Default method ICE when (unused) parameter is unnamed.

[pnkfelix]

tic.rs:

trait T {
    fn foo(hmm:int, int) -> int { hmm + 1 }
}

fn main() {
}

% rustc --version
/Users/fklock/opt/rust-dbg/bin/rustc 0.8-pre (7b2163d 2013-08-05 00:56:00 -0700)
host: x86_64-apple-darwin
% RUST_LOG=rustc=1,::rt::backtrace rustc  /tmp/tic.rs
rust: task failed at 'index out of bounds: the len is 0 but the index is 0', /Users/fklock/Dev/Mozilla/rust.git/src/librustc/middle/liveness.rs:1604
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug
note: try running with RUST_LOG=rustc=1,::rt::backtrace to get further details and report the results to github.com/mozilla/rust/issues
rust: task failed at 'explicit failure', /Users/fklock/Dev/Mozilla/rust.git/src/librustc/rustc.rs:358
rust: domain main @0x7fc112008410 root task failed
%

(an easy work-around is to put a name on the second parameter above. But it would be better to either support this, or issue a proper error message for the erroneous input.)

[pnkfelix]

Part of #2794

[msullivan]

This probably ought to be just disallowed. It's not allowed for regular function defs. Closely related to #8468.

[msullivan]

I may just fix the crash, though, to close this part, and leave the decision about whether to disallow it as part of #8468.