What is the structure of the sentence 'const to the rescue'?

The following is the context:

Despite my fastidious coding habits, I have made a silly mistake and typed += when I meant to type +. As a result, when concatUnsafe is called, it will modify the arguments out and s1, which may come as surprise to the user who would expect a concatenation function to modify one of the source strings?

const to the rescue.

"X to the rescue" is an idiom that means that X is coming to save us from a bad situation. It means that const will be the solution to the problem. Using this idiom is a more playful and informal way of saying this.

It can be thought as omitting a couple of words:

const [is coming] to the rescue.

In other words, const will save you from making the programming mistake described in the text. (By the look of it, the author is talking about strings in C, which are indeed a potential minefield - so tread carefully!)