I thought Rust was that better C++. It doesn't help existing large projects because nobody will ever rewrite them. However, if you're starting a C++ project in 2021 and have the option of using Rust, you might want to think twice.
On the other hand, if the thing you don't like about C++ is that it keeps changing you may be unsatisfied in Rust too because Rust has in some ways an even faster pace of changes. Rust as it was written in 2015 still compiles on a modern Rust compiler (with edition set to "2015" and modulo any security issues) but it is no longer idiomatic Rust.
This is precisely what the Rust edition are there to solve: your legacy code will work but the language will change with each edition to reflect the state of the art, and if needed, it will break backwards compatibility.
I'm not sure there are any security issues associated with using an older edition. I think when soundness holes and other things like that are fixed, they're usually fixed across all editions. Sometimes that's technically a backwards-incompatible change/fix, but Rust's back compat policy considers that acceptable if the breakage isn't widespread.
The changes since the 2015 edition have all been relatively minor in terms of idiomatic code I think. I mean sure, you can use the '?' operator instead doing a manual match and return, but most of the changes have just been making the compiler more permissive in terms of what it accepts.
The only new concept to be learnt would be async-await, and that should be familiar to most from other languages and only applies if you're doing IO.
fn twiddle(foozles: impl IntoIterator<Item=Foozle>)
But in 2015 you couldn't have written that. It works in the 2015 edition today, sure enough, but it would not have worked in any 2015 Rust compiler, and so it was not idiomatic Rust in 2015.
Instead in 2015 you'd have either chosen to specify what sort of container the Foozles live in, or, if you believe freedom to choose different containers is important to your users, you have to ask the caller to Box up an Iterator over their Foozles so you don't need to care what container they used.
so e.g.
fn twiddle(foozles: Vec<Foozle>) // Hope your foozles are in a Vector or else you'll need to write an adaptor function
or
fn twiddle(foozles: Box<Iterator<Item=Foozle>>) // Now we're incurring a heap allocation
fn twiddle<T: Foozle, IterT: IntoIterator<Item=T>>(foozles: IterT)
So sometimes you still want to use the "old" syntax in your public APIs, even when you could be using an impl trait
fn twiddle<I>(foozles: I)
where
I: Iterator<Item=Foozle>
Generics and trait bounds are in the language from the start.
So you can write this function like this:
fn twiddle<T: IntoIterator<Item = Foozle>>(foozles: T)
GADTs are useful for extremely generic code and are required if you want to write a general Monad trait. There is currently no way to write a trait which says return Self, but with a different generic argument. And not just Self, any associated type on a trait is a single type so you can't have a requirement for an associated type to be generic.
Finally, the procedural macro system is kind of a mess at the moment. Last time I checked, you really couldn't write a hygienic procedural macro (though I think this can be added later on without. A bigger limitation is that you don't really have any way to access type information in a procedural macro. Obviously type information is only available after parsing unless you make the macro system in Rust way more complicated, but it would be nice to have static type info for other modules. Having this information would allow you to do stuff like generating serialization code for some library that you don't control.