Ferrocene has donated their specification to the project, so there absolutely is a specification now. What you can argue is that the memory model isn‘t fully defined, but it‘s almost certainly going to land somewhere around stacked borrows or tree borrows. Arguably C doesn‘t fare much better in that regard though as it doesn‘t even properly define its pointer provenance model either and Rust is much closer to defining its.

Oh something has changed in the last 6 months? glad they are making progress on the spec.

Note that, in compiler lingo, unspecified and undefined are two different things. C++ is specified to death, but full of undefined behavior (and also some unspecified behavior).

Rust is largely not specified, but aims to have no undefined behavior (outside of unsafe blocks).

I am aware but without a spec we don’t know which is which. You can’t say it has no undefined behavior because what happens is you try to specify it and find gaps or challenges.

In C undefined is used primarily when there is not a reliable and efficient mechanism for detecting a problem is happening. For example a C implementation may check every single invalid pointer deref, but more realistically it only detects only extreme out of range. So it’s undefined what happens.

Good point.

That being said, at least in C++, undefined has been used largely as a joker for compiler optimizations. In Rust, if my memory serves, having the same code produce different results depending on the optimization level would be considered a pretty serious bug. In C++, it's par for the course.

> I am aware but without a spec we don’t know which is which.

I don't think I fully agree with this. A more accurate statement might be that "without a spec we don't always know which is which". You obviously don't need a spec to pin down at least some of a language's behavior, and even then the presence of a spec doesn't necessarily mean that you can answer arbitrary questions about what behavior is intended (c.f., various defect reports in the C/C++ spec asking clarifying questions, pointing out holes/deficiencies, or even admitting that certain behavior is not intended but being unsure about how to forbid it)