Well of course a hello world Rust project would just compile fine since Rust 1.0, unlike my recent Rust project which broke again and that just broke on a minor version bump.

As for another ancient Rust project which was also on 1.0 when I was trying the language out, the borrow checker on 1.52.1 screeched at every line of my code.

While C++ is just as bad, the fact that Firefox must always use the latest Rust compiler gives you a hint that it will not work on lower versions; hence the need of a MSRV. (Minimum Supported Rust Version)

If the borrow checker of 1.52.1 "screeches" on every LOC but it compiles on 1.0 then very likely either there's some bug in 1.52.1 that you should report or you code was never correct and should never have compiled in 1.0 in the first place but did due to a compiler bug that since got fixed.

Of course, without more info nobody can tell which of these or even whether it is one of these two or something else.

My project had about 40kloc and acted as a low latency ad server. Far from a hello world.