Well, i dont use C++ much(i'm FW engineer, most of my stuff is in C).

The std in C is simple and explicit. For Ex: I can make an educated guess how memcpy() work by looking at its signature. It takes pointer to src and destination, and size, so i can guess it does not allocate any new memory(or if it has, it has to be some kind of optimization reason).

Another example is strstr(), it returns pointer to a piece of memory i provided to it, so i can safely do some pointer math with the return value.

It's true that i do not spend much time in Rust, so maybe i'm missing some fundamental things. I guess my mistake is trying to apply my knowledge in C to rust.

But still, it's kind of irritating now knowing (or guessing) how does function work just by looking at at its signature.

You can surmise the behavior from the signature of clone in the same way that you can memcpy(), it returns an owned type so it will allocate. If you want to match the signature of memcpy then you don't want the allocation and you can instead call clone_from() which takes a mutable borrow of something you had to allocate.

> piece of memory i provided to it, so i can safely do some pointer math with the return value.

Except pointer math is never going to be safe in C, a particular case might be bugfree but it is not safe. Moreover, nothing in the C language says that the provenance of the output pointer matches the provenance of the input pointer (it could take a reference to a pointer from some other thread while that other thread is freeing that memory). In rust you will pass a pointer and get a pointer back with the same lifetime, then you can safely use the pointer in that lifetime bound: the provenance is part of the signature. So in this case, you are incorrectly assuming things from the C signature while the corresponding rust signature definitively tells you.

So yeah, if you learn more about rust then you will see that in fact it tells you more than the corresponding C signatures.

Isn't guessing what a function does based purely on its name pretty risky? There's usually nuance, so if I'm not already familiar with a function, I'm looking up its docs at least once.

> It takes pointer to src and destination, and size, so i can guess it does not allocate any new memory

Does the pointer provided by src get altered in any way? Might is be NULL after calling memcpy? What happens if I pass NULL to dst? Is size in bytes or whatever the pointer is pointing to?

The moment you need to read a man page to get any of those mab pages you can read the docs for clone and get all the information you would need.

> Another example is strstr(), it returns pointer to a piece of memory i provided to it

This is not at all clear from the signature, from the signature it might allocate and return a new string entirely that you would need the deallocate, the only way to know that is to read the docs which runs into the problem again.

And again there is no indications that the pointers passed into the function are not mutated in any way other than convention and documentation.

Rust makes all of these things explicit with a compile error if you fail an invariant of the type systems.

Btw it's possible to encode the same invariants in C++ but isn't the default most of the time.