The problem with this is that pollster doesn't, by itself, provide a signalling mechanism by which bottom-level futures (such as OS-provided IO primitives) can

The problem with this is that pollster doesn't, by itself, provide a signalling mechanism by which bottom-level futures (such as OS-provided IO primitives) can signal the executor. It isn't a "light-weight replacement" for tokio, tokio does a lot of stuff that pollster simply can't because tokio is a reactor + a runtime, whereas pollster is just a runtime. [This page](https://rust-lang.github.io/async-book/08_ecosystem/00_chapter.html#determining-ecosystem-compatibility https://rust-lang.github.io/async-book/08_ecosystem/00_chapt...) gives some nice details about the difference, and hopefully some sense of the sheer difficult that comes with trying to integrate a default reactor into `std` without it falling well short when it comes to extensibility.