ICE when invoking attribute macro on crate root

[abonander]

An ICE occurs when invoking a procedural macro attribute on the root of a crate with #![my_proc_macro].

Relevant backtrace frames:

thread 'rustc' panicked at 'called `Option::unwrap()` on a `None` value', src\libcore\option.rs:329
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: syntax::ext::expand::MacroExpander::expand_crate
             at C:\Users\cyber\Rust\rust\src\libsyntax\ext\expand.rs:225
   1: rustc_driver::driver::phase_2_configure_and_expand::{{closure}}<closure>
             at C:\Users\cyber\Rust\rust\src\librustc_driver\driver.rs:699

This happens on the latest nightly [rustc 1.18.0-nightly (ad36c2f55 2017-04-09)], however because I'm lazy I got this backtrace from the last debug build from my last PR so the lines have moved. I suspect it's one of the two unwraps() at line 217.

cc @jseyfried

[abonander]

@jseyfried If you haven't started on this one I'll go ahead and take a crack at it since people are stumbling on it.

[abonander]

That second unwrap is syntax::ptr::P::unwrap() which is infallible. That's kind of misleading, but okay.