rustdoc crashes on weird macro `assertion failed: begin <= end', src/libcore/str.rs:1002`

[apoelstra]

I noticed this in the rust-crypto code base. Consider the following program temp.rs:

#![feature(macro_rules)]
macro_rules! define_struct(
 ($rounds:expr) => (
    struct Struct {
      sk: [u32, ..$rounds + 1]
    }
  )
)

define_struct!(2)

Running rustdoc temp.rs gets you

task '<unnamed>' failed at 'assertion failed: begin <= end', /home/apoelstra/dload/code/rust/src/libcore/str.rs:1002
stack backtrace:
   1:     0x7f75bff39fc0 - rt::backtrace::imp::write::h5e8bb7d7d4e816f7eVp
   2:     0x7f75bff3cfc0 - failure::on_fail::h97df37e59a66e1b5bgq
   3:     0x7f75c494a9a0 - unwind::begin_unwind_inner::h7ca997cbfa47d085lTd
   4:     0x7f75c494a5f0 - unwind::begin_unwind_fmt::h7cd358ab9a21219duQd
   5:     0x7f75c494a5a0 - rust_begin_unwind
   6:     0x7f75c4995bb0 - failure::begin_unwind::ha6e2bb86cd385c5crWj
   7:     0x7f75c0ad91f0 - str::_&'a str.StrSlice<'a>::slice::h5e51c1396a0001e6evh
   8:     0x7f75c0adbd80 - codemap::CodeMap::span_to_snippet::hc28fdd42f889b11aRkC
   9:     0x7f75c4f487c0 - clean::syntax..codemap..Span.ToSource::to_src::hbcbc3ed17f478af4k2g
  10:     0x7f75c4f39920 - clean::ast..Ty.Clean<Type>::clean::h2b85767de491c743wWe
  11:     0x7f75c4f63f20 - clean::ast..StructField.Clean<Item>::clean::hdc53298eafb6c10fkbf
  12:     0x7f75c4f641f0 - clean::Vec<T>.Clean<Vec<U>>::clean::h3601407636379359189
  13:     0x7f75c4f30120 - clean::doctree..Struct.Clean<Item>::clean::ha00405932417ad80ijf
  14:     0x7f75c4f29410 - clean::doctree..Module.Clean<Item>::clean::hafd601e9a5a4f5db1vb
  15:     0x7f75c4f27dc0 - clean::visit_ast..RustdocVisitor<'a>.Clean<Crate>::clean::h883b248e43fece75SHa
  16:     0x7f75c4f7a3d0 - core::run_core::h3a0e665b6465cf98rLh
  17:     0x7f75c5048150 - rust_input::closure.$x22closure$x22$LP$30020$RP$
  18:     0x7f75c503f790 - task::TaskBuilder<S>::try_future::closure.$x22closure$x22$LP$29784$RP$
  19:     0x7f75c4cacb20 - task::spawn_opts::closure.$x22closure$x22$LP$8587$RP$
  20:     0x7f75c49ac990 - rust_try
  21:     0x7f75c4947910 - unwind::try::h5e208c1a32550db4qHd
  22:     0x7f75c4947630 - task::Task::run::h0316a0f15219b9f2gSc
  23:     0x7f75c4cac910 - task::spawn_opts::closure.$x22closure$x22$LP$8531$RP$
  24:     0x7f75c49499f0 - thread::thread_start::h85e2b2ae4067e775kfd
  25:       0x38d9807e70 - start_thread
  26:       0x38d90f4db9 - clone
  27:                0x0 - <unknown>
task '<main>' failed at 'called `Result::unwrap()` on an `Err` value: Box<core::any::Any:Send>', /home/apoelstra/dload/code/rust/src/libcore/result.rs:548
stack backtrace:
   1:     0x7f75bff39fc0 - rt::backtrace::imp::write::h5e8bb7d7d4e816f7eVp
   2:     0x7f75bff3cfc0 - failure::on_fail::h97df37e59a66e1b5bgq
   3:     0x7f75c494a9a0 - unwind::begin_unwind_inner::h7ca997cbfa47d085lTd
   4:     0x7f75c494a5f0 - unwind::begin_unwind_fmt::h7cd358ab9a21219duQd
   5:     0x7f75c494a5a0 - rust_begin_unwind
   6:     0x7f75c4995bb0 - failure::begin_unwind::ha6e2bb86cd385c5crWj
   7:     0x7f75c502ecd0 - rust_input::hd8f276d4bba8946aU9s
   8:     0x7f75c5023ef0 - main_args::h7efb7f1389094a38DKs
   9:     0x7f75c5023dd0 - main::h6062a3701b4ba015bFs
  10:     0x7f75c4cad250 - start::closure.$x22closure$x22$LP$8657$RP$
  11:     0x7f75c49ac990 - rust_try
  12:     0x7f75c4947910 - unwind::try::h5e208c1a32550db4qHd
  13:     0x7f75c4947630 - task::Task::run::h0316a0f15219b9f2gSc
  14:     0x7f75c4cacee0 - start::h29522b57e3b2e373ake
  15:     0x7f75c4cace50 - lang_start::hc4a60e172b398ff6uje
  16:       0x38d9021c70 - __libc_start_main
  17:           0x4006f0 - <unknown>
  18:                0x0 - <unknown>

A workaround is to put parens around $round + 1.