title: Stack trace with line numbers when running cargo run
body: <p>Doing <code>RUST_BACKTRACE=1 cargo run</code> gives a stack trace when an error occurs as shown below. Is there any way to get the file &amp; line number instead of the hex address?</p>
  
  <pre><code>thread '&lt;main&gt;' panicked at 'called `Result::unwrap()` on an `Err` value: Parsing: invalid encoding byte', /Users/rustbuild/src/rust-buildbot/slave/stable-dist-rustc-mac/build/src/libcore/result.rs:729
  stack backtrace:
     1:        0x104c9403f - sys::backtrace::write::h7807ec07859fb503t1r
     2:        0x104c980e4 - panicking::on_panic::ha0ed2b9b562a7f9ctZv
     3:        0x104c7c4d5 - rt::unwind::begin_unwind_inner::hbfb8d99cb52be7a1cHv
     4:        0x104c7cd66 - rt::unwind::begin_unwind_fmt::hac7eda7c3f3b8498QFv
     5:        0x104c979bc - rust_begin_unwind
     6:        0x104cb75f5 - panicking::panic_fmt::h051633da0da2e362wwy
     7:        0x104be0877 - result::Result&lt;T, E&gt;::unwrap::h15040486031244389916
     8:        0x104bdc6f1 - main::h393644ca2d1fdb82uLa
     9:        0x104c99e18 - rust_try_inner
    10:        0x104c99e05 - rust_try
    11:        0x104c988e8 - rt::lang_start::h5324dae87dacdac8YTv
    12:        0x104be500e - main
  An unknown error occurred
  </code></pre>