I actually would love this to be built in to a language/compiler. A lot of times when I’m building a single-threaded program but I’m using libraries written by other people. These libraries don’t know whether they are being incorporated into programs with single thread or not. So they either take the performance penalty of assuming multi-threaded (the approach by std::shared_ptr) or they give callers choice by making two implementations (Rust Arc and Rc). But the latter doesn’t actually work because this needs to be a global setting, not just a decision made at a local call site. It won’t work if such a library is a transitive dependency.