> If I find a Rust program that is (perforce) not using powerful libraries, can I be confident that it does not harbor grave errors?

You can be confident that it doesn't harbour memory errors. You can be confident that it doesn't contain arbitrary code execution bugs, which is a much better circumstance than with any C++ project I've seen (C++ by its nature turns almost any bug into a security bug).

IME you can also have a much higher level of confidence that it does what you expect (including not having bugs) than you would for a C++ project, because of Rust's more expressive type system.

> C++ enables capturing bug-prone code in well-tested libraries, eliminating whole families of bugs, including, in my recent experience, memory bugs.

And yet in practice you can neither be confident that there are no memory bugs, nor that there are no other bugs. Even the big name C++ libraries are riddled with major bugs. Perhaps libraries that are written in a certain fashion avoid this bugginess, but that's of little use when it's not possible to tell from a glance whether a given library is one of the buggy ones or not.