#[no_panic] --> #[no_panic_if(cond)]

[ninehusky]

Implements conditional panics, which is the starter step for us to target things like no_panic on traits/closures/etc.

The rough implementation roadmap in my head:

• Change the no_panic attribute so that it's accompanied with an Expr describing some condition. no_panic without an expr desugars to no_panic_if(true).
• In the same place where we check preconditions/refinements, if a function is annotated with no_panic_if(cond), then append that as a precondition. e.g., if I have some function which requires len > 0, and a no_panic spec saying idx < len, then the precondition to prove is len > 0 && idx < len.

EDIT: Freezing this for now so that we can work on the more specific solution of marking closures as no_panic. We can come back to this later.

[ninehusky]

It's still not 100% clear to me why this is on the critical path to checking closures/traits with no_panic.

One thing me and Evan chatted about today was that this could empower users to have a more ergonomic interface for Flux: this allows us to say something like "scream at the user if they care about one specific flavor of checks, otherwise don't care." That seems a bit orthogonal to checking traits, though -- I think I'm forgetting the motivation behind this change!

To be clear: I don't have any specific concerns. I just forget why this is the next step!

Edit: oh, it's because this is no_panic_bikeshed from this discussion.

[ninehusky]

Thanks for the help @nilehmann!! Ready for more comments :)

[nilehmann]

@ninehusky, I fixed some details and merged your changes in #1492