2018 edition allows ungated access to unstable crates

[alexcrichton]

When compiled with --edition 2018 this code compiles just fine:

use rustc;

fn main() {}

That's bad!

[Mark-Simulacrum]

@rust-lang/compiler Can we get some eyes on this, not necessarily to fix but at least to confirm that we believe a fix can be made and landed in time for RC? This is a hard-stop blocker for RC.

[eddyb]

So this is generally harmless (you can't actually get anything from the unstable crate), except for it being a minor dead-code-but-allowed hazard.

The culprit appears to be this special-casing:

rust/src/librustc_resolve/resolve_imports.rs

Lines 635 to 641 in e94df4a

	// FIXME: Last path segment is treated specially in import resolution, so extern crate
	// mode for absolute paths needs some special support for single-segment imports.
	if module_path.len() == 1 && (module_path[0].name == keywords::CrateRoot.name() ||
	module_path[0].name == keywords::Extern.name()) {
	let is_extern = module_path[0].name == keywords::Extern.name() ||
	(self.session.features_untracked().extern_absolute_paths &&
	self.session.rust_2018());

Which loads the crate (producing an error if no crate with that name exists), but doesn't record the resolution. So there's nothing to check - and presumably no way to use that broken import.

The part that's missing is this (recording the import resolutions):

rust/src/librustc_resolve/resolve_imports.rs

Lines 878 to 884 in e94df4a

	// Record what this import resolves to for later uses in documentation,
	// this may resolve to either a value or a type, but for documentation
	// purposes it's good enough to just favor one over the other.
	self.per_ns(|this, ns| if let Some(binding) = result[ns].get().ok() {
	let import = this.import_map.entry(directive.id).or_default();
	import[ns] = Some(PathResolution::new(binding.def()));
	});

But the special-casing exits early:

rust/src/librustc_resolve/resolve_imports.rs

Lines 693 to 694 in e94df4a

	let _ = self.try_define(directive.parent, target, TypeNS, binding);
	return None;

Expected minimal diff (between the two lines in the last snippet):

let import = self.import_map.entry(directive.id).or_default(); 
import[TypeNS] = Some(PathResolution::new(binding.def()));