vec::test_drain_range exposes overflow "bug" in slice code

[pnkfelix]

While doing a full make check-stage1 on a rust build configured with --enable-debug, I encountered this failure in collectionstest:

---- vec::test_drain_range stdout ----
    thread 'vec::test_drain_range' panicked at 'arithmetic operation overflowed', /Users/fklock/Dev/Mozilla/rust-span_to_lines/src/libcore/slice.rs:639

That line is here in this macro:

macro_rules! slice_offset {
    ($ptr:expr, $by:expr) => {{
        let ptr = $ptr;
        if size_from_ptr(ptr) == 0 {
            transmute(ptr as usize + $by) // <=== this line
        } else {
            ptr.offset($by)
        }
    }};
}

According to preliminary analysis, this arises because of this code elsewhere in slice:

                        self.end = slice_offset!(self.end, -1);

The -1 there is going to get interpreted as a usize, and thus its going to cause the overflow check to fire.

Note that there is no actual bug being caught in this case; in fact the cases where it is signaling overflow (namely where self.end is nonzero) are exactly the only cases that should be treated as "working."

---

Anyway, an easy short-term fix for this is to introduce distinct slice_add_offset! and slice_sub_offset! macros that both take non-negative input values. I'm testing that change locally now.

[pnkfelix]

(I guess if I were keeping track of false positives signaled by arithmetic overflow, then this would qualify as one.)