pretty printer for uints sometimes drops the "u" suffix

[nikomatsakis]

If I run rustc --pretty normal with this input (tag-disr-val-shape.rs):

// xfail-pretty Issue #1510

tag color {
    red = 0xff0000;
    green = 0x00ff00;
    blue = 0x0000ff;
    black = 0x000000;
    white = 0xFFFFFF;
}

fn main() {
    assert uint::to_str(red as uint, 10u) == #fmt["%?", red];
    assert uint::to_str(green as uint, 10u) == #fmt["%?", green];
    assert uint::to_str(white as uint, 10u) == #fmt["%?", white];
}

then the 10us which appear in the asserts become 10. Looking at the code, it's fairly clear that ast_util::uint_ty_to_str() is wrong:

fn uint_ty_to_str(t: uint_ty) -> str {
    alt t {
      ty_u. { "" } ty_u8. { "u8" } ty_u16. { "u16" }
      ty_u32. { "u32" } ty_u64. { "u64" }
    }
}

I was just going to fix this function... but then I found that almost every other example of printing 10u which I have tried works! I cannot for the life of me figure out why. I believe I've even seen the output vary from run to run. So I'm wondering if there is a deeper bug: I've been digging into it but haven't found it yet. I'm getting a bit bored of this bug though and want to get hacking on other things, so I'm filing an issue. Anyhow, maybe someone else has some idea what's going on?