Better assists for `type annotations needed, consider specifying the generic argument`

For example: given an iter: Iterator<Item=f64> if i call iter.sum() isn't it most likely that I want to sum them up to f64? In fact I believe that is the only possible output, yet sometimes the rustc type inference engine refuses to use that (iter.sum() / x). I kind of understand why, but maybe rust-analyzer can make smarter suggestions?

We could either program in a few common patterns like the one I mentioned above (maybe ask the community?) . Or implement more complete exhaustive checking on-demand as part of assists where the rustc trait resolver/type inference engine failed. Maybe both as I imagine the former is a lot more performant.

Note: I would actually be open to doing the implementation legwork on this if its a good beginner issue to get into the rust-analyzer codebase.

Start by examining rust-analyzer's assists and how they interact with the rustc type inference engine and trait resolver. Define whether the first milestone is support for common patterns such as Iterator<Item=f64>.sum() or more exhaustive on-demand checking, then establish tests for the proposed suggestions.