pretty printer ICEs on large expressions: assertion failed: (self.right != self.left)

Consider the following code:

``` rust
#![feature(macro_rules)]

macro_rules! gen {
    ($i:expr;) => {};
    ($i:expr; $n0:ident, $($n:ident,)*) => {
        static $n0: int = $i; gen!($i + 1; $($n,)*)
    }
}

gen! { 0;
    I0,
    I1,
    I2,
    // lots of good stuff here
    I199,
}

fn main() {
}
```

Output with --pretty expanded:

```
#![feature(macro_rules)]
#![feature(phase)]
#![no_std]
#![feature(globs)]
#[phase(plugin, link)]
extern crate "std" as std;
extern crate "native" as rt;
#[prelude_import]
use std::prelude::*;

static I0: int =

    0;
static I1: int = 0 + 1;
static I2: int = 0 + 1 + 1;
// lots of good stuff here
static I103: int =
    0 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1
        + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 +
        1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1
        + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 +
        1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1
        + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1;
static I104: int =
error: internal compiler error: unexpected failure
note: the compiler hit an unexpected failure path. this is a bug.
note: we would appreciate a bug report: http://doc.rust-lang.org/complement-bugreport.html
note: run with `RUST_BACKTRACE=1` for a backtrace
task 'rustc' failed at 'assertion failed: (self.right != self.left)', /build/rust-git/src/rust/src/libsyntax/ast_util.rs:694

stack backtrace:
   1:     0x7f5dbdbb4b50 - rt::backtrace::imp::write::h7677f84be452f7e2iHq
   2:     0x7f5dbdbb7d10 - <unknown>
   3:     0x7f5dbe3683f0 - unwind::begin_unwind_inner::hed657adb365550c0MTd
   4:     0x7f5dba237660 - <unknown>
   5:     0x7f5dba39a0d0 - print::pp::Printer::pretty_print::h1865774e32c541f5LpW
   6:     0x7f5dba3abfa0 - print::pprust::State<'a>::word_space::h454016a946c40a4038X
   7:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
   8:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
   9:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  10:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  11:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  12:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  13:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  14:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  15:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  16:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  17:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  18:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  19:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  20:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  21:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  22:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  23:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  24:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  25:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  26:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  27:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  28:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  29:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  30:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  31:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  32:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  33:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  34:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  35:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  36:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  37:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  38:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  39:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  40:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  41:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  42:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  43:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  44:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  45:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  46:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  47:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  48:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  49:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  50:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  51:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  52:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  53:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  54:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  55:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  56:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  57:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  58:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  59:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  60:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  61:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  62:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  63:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  64:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  65:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  66:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  67:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  68:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  69:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  70:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  71:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  72:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  73:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  74:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  75:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  76:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  77:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  78:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  79:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  80:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  81:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  82:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  83:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  84:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  85:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  86:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  87:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  88:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  89:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  90:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  91:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  92:     0x7f5dba2ab530 - print::pprust::State<'a>::print_expr::h7be58717952ade00GRZ
  93:     0x7f5dba2a75d0 - print::pprust::State<'a>::print_item::ha8b4b7caf20d57edAIY
  94:     0x7f5dba39fce0 - print::pprust::State<'a>::print_mod::h7d36275f9c448b20vnY
  95:     0x7f5dba39f690 - print::pprust::print_crate::h9e4149b4e3dac92f3FX
  96:     0x7f5dbefd92c0 - <unknown>
  97:     0x7f5dbefd2a70 - driver::pretty::pretty_print_input::hc71b3b2ff868053faxy
  98:     0x7f5dbefe2cd0 - <unknown>
  99:     0x7f5dbefe2bb0 - <unknown>
  100:     0x7f5dbe7722b0 - <unknown>
 ... <frames omitted>
```

Full code here: https://gist.github.com/anonymous/87527e1e822bdb15bfdb

```
$ rustc -v
rustc 0.12.0-pre (8067f4425 2014-09-17 21:15:38 +0000)
```


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pretty printer ICEs on large expressions: assertion failed: (self.right != self.left) #17406

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pretty printer ICEs on large expressions: assertion failed: (self.right != self.left) #17406

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