[Bug rust/107633] New: [13 regression] Bootstrap failure due to -Werror=unused-parameter and -Werror=dangling-reference

["ro at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107633

            Bug ID: 107633
           Summary: [13 regression] Bootstrap failure due to
                    -Werror=unused-parameter and
                    -Werror=dangling-reference
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rust
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ro at gcc dot gnu.org
                CC: dkm at gcc dot gnu.org
  Target Milestone: ---
              Host: i386-pc-solaris2.11
            Target: i386-pc-solaris2.11
             Build: i386-pc-solaris2.11

I just tried bootstrapping the current devel/rust/master branch on Solaris/x86,
but ran into quite number of failues:

* For one, there were hundreds of instances of -Werror=unused-parameter, e.g.

/vol/gcc/src/hg/master/rust/gcc/rust/lex/rust-lex.cc: In member function
'std::tuple<Rust::Codepoint, int, bool> Rust::Lexer::parse_utf8_escape(char)':
/vol/gcc/src/hg/master/rust/gcc/rust/lex/rust-lex.cc:1326:32: error: unused
parameter 'opening_char' [-Werror=unused-parameter]
 1326 | Lexer::parse_utf8_escape (char opening_char)
      |                           ~~~~~^~~~~~~~~~~~
/vol/gcc/src/hg/master/rust/gcc/rust/ast/rust-ast-dump.cc: In member function
'virtual void Rust::AST::Dump::visit(Rust::AST::AttrInputMetaItemContainer&)':
/vol/gcc/src/hg/master/rust/gcc/rust/ast/rust-ast-dump.cc:243:42: error: unused
parameter 'input' [-Werror=unused-parameter]
  243 | Dump::visit (AttrInputMetaItemContainer &input)
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

* With those worked around (see below), there still a couple of
-Werror=dangling-reference
  errors:

/vol/gcc/src/hg/master/rust/gcc/rust/typecheck/rust-hir-type-check-type.cc: In
member function 'Rust::TyTy::BaseType*
Rust::Resolver::TypeCheckType::resolve_segments(Rust::NodeId, Rust::HirId,
std::vector<std::unique_ptr<Rust::HIR::TypePathSegment> >&, std::size_t,
Rust::TyTy::BaseType*, const Rust::Analysis::NodeMapping&, Location)':
/vol/gcc/src/hg/master/rust/gcc/rust/typecheck/rust-hir-type-check-type.cc:465:13:
error: possibly dangling reference to a temporary [-Werror=dangling-reference]
  465 |       auto &candidate = *candidates.begin ();
      |             ^~~~~~~~~
/vol/gcc/src/hg/master/rust/gcc/rust/typecheck/rust-hir-type-check-type.cc:465:44:
note: the temporary was destroyed at the end of the full expression
'candidates.std::set<Rust::Resolver::PathProbeCandidate>::begin().std::_Rb_tree_const_iterator<Rust::Resolver::PathProbeCandidate>::operator*()'
  465 |       auto &candidate = *candidates.begin ();
      |                    

I've hacked around both by adding -Wno-error=unused-parameter
-Wno-error=dangling-reference
to gcc/rust/Make-lang.in (RUST_CXXFLAGS), but this is supposed to work out of
the box. The build is in stage 2 now.