That depends a lot on how you define “correct C”.

It is harder to write rust code than C code that the compiler will accept. It is IMHO easier to write rust code than to write correct C code, in the sense it only uses well defined constructs defined in the C standard.

The difference is that the rust compiler is much stricter, so you need to know a lot about details in the memory model, etc. to get your code past the compiler. In C you need the same knowledge to debug the program later.

Yes Rust is harder to write than C, that’s basically by design as it’s due to the statically guaranteed memory safety. That’s pretty magical. C doesn’t have that and neither does C++ even with smart pointers and such. Rusts unsafe keyword is poorly named, what it actually does is tell the compiler that you the programmer guarantee Rusts rules are upheld within the unsafe block.

For example

Access or modify a mutable static variable

That is a global, that’s incredibly hard to impossible to statically prove it’s safely done, so you have to do it in an unsafe block. So you violating Rusts rules within an unsafe block is actually using the unsafe block wrong. That’s not what it’s for

That’s true in C as well, though. This is what people mean when they say things like “undefined behavior can result in time travel”.

The difference is twofold:

• Rust’s rules for valid unsafe code are not completely formalized yet. This means that there are open questions about whether particularly complex patterns in unsafe code will be guaranteed by future versions of the compiler to be sound. Conversely, the C and C++ spec are generally sufficient to determine whether any particular piece of code has undefined behavior, even if actually analyzing it to find out is not possible automatically using existing static analysis tools.
• Because safe Rust is so strict about what it permits, the compiler is able to make more aggressive optimizations; in theory, this could indeed cause undefined behavior to be “worse” at runtime than a comparable situation in a globally-unsafe language. I’m unaware of any actual examples of that phenomenon, though.

What Rust provides is statically guaranteed memory safety. Some C++ types will prevent memory issues however the language itself is unsafe. Playing with raw pointers is just as valid as using std::unique_ptr. In Rust however you must sign a contact (using unsafe) in order to play with raw pointers. Unsafe is you the programmer promising that you followed the rules. This is like how C++ says it’s illegal to write UB and your program will break (and it’s your fault) but enforced through a special type of block

I would still like to take a moment to answer your specific questions more directly:

And what does that exclude that C or C++ has that’s memory unsafe? I suppose use after free?

I think indeed use after free is the main one, although data races are another. Both are prevented in Rust by the borrow checker.

Dereference a pointer without a bounds check is the major problem when we’re talking about memory safety.

I think that’s only half of the issue, with the other half indeed being use after free. After all, using a reference isn’t much different from dereferencing a pointer. But doing bounds check on all your pointers is relatively easy to do by humans; you see where the pointer is being used and you see if there’s a check or not. But proving liveness of your references before you use them is much harder, because it often requires whole-program analysis to understand when the target may be destroyed. And in terms of danger, use after free is just as dangerous as unbound pointer access.

Thread safe code isn’t the issue otherwise Java, Python, etc would all be on the list of languages to run from.

Thread safe code is also the issue. The reason people don’t have to run from Java is because data races there don’t escalate to memory unsafety; they’re just caught as “ordinary” exceptions and thus manifest as ordinary (but still hard-to-debug) bugs. But in C++ those too can create invalid memory accesses, with a possibility for exploitation. In fact, even Go has a memory unsafe data race in its threading model that occurs because of its use of fat pointers that embed both a data type and an address.

Point being, that is still a very dangerous subset. Off-by one errors have done in a lot of C code (and C++ code that isn’t using range-based loops).

It is indeed a dangerous subset, but as I mentioned elsewhere, Rust’s borrow-checker, which prevents use after free with references is still active, even in unsafe code. Off-by-one errors are still bound-checked even in unsafe code, unless you explicitly use the non-bound-checked versions. Any Rust code that is valid safe Rust is just as safe when wrapped in an unsafe block. It is only when you explicitly use the unsafe features that you’re on your own when it comes to safety.

A lot of these issues can be avoided in C++ by just using existing types like std::variant, std::unique_ptr, std::shared_ptr, std::array, and std::vector (with the at based accessor) instead of lower level constructs.

They indeed avoid some of the issues, but notably don’t protect against use after free at all. And take std::vector as an example:

std::vector v;
v[2]; // out-of-bounds access

vs.

unsafe {
    let v = Vec::new();
    v[2]; // panic
}

Even wrapped in unsafe, the Rust equivalents are still safer.