`HirId`-ification initiative

[zackmdavis]

HirIds were introduced in bc259ee (March 2017) as a replacement for ast::NodeIds after HIR lowering; this is said to be good for incremental compilation. The commentary for pull request #43740 (August 2017) says:

In the future the HirId should completely replace the NodeId in HIR nodes. [...] Ideally we convert more and more code from NodeId to HirId in the future so that there are no more NodeIds after HIR lowering anywhere.

The HIR chapter of the rustc development guide similarly says "while [NodeIds] are still in common use, they are being slowly phased out" (emphasis in original).

However, a drawback of the "gradually migrate over to the new way of doing things in the course of addressing other issues" strategy is that it's all too easy for the codebase to remain in a transitional state indefinitely. Thus, perhaps we should have this dedicated issue to track progress towards not using NodeIds after HIR lowering.

[michaelwoerister]

Thanks, @zackmdavis!

However, a drawback of the "gradually migrate over to the new way of doing things in the course of addressing other issues" strategy is that it's all too easy for the codebase to remain in a transitional state indefinitely.

Yes, that's indeed what has been happening over the last year. We've done enough to make incremental compilation work and then pretty much stopped.

Btw, it might be a good idea to add a custom implementation of Hash for HirId, something like:

impl Hash for HirId {
    fn hash<H: Hasher>(&self, state: &mut H) {
        let data: u64 = unsafe { mem::transmute(*self) };
        state.write_u64(data);
    }
}

It would have to be tested if that actually speeds things up.