Path to enabling build script and proc macro support by default

[lnicola]

We've had a couple issue reports that turned out to be fixed by enabling build script and/or proc macro support. I think the reason these are off by default is that they can slow down the first load of a project by a lot. On the other hand, it's clear that they are needed to provide a good IDE experience.

So I think we'll want to turn them on in the future. The question, then, is when we should do that. We could:

• enable them right now
• wait for better proc macro support (to offset the loading cost with even better IDE support)
• wait for a sandboxing story (which won't happen on a reasonable timeline)
• anything else?

[bjorn3]

Another option would be to add a diagnostic for env!("OUT_DIR") and #[derive(SomeProcMacroDerive)] when the respective features are disabled to enable them. This won't help for dependencies, but it could for the current crate without the drawbacks associated with always enabling them.

[lnicola]

Closing in favor of #6448.