For me personally, I rarely perform site:github.com or site:stackoverflow.com searches in my browser's default search engine. I typically just enter a few keywords and try to make sense of what I find.
When I personally think of a search engine built for devs, I think of one that parses queries differently from the natural language processing that seems to be all the rage these days. For instance, when I search "?. C#" I want to know what this operator actually is. Instead, the algorithm omits the ?. and just returns the most generic results relating to C#. https://you.com/search?q=%3F.%20C%23&fromSearchBar=true This is true even when I put the "?." in quotes. One of the results is titled "Null coalescing operator" from Wikipedia, but this is actually misleading since ?. is actually the null-conditional operator. The information is there but it is not easy to find.
The other item on my wishlist for a dev-centric search engine is to be able to persist the main language(s) I am trying to search about. Its mildly tedious to type C3 and then my keywords (..... dammit, C#, this happens ALL the time), in each query to narrow things down and not get javascript results.