ICE `in get_locus, at rust/ast/rust-expr.h:303` with botched feature gate

[matthiaskrgr]

auto-reduced (treereduce-rust):

//@compile-flags: -frust-incomplete-and-experimental-compiler-do-not-use
#![feature(unused_variables, server = b"\0")]

original:

#![forbid(unstable_features)]
#![feature(unused_variables, server = b"\0")] //~ ERROR unstable feature

fn main() { }

Version information:

f942905

Possibly related line of code:

gccrs/gcc/rust/ast/rust-expr.h

Lines 297 to 309 in f942905

	// There are two Locations in MetaItemPathExpr (path and expr),
	// we have no idea use which of them, just simply return UNKNOWN_LOCATION
	// now.
	// Maybe we will figure out when we really need the location in the future.
	location_t get_locus () const override
	{
	rust_unreachable ();
	return UNKNOWN_LOCATION;
	}
	void accept_vis (ASTVisitor &vis) override;
	bool check_cfg_predicate (const Session &session) const override;

Command:
/home/matthias/vcs/github/gccrs/gccrs-build/gcc/crab1 -frust-incomplete-and-experimental-compiler-do-not-use

Program output

/tmp/icemaker_global_tempdir.TpCXxwJbOGok/rustc_testrunner_tmpdir_reporting.y2x4zrYuNwMc/mvce.rs:1:12: error: unknown feature ‘unused_variables’ [E0635]
    1 | #![feature(unused_variables, server = b"\0")]
      |            ^~~~~~~~~~~~~~~~
crab1: internal compiler error: in get_locus, at rust/ast/rust-expr.h:303
0x309a879 internal_error(char const*, ...)
	../../gcc/diagnostic-global-context.cc:787
0xc38387 fancy_abort(char const*, int, char const*)
	../../gcc/diagnostics/context.cc:1805
0x90bb44 Rust::AST::MetaItemPathExpr::get_locus() const
	../../gcc/rust/ast/rust-expr.h:303
0x11ef69d Rust::FeatureGate::visit(Rust::AST::Crate&)
	../../gcc/rust/checks/errors/feature/rust-feature-gate.cc:69
0xeae3a1 Rust::Session::compile_crate(char const*)
	../../gcc/rust/rust-session-manager.cc:640
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