My Rust 1.0 code on the other hand has compiled just fine since the dawn of stable rust.
The only really stable languages might be C89, C99, some fortran, and Cobol.
I think the Rust team does a fantastic job of supporting old code bases with new compilers. Crater is a testament to how much they care in fact.
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)
Of course, without more info nobody can tell which of these or even whether it is one of these two or something else.