Most "modern" C++ involves template metaprogramming of a sort that is in header file, so the modern aspect doesn't stand to increase the need for LTO that much.

Most "modern" C++ involves template metaprogramming of a sort that is in header file, so the modern aspect doesn't stand to increase the need for LTO that much. I would imagine that Rust programs benefit from LTO by removing static assertions necessary for memory safety that C++ programs simply omit, and by unraveling functional style code that Rust programs often get shoehorned into. Or maybe not, but that's one non-LTO that's needed.