Not to dismiss the importance of language stability, but forward porting in Rust should be significantly easier than Ruby for a few reasons:
* Rust's strong type system means the pieces fit together only in specific ways - roughly, if you can get it to compile again it will work; in more dynamic languages you have little confidence that the code works until runtime.
* Co-evolving the language with the downstream community is such a critical issue that Rust is developing several tools and processes to help, and this should set it apart from other open source languages that have gone down this path:
The Rust process already attempts to tag all breaking changes in the commit log with `[breaking-change]` and we've heard anecdotally that this has made forward-porting Servo much easier. This log isn't published anywhere besides the commit log yet, but it will be.
Secondly, Rust has a [stability](http://doc.rust-lang.org/rust.html#stability) system that tracks API stability at a fine level. This is influenced by node.js, but in Rust stability is detected and use of unstable API's can be enforced by the tooling. This is still in development but you can see it in the [docs](http://doc.rust-lang.org/std/intrinsics/).
* Rust's strong type system means the pieces fit together only in specific ways - roughly, if you can get it to compile again it will work; in more dynamic languages you have little confidence that the code works until runtime.
* Co-evolving the language with the downstream community is such a critical issue that Rust is developing several tools and processes to help, and this should set it apart from other open source languages that have gone down this path:
The Rust process already attempts to tag all breaking changes in the commit log with `[breaking-change]` and we've heard anecdotally that this has made forward-porting Servo much easier. This log isn't published anywhere besides the commit log yet, but it will be.
Secondly, Rust has a [stability](http://doc.rust-lang.org/rust.html#stability) system that tracks API stability at a fine level. This is influenced by node.js, but in Rust stability is detected and use of unstable API's can be enforced by the tooling. This is still in development but you can see it in the [docs](http://doc.rust-lang.org/std/intrinsics/).