Panic on accessing codemap of size 0

[cassiersg]

Using rustc_driver::run_compiler(["empty_file.rs"], ...), the compilation is stopped after parsing.
The codemap is retrieved, and the call to lookup_char_pos panics when we visit the AST:

impl visit::Visitor for FmtVisitor {
    fn visit_mod(&mut self, m: &'v ast::Mod, s: Span, _: ast::NodeId) {
        if self codemap.lookup_char_pos(s.lo).file.name
[...]

I'm wondering if it's an unexpected behavior or if this situation violates an invariant of codemap.

Backtrace:

thread '<main>' panicked at 'position 0 does not resolve to a source location', ../src/libsyntax/codemap.rs:908
stack backtrace:
   1:     0x7fb91319f92e - sys::backtrace::write::he24e072826f78dbbpqs
   2:     0x7fb9131a79ec - panicking::on_panic::hfe8dee88ad5f1f8f69w
   3:     0x7fb91316a53e - rt::unwind::begin_unwind_inner::h30a8282e6074626cKPw
   4:     0x7fb91316b26c - rt::unwind::begin_unwind_fmt::h0d67ce05a74a7ba4QOw
   5:     0x7fb913987313 - codemap::CodeMap::lookup_filemap_idx::hdb5a6d4b9a382f2by4z
   6:     0x7fb913986e11 - codemap::CodeMap::lookup_char_pos::hcfccdd83504b914fJJz
   7:     0x7fb9152b24c8 - visitor::FmtVisitor<'a>.visit..Visitor<'v>::visit_mod::hb8cb15535f0f5ffbZUa
                        at /home/cassiersg/rustfmt/src/visitor.rs:249
   8:     0x7fb9152f174b - visit::walk_crate::h11272277405317629321
                        at ../src/libsyntax/visit.rs:152
   9:     0x7fb9152f1489 - fmt_ast::h0042e145cf43e2281Jd
                        at /home/cassiersg/rustfmt/src/lib.rs:165
  10:     0x7fb9152fbc3d - RustFmtCalls.CompilerCalls<'a>::build_controller::closure.12943
                        at /home/cassiersg/rustfmt/src/lib.rs:309
  11:     0x7fb914ce8a88 - driver::compile_input::hc2338ec138000972Tba
  12:     0x7fb914dc67d3 - run_compiler::h64b107044b742926x7b
  13:     0x7fb9152fc5d2 - run::h1118ec9f2f6cbfc82Wd
                        at /home/cassiersg/rustfmt/src/lib.rs:340
  14:     0x7fb91525a5c3 - main::h78aa2f0fe725515fkaa
                        at bin/rustfmt.rs:27
  15:     0x7fb91321f758 - rust_try_inner
  16:     0x7fb91321f745 - rust_try
  17:     0x7fb9131a9527 - rt::lang_start::hdadb5fc58c5eeff334w
  18:     0x7fb91526159b - main
  19:     0x7fb91282078f - __libc_start_main
  20:     0x7fb91525a2a8 - _start
  21:                0x0 - <unknown>

rustc:

$ rustc --version --verbose
rustc 1.2.0-nightly (9ad0063a1 2015-06-20)
binary: rustc
commit-hash: 9ad0063a1772edca3299e2dc76187bbfa74e8b16
commit-date: 2015-06-20
host: x86_64-unknown-linux-gnu
release: 1.2.0-nightly

cc nrc/rustfmt#115

[Stebalien]

Sounds like #23301.

[marcusklaas]

This has been resolved, I think.

cc @cassiersg @nrc @steveklabnik