Rust Specification

Compile-time classification of types according to Rust specification.

Classification Axes

RustSpec describes a type across the following axes:

• Layout: representation stability.
• Size: statically sized, metadata-sized, or extern-type-like shape.
• Alignment: ABI alignment in bytes as a typenum unsigned integer.
• Trap: whether the value representation has trap values.
• Niche: stable, unstable, or absent niche value.

Layout

Describes total reachable (through pointer indirection) representation stability:

• Stable: compiler-guaranteed representation.
• Unstable: representation is not guaranteed.

Size

Describes compile-time size and pointer metadata shape:

• Sized<Zst>: compile-time known zero-sized type.
• Sized<NonZst>: compile-time known non-zero-sized type.
• MetaSized<SliceLike>: dynamically sized slice-like type.
• MetaSized<DynTraitLike>: dynamically sized trait-object-like type.
• ExternTypeLike: dynamically sized extern-type-like type.

Alignment

The ABI alignment in bytes as a power-of-two typenum value.

Trap

Describes total reachable (through pointer indirection) value-representation validity:

• Robust: every bit pattern is valid.
• NonRobust: some bit patterns are trap/invalid values.

Niche

Describes whether and what kind of niche is available for the type:

• WithoutNiche: no niche is available.
• WithNiche<Stable>: compiler-guaranteed niche.
• WithNiche<Unstable>: niche exists but is not guaranteed.

How to Use

Derive RustSpec for your types, then use its associated marker families as bounds when implementing other traits:

use disjoint_impls::disjoint_impls;
use rust_spec::{
    RustSpec,
    niche::{self, WithNiche, WithoutNiche},
};

trait NullableEncoding {
    const NEEDS_TAG: bool;
}

#[derive(RustSpec)]
struct Header {
    id: u32,
    flags: u16,
}

disjoint_impls! {
    impl<T: RustSpec<Niche = WithoutNiche>> NullableEncoding for T {
        const NEEDS_TAG: bool = true;
    }

    impl<T: RustSpec<Niche = WithNiche<niche::Stable>>> NullableEncoding for T {
        const NEEDS_TAG: bool = false;
    }

    impl<T: RustSpec<Niche = WithNiche<niche::Unstable>>> NullableEncoding for T {
        const NEEDS_TAG: bool = false;
    }
}

const HEADER_OPTION_NEEDS_TAG: bool = Header::NEEDS_TAG;