"bananas".contains("nana") returns false

[nham]

Currently, "bananas".contains("nana") returns false. It is the only substring of "bananas" for which this is true:

fn main() {
    let b = "bananas";
    for i in range(0, b.len()) {
        for j in range(i, b.len() + 1) {
            let curr = b.slice(i, j);
            println!("{} - {}", b.contains(curr), curr);
        }
    }
}

results in:

true - 
true - b
true - ba
true - ban
true - bana
true - banan
true - banana
true - bananas
true - 
true - a
true - an
true - ana
true - anan
true - anana
true - ananas
true - 
true - n
true - na
true - nan
false - nana
true - nanas
true - 
true - a
true - an
true - ana
true - anas
true - 
true - n
true - na
true - nas
true - 
true - a
true - as
true - 
true - s

I've done some preliminary work tracking this down. The problem seems to be in core::str::TwoWaySearcher.

[Gankra]

"(x^i)anana".contains("nana") only passes for i = 0 (mod 4), for when the underflow is fixed.

So e.g. "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxanana".contains("nana") fails.

Edit: You can pad the tail with an arbitrary number of x's, it doesn't change the behaviour.