You may be running into forwards compatibility issues, not backwards compatibility issues, which is what nightly is about.
The Rust Project releases a new stable compiler every six weeks. Because it is backwards compatible, most people update fairly quickly, as it is virtually always painless. So this may mean, if you don’t update your compiler, you may try out a new package version and it may use features or standard library calls that don’t exist in the version you’re using, because the authors updated regularly. There’s been some developments in Cargo to try and mitigate some of this, but since it’s not what the majority of users do, it’s taken a while and those features landed relatively recently, so they’re not widely adopted yet.
Nightly features are ones that aren’t properly accepted into the language yet, and so are allowed to break in backwards incompatible ways at any time.
The Rust Project releases a new stable compiler every six weeks. Because it is backwards compatible, most people update fairly quickly, as it is virtually always painless. So this may mean, if you don’t update your compiler, you may try out a new package version and it may use features or standard library calls that don’t exist in the version you’re using, because the authors updated regularly. There’s been some developments in Cargo to try and mitigate some of this, but since it’s not what the majority of users do, it’s taken a while and those features landed relatively recently, so they’re not widely adopted yet.
Nightly features are ones that aren’t properly accepted into the language yet, and so are allowed to break in backwards incompatible ways at any time.