> It's too bad that there's not a good way to have a broader way to switch between dialects within a project where in one place it's "this portion of the code is perf-critical and carefully reviewed" and in another "this portion of the code is perf-oblivious and has lots of async state that's easy to get wrong". I've wondered if it's almost worth mixing two separate languages (like a GCed one for the latter) just to make the distinction clear.

You are describing Rust's "unsafe" keyword.

And this kind of code is very literally the original impetus of it. The language was sort of originally designed to implement a browser, after all.