I would highlight `std::launder` as an example. It was added in C++17. Famously, most people have no idea what it is used for or why it exists. For low-level systems it was a godsend because there wasn’t an official way to express the intent, though compilers left backdoors open because some things require it.
It generates no code, it is a compiler barrier related to constant folding and lifetime analysis that is particularly useful when operating on objects in DMA memory. As far as a compiler is concerned DMA doesn’t exist, it is a Deus Ex Machina. This is an annotation to the compiler that everything it thinks it understands about the contents and lifetime of a bit of memory is now voided and it has to start over. This case is endemic in high-end database engines.
It should be noted that `std::launder` only works for different instances of the same type. If you want to dynamically re-type memory there is a different set of APIs for informing the compiler that DMA dropped a completely different type in the same memory address.
All of this is compiled down to nothing. It annotates for the compiler things it can’t understand just by inspecting the code.