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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
by Rusky 10mo ago
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.
- timClicks 10mo agoReferences 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#representation 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.
- Rusky 10mo agoThere 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 https://github.com/rust-lang/rust/issues/135996
- scottlamb 10mo agoThere 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-size/17933/35 https://internals.rust-lang.org/t/pre-rfc-allow-array-stride...