Rust's three stability caveats are as follows:

"We reserve the right to fix compiler bugs, patch safety holes, and change type inference in ways that may occasionally require new type annotations."

https://blog.rust-lang.org/2014/10/30/Stability/#what-are-th...

The breakage in question here falls under changes to type inference, which are allowed within reason when they're relatively minor, when steps have been taken to mitigate any breakage in the broader ecosystem, and when the compiler has emitted warnings regarding the imminent change during a grace period, all of which have been done here.