Hypothetically Rust could make `Mutex<InnerBlah>` work with just two bits in the same way it makes `Option<&T>` the same size as `&T`. Annotate `InnerBlah` with the information about which bits are available and let `Mutex` use them.
There was talk of Rust allowing stride != alignment. [1] I think this would mean if say `InnerBlah` has size 15 and alignment 8, `parking_lot::Mutex<InnerBlah>` can be size 16 rather than the current 24. Same would be true for an `OuterBlah` the mutex is one field of. But I don't think it'll happen.
In principle, you Rust could create something like std::num::NonZero and its corresponding sealed trait ZeroablePrimitive to mark that two bits are unused. But that doesn't exist yet as far as I know.
There are also currently the unstable rustc_layout_scalar_valid_range_start and rustc_layout_scalar_valid_range_end attributes (which are used in the definition of NonNull, etc.) which could be used for some bit patterns.
