> The resulting constraint is roughly similar to that of never ever breaking ABI in C++.

No, not even remotely. ABI-stability in C++ means that C++ is stuck with suboptimal implementations of stdlib functions, whereas Rust only stabilizes the exposed interface without stabilizing implementation details.

> Unfortunately editions don't allow breaking changes in the standard library

Surprisingly, this isn't true in practice either. The only thing that Rust needs to guarantee here is that once a specific symbol is exported from the stdlib, that symbol needs to be exported forever. But this still gives an immense amount of flexibility. For example, a new edition could "remove" a deprecated function by completely disallowing any use of a given symbol, while still allowing code on an older edition to access that symbol. Likewise, it's possible to "swap out" a deprecated item for a new item by atomically moving the deprecated item to a new namespace and making the existing item an alias to that new location, then in the new edition you can change the alias to point to the new item instead while leaving the old item accessible (people are exploring this possibility for making non-poisoning mutexes the default in the next edition).