Supporting different Rust versions (not just channels).

[iddm]

Hi there! We all have different projects, and sometimes many. All of our projects may not necessarily use the latest Rust compiler (or even the nightly one) but maybe a little bit older or something we just agreed on for some reason. Due to that and not just that, we all specify the "Minimum Required Rust Version" in our README files. Sometimes, the MSRV CI breaks, and sometimes, we specify the MSRV as low as it could be in order to compile a runnable program. We may be using Rust 1.72 while the MSRV is 1.56. Unfortunately, it is not really convenient to recompile the whole project every time this happens just to fix one tiny little thing. For example, right now, the std::str::split method doesn't work correctly (as per documentation) in 1.56 but works everywhere else. It would have been really nice if we could just use the playground to test these two lines on the older compiler to confirm this is a problem just there and not somewhere else:

let v: Vec<&str> = "2020-11-03 23:59".split(&['-', ' ', ':', '@'][..]).collect();
assert_eq!(v, ["2020", "11", "03", "23", "59"]);

Wouldn't it be great if the playground supported older Rust versions rather than just the stable, beta and nightly? Thank you!