What do you mean this isn't one of Rust's limitations? It's a common, well-understood feature of systems languages, which Rust doesn't have. Look for yourself on Rosetta Code[0]. There are many languages that feature a way to define a structure with individual bit fields: Basic, Fortran, Ada, etc.

[0]https://rosettacode.org/wiki/Memory_layout_of_a_data_structu...