Compiling with debug logging is never end.

When there is `println!` macro in the code, compiling is never end.

```rust
// print.rs
fn main() {
  println!("Hello!");
}
```

```bash
$ RUST_LOG=rustc rustc print.rs
```

And this is a part of output from stderr.

```bash
# ...
Looking up syntax::ast::DefId{crate: 1u32, node: 176234u32}
looking up syntax::ast::DefId{crate: 1u32, node: 176235u32} : extra::ebml::Doc<>{data: &[168u8, 16u8, 0u8, 0u8, 16u8, 51u8, 101u8, 53u8, 97u8, 101u8, 98u8, 56u8, 51u8, 55u8, 97u8, 101u8, 49u8, 54u8, 50u8
# ...
# vector which has infinite length.
```

* note : rust 0.9, 0.10-pre