Rust must statically prove lifetime of references to stack allocated variables does not exceed lifetime of the variables they point to at compile time, in order to be 100% memory safe. How is that different than just an advanced escape anlysis? Theoretically a VM could do much more, because it could do speculative escape analysis (I heard Azul guys were working on such experimental thing called escape detection) and even move variables from stack to heap once it turns out they do escape.
You could do escape analysis equivalent to Rust's only if you inlined all the functions. Sometimes, that's simply not possible, e.g. with separate compilation.
On the other hand, Rust is still able to perform its kind of escape analysis (via lifetime-tracking type system), because the relevant facts are embedded in type signatures of Rust functions, and as such must be present even for separate compilation (even if the actual implementation of the function is unknown).
Not necessarily. As you said, Rust encodes that information in type signatures. Exactly the same information can be used in a VM and it could do escape analysis one method at a time then.
For one thing, Rust can reject programs at compile time if it's not satisfied with its analysis. For Ruby to get that, you might have to restrict the language in a similar way.
