could be tricky to do correctly because there are some places outside of constructors that run in an injection context, which allows you to use inject.

• HTTP interceptors
• Route guards
• Resolvers
• App initializers
• Environment initializers
• Platform initializers

There's potentially other places that I've missed. Unfortunately, a lot of those things are just normal functions, which would make it difficult to reliably detect that they are running in an injection context.

It's a shame, because this would be a nice rule. ☹

PR here: #2892