ICE: NR2: segfault with edition 2018

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auto-reduced (treereduce-rust):

//@compile-flags: -frust-edition=2018 -frust-incomplete-and-experimental-compiler-do-not-use
mod a {

    use ::*;
}

original:

// { dg-additional-options "-frust-edition=2015" }

#![feature(no_core)]
#![no_core]

mod a {
    #[allow(unused)]
    use ::*;
}

Version information:

696cb485cfa9969c952d6765889d82ed548ca15c

Command:
/home/matthias/vcs/github/gccrs/gccrs-build/gcc/crab1 -frust-edition=2018 -frust-incomplete-and-experimental-compiler-do-not-use

Program output

crab1: error: failed to locate crate ‘core’
crab1: error: failed to locate crate ‘std’
crab1: error: unknown crate ‘core’
crab1: error: unknown crate ‘std’
crab1: internal compiler error: Segmentation fault
0x32c0107 internal_error(char const*, ...)
	../../gcc/diagnostic-global-context.cc:787
0x1a7e17f crash_signal
	../../gcc/toplev.cc:325
0x10026f8 Rust::Resolver2_0::Early::build_import_mapping(std::pair<unsigned int, std::vector<Rust::Resolver2_0::TopLevel::ImportKind, std::allocator<Rust::Resolver2_0::TopLevel::ImportKind> > >&&)
	../../gcc/rust/resolve/rust-early-name-resolver-2.0.cc:228
0x10029fa Rust::Resolver2_0::Early::visit(Rust::AST::UseDeclaration&)
	../../gcc/rust/resolve/rust-early-name-resolver-2.0.cc:584
0xe81450 void Rust::AST::DefaultASTVisitor::visit<Rust::AST::Item>(std::unique_ptr<Rust::AST::Item, std::default_delete<Rust::AST::Item> >&)
	../../gcc/rust/ast/rust-ast-visitor.h:421
0xe81450 Rust::AST::DefaultASTVisitor::visit(Rust::AST::Module&)
	../../gcc/rust/ast/rust-ast-visitor.cc:843
0xfee181 operator()
	../../gcc/rust/resolve/rust-default-resolver.cc:66
0xfee181 scope_inner<Rust::Resolver2_0::CanonicalPathCtx::scope<Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)::<lambda()> >(Rust::NodeId, std::string, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)::<lambda()>&&)::<lambda()>, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)::<lambda()> >
	../../gcc/rust/resolve/rust-name-resolution-context.h:456
0xfee181 scope<Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)::<lambda()> >
	../../gcc/rust/resolve/rust-name-resolution-context.h:399
0xfee181 scope<Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)::<lambda()> >
	../../gcc/rust/resolve/rust-name-resolution-context.h:392
0xfee181 operator()
	../../gcc/rust/resolve/rust-default-resolver.cc:69
0xfee181 __invoke_impl<void, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)::<lambda()>&>
	/usr/include/c++/16.1.1/bits/invoke.h:63
0xfee181 __invoke_r<void, Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)::<lambda()>&>
	/usr/include/c++/16.1.1/bits/invoke.h:113
0xfee181 _M_invoke
	/usr/include/c++/16.1.1/bits/std_function.h:295
0xfe4120 std::function<void ()>::operator()() const
	/usr/include/c++/16.1.1/bits/std_function.h:581
0xfe4120 Rust::Resolver2_0::NameResolutionContext::scoped(Rust::Resolver2_0::Rib::Kind, unsigned int, std::function<void ()>, tl::optional<Rust::Identifier>)
	../../gcc/rust/resolve/rust-name-resolution-context.cc:341
0xfeb405 Rust::Resolver2_0::DefaultResolver::visit(Rust::AST::Module&)
	../../gcc/rust/resolve/rust-default-resolver.cc:73
0xffe82c Rust::Resolver2_0::Early::visit(Rust::AST::Module&)
	../../gcc/rust/resolve/rust-early-name-resolver-2.0.cc:308
0xe7f21f void Rust::AST::DefaultASTVisitor::visit<Rust::AST::Item>(std::unique_ptr<Rust::AST::Item, std::default_delete<Rust::AST::Item> >&)
	../../gcc/rust/ast/rust-ast-visitor.h:421
0xe7f21f Rust::AST::DefaultASTVisitor::visit(Rust::AST::Crate&)
	../../gcc/rust/ast/rust-ast-visitor.cc:39
/home/matthias/vcs/github/gccrs/gccrs-build/gcc/crab1 /tmp/icemaker_global_tempdir.Y6DWmzQt0PsB/rustc_testrunner_tmpdir_reporting.5kTXHAOMcrfb/mvce.rs -o/tmp/icemaker_global_tempdir.Y6DWmzQt0PsB/rustc_testrunner_tmpdir_reporting.5kTXHAOMcrfb/binaryname -frust-edition=2018 -frust-incomplete-and-experimental-compiler-do-not-use
Please submit a full bug report, with preprocessed source (by using -freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

Start by running the reduced Rust reproducer with -frust-edition=2018 and the incomplete compiler flag. Inspect build_import_mapping in gcc/rust/resolve/rust-early-name-resolver-2.0.cc at line 228, where the backtrace reports the crash. Done means the reproducer no longer segfaults and reports the invalid import or missing crate without an internal compiler error.

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