Looks nice. A lot of people don’t realize how expensive Linq can be. That said, my preference for most (source) codegen is in the editor – this would be nice as an editor plugin.
Worth bearing in mind that this only helps if you are using LINQ against in-memory objects (i.e. LINQ to Objects). Depending on how this detects IQueryable providers it will either skip it, break it or worse of all silently 'work' but pull all the remote data into memory for evaluation. I'm hoping it's skip.
It would be more interesting to apply it selectively to LINQ on hot code paths and keep LINQ's readability where performance and allocations are not a concern