Investigate if proc-macro crates can use regular crate metadata.

[michaelwoerister]

Currently, proc-macro crates require a lot of special-casing like the following:

rust/src/librustc_metadata/decoder.rs

Lines 503 to 510 in b176285

	pub fn get_def(&self, index: DefIndex) -> Option<Def> {
	if !self.is_proc_macro(index) {
	self.entry(index).kind.to_def(self.local_def_id(index))
	} else {
	let kind = self.proc_macros.as_ref().unwrap()[index.to_proc_macro_index()].1.kind();
	Some(Def::Macro(self.local_def_id(index), kind))
	}
	}

I wonder if this is really necessary. It would be preferable if proc-macro crates could be treated as regular crates.

cc @rust-lang/compiler @jseyfried

[bjorn3]

I believe this has been fixed for a while.

[oli-obk]

looks like it. There's mostly special casing in the metadata encoder to just skip serializing entire kinds of items