Make all submodules in the rustc repo optional

[RalfJung]

With stdarch being a subtree, we have almost reached the point where the majority of contributors do not need to have any submodules checked out. There's just one left: library/backtrace. Given how poorly submodules interact with a lot of the rest of git, it would be great to also get rid of this final mandatory submodule somehow. Is that a possibility?

Cc @alexcrichton @tgross35

[tgross35]

@workingjubilee does much more with backtrace than I do. Though, I don’t see any reason it couldn’t also be a subtree.

[tgross35]

That being said, backtrace updates are pretty infrequent and it doesn’t typically rely on unstable features. Maybe we should look into just using the crates.io version, to avoid additional sync overhead? No idea what the history is here.

[RalfJung]

AFAIK that's hard because the crate needs access to std internals.

[ChrisDenton]

Yes to be clear backtrace is currently used as a module within library/std not as a separate crate:

rust/library/std/src/lib.rs

Lines 697 to 699 in df32e15

	#[path = "../../backtrace/src/lib.rs"]
	#[allow(dead_code, unused_attributes, fuzzy_provenance_casts, unsafe_op_in_unsafe_fn)]
	mod backtrace_rs;

[tgross35]

Yeah I got that answer in Zulip and forgot to follow up here, too bad.

One downside of making this a subtree is that it means we might have to sync both directions, which is complexity I don't think we need for backtrace. We could move the crate entirely and publish from r-l/r, but that also doesn't seem ideal.