Rusky parent
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.
[1] e.g. https://internals.rust-lang.org/t/pre-rfc-allow-array-stride...
References only have a single bit available as a niche (the null byte), which Option makes use of for null pointer optimization (https://doc.rust-lang.org/std/option/index.html#representati...).
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.
Also aspirations to use pattern types for this sort of thing: https://github.com/rust-lang/rust/issues/135996