rustc --cfg parsing is lax; ICEs on --cfg ""

[rprichard]

$ RUST_BACKTRACE=1 rustc --cfg ""
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'position 0 does not resolve to a source location', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/codemap.rs:773

stack backtrace:
   1:     0x7fe3b3c72752 - sys::backtrace::write::haa06f2eb4ff4d5e5ADA
   2:     0x7fe3b3c98792 - panicking::on_panic::hdcb20beee702c0b03IJ
   3:     0x7fe3b3bded69 - rt::unwind::begin_unwind_inner::hbe159b4d97e14ee66oJ
   4:     0x7fe3b3bdf111 - rt::unwind::begin_unwind_fmt::haee4ef7e01d3657fHnJ
   5:     0x7fe3b1196a38 - codemap::CodeMap::lookup_filemap_idx::h96ac1c59c75dc900u7C
   6:     0x7fe3b1194bc1 - codemap::CodeMap::lookup_char_pos::h51279553f4627f7bSUC
   7:     0x7fe3b11950f5 - codemap::CodeMap::lookup_char_pos_adj::hcaba10a387a5285a5UC
   8:     0x7fe3b119540d - codemap::CodeMap::span_to_string::hfafeb005cfdb9b99yVC
   9:     0x7fe3b11edca2 - diagnostic::emit::h3699619c2b53fd34oCE
  10:     0x7fe3b11eb5b7 - diagnostic::EmitterWriter.Emitter::emit::h192eeee6a59af027ZyE
  11:     0x7fe3b11e9d67 - diagnostic::Handler::emit::h8baf6fed67201d3d3eE
  12:     0x7fe3b119168a - diagnostic::SpanHandler::span_fatal::h087dc0374fe394cem2D
  13:     0x7fe3b11fe744 - parse::parser::Parser<'a>::parse_ident::hdcb90b6ba49d1733g3H
  14:     0x7fe3b124ebc6 - parse::attr::Parser<'a>.ParserAttr::parse_meta_item::h7d7f52f8f6262767m9T
  15:     0x7fe3b1251b6e - parse::parse_meta_from_source_str::hbce91171fcad4c3axvU
  16:     0x7fe3b1e62ec4 - iter::Map<I, F>.Iterator::next::h4056714858292786413
  17:     0x7fe3b1e62a47 - session::config::parse_cfgspecs::h6d2932293b2e7b8azip
  18:     0x7fe3b1e65459 - session::config::build_session_options::hbe60bdcd2bec83684ip
  19:     0x7fe3b4336cdc - run_compiler::h6c6deadca3c1da44N6b
  20:     0x7fe3b43356fc - thunk::F.Invoke<A, R>::invoke::h17587398869216592240
  21:     0x7fe3b4334350 - rt::unwind::try::try_fn::h13136719022791656042
  22:     0x7fe3b3d045b8 - rust_try_inner
  23:     0x7fe3b3d045a5 - rust_try
  24:     0x7fe3b4334adb - thunk::F.Invoke<A, R>::invoke::h15245498111397843904
  25:     0x7fe3b3c85f95 - sys::thread::thread_start::hf986f8b6540d7d70daF
  26:     0x7fe3adccf181 - start_thread
  27:     0x7fe3b385747c - __clone
  28:                0x0 - <unknown>

The compiler is trying to report that the --cfg argument is invalid, but I guess it's not prepared to handle 0-length source code?

$ rustc --version --verbose
rustc 1.0.0-nightly (cfea8ec41 2015-03-10) (built 2015-03-11)
binary: rustc
commit-hash: cfea8ec41699e25c8fb524d625190f0cb860dc71
commit-date: 2015-03-10
build-date: 2015-03-11
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly

I also think it's a little strange that I can pass arbitrary source code to the --cfg command-line argument, and trailing source code is frequently ignored:

e.g. I think these two invocations are equivalent:

rustc --cfg foo,bar --cfg '/*foo*/ feature="A","B" <-THIS IS IGNORED->' hello.rs
rustc --cfg foo     --cfg 'feature="A"' hello.rs

I'm also wondering if the syntax could be just --cfg feature=A. The quoting is inconvenient.

[Stebalien]

IMHO, this is a bug in libsyntax/codemap.rs:lookup_filemap_idx (reproduced below). The comment incorrectly states that we can't possibly be looking for a length zero filemap but, in this case, we're looking for a zero-length span in a zero-length filemap. I don't know if there's any clean way to fix this because of the way filemaps work. The simple fix is to say that one must not pass empty strings to parse_*_from_source_str functions. Something like (untested):

diff --git a/src/librustc/session/config.rs b/src/librustc/session/config.rs
index b999929..dc8f9fe 100644
--- a/src/librustc/session/config.rs
+++ b/src/librustc/session/config.rs
@@ -849,6 +849,9 @@ pub fn rustc_optgroups() -> Vec<RustcOptGroup> {
 // Convert strings provided as --cfg [cfgspec] into a crate_cfg
 pub fn parse_cfgspecs(cfgspecs: Vec<String> ) -> ast::CrateConfig {
     cfgspecs.into_iter().map(|s| {
+        if &s == "" {
+            early_error("empty --cfg argument");
+        }
         parse::parse_meta_from_source_str("cfgspec".to_string(),
                                           s.to_string(),
                                           Vec::new(),

    fn lookup_filemap_idx(&self, pos: BytePos) -> usize {
        let files = self.files.borrow();
        let files = &*files;
        let len = files.len();
        let mut a = 0;
        let mut b = len;
        while b - a > 1 {
            let m = (a + b) / 2;
            if files[m].start_pos > pos {
                b = m;
            } else {
                a = m;
            }
        }
        // There can be filemaps with length 0. These have the same start_pos as
        // the previous filemap, but are not the filemaps we want (because they
        // are length 0, they cannot contain what we are looking for). So,
        // rewind until we find a useful filemap.
        loop {
            let lines = files[a].lines.borrow();
            let lines = lines;
            if !lines.is_empty() {
                break;
            }
            if a == 0 {
                panic!("position {} does not resolve to a source location",
                      pos.to_usize());
            }
            a -= 1;
        }
        if a >= len {
            panic!("position {} does not resolve to a source location",
                  pos.to_usize())
        }

        return a;
    }