AST: segfault with garbage cfg attr

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auto-reduced (treereduce-rust):

//@compile-flags: -frust-incomplete-and-experimental-compiler-do-not-use
#[cfg(|_: fn() -> B| C | D)]
mod foo {}

original:

// Expansion drives parsing, so conditional compilation will strip
// out outline modules and we will never attempt parsing them.

//@ check-pass

fn main() {}

#[cfg(|_: fn() -> B| C | D)]
mod foo {
    mod bar {
        mod baz; // This was an error before.
    }
}

Version information:

c742973c23448e321fba4d9ff98a9092dc0df7ba

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

Program output

/tmp/icemaker_global_tempdir.4jbydGKfmYZn/rustc_testrunner_tmpdir_reporting.8IbjznFHEQef/mvce.rs:1:7: error: unrecognised token '|' in meta item
    1 | #[cfg(|_: fn() -> B| C | D)]
      |       ^
/tmp/icemaker_global_tempdir.4jbydGKfmYZn/rustc_testrunner_tmpdir_reporting.8IbjznFHEQef/mvce.rs:1:7: error: failed to parse inner meta item in attribute
crab1: internal compiler error: Segmentation fault
0x300c7d8 internal_error(char const*, ...)
	../../gcc/diagnostic-global-context.cc:534
0x199d0df crash_signal
	../../gcc/toplev.cc:323
0xdf23c1 std::__uniq_ptr_impl<Rust::AST::MetaItemInner, std::default_delete<Rust::AST::MetaItemInner> >::_M_ptr() const
	/usr/include/c++/15.2.1/bits/unique_ptr.h:193
0xdf23c1 std::unique_ptr<Rust::AST::MetaItemInner, std::default_delete<Rust::AST::MetaItemInner> >::get() const
	/usr/include/c++/15.2.1/bits/unique_ptr.h:473
0xdf23c1 std::unique_ptr<Rust::AST::MetaItemInner, std::default_delete<Rust::AST::MetaItemInner> >::operator->() const
	/usr/include/c++/15.2.1/bits/unique_ptr.h:466
0xdf23c1 Rust::AST::Attribute::check_cfg_predicate(Rust::Session const&) const
	../../gcc/rust/ast/rust-ast.cc:4196
0xead14e Rust::CfgStrip::fails_cfg_with_expand(std::vector<Rust::AST::Attribute, std::allocator<Rust::AST::Attribute> >&) const
	../../gcc/rust/expand/rust-cfg-strip.cc:75
0xeaf41b Rust::CfgStrip::visit(Rust::AST::Module&)
	../../gcc/rust/expand/rust-cfg-strip.cc:1769
0xe4262f 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:422
0xe4262f Rust::AST::DefaultASTVisitor::visit(Rust::AST::Crate&)
	../../gcc/rust/ast/rust-ast-visitor.cc:39
0xeafc39 Rust::CfgStrip::visit(Rust::AST::Crate&)
	../../gcc/rust/expand/rust-cfg-strip.cc:165
0xe7aed8 Rust::Session::expansion(Rust::AST::Crate&, Rust::Resolver2_0::NameResolutionContext&)
	../../gcc/rust/rust-session-manager.cc:943
0xe87f25 Rust::Session::compile_crate(char const*)
	../../gcc/rust/rust-session-manager.cc:618
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.

Reproduce the crash with the reduced Rust input and command shown in the issue. Start at Rust::AST::Attribute::check_cfg_predicate in gcc/rust/ast/rust-ast.cc:4196, then follow the cfg-strip callers in gcc/rust/expand/rust-cfg-strip.cc. Done means the malformed cfg attribute still reports its parse errors without causing a segmentation fault.

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