Enum ConstValue

pub enum ConstValue<'tcx> {
    Scalar(Scalar),
    ZeroSized,
    Slice {
        data: ConstAllocation<'tcx>,
        meta: u64,
    },
    Indirect {
        alloc_id: AllocId,
        offset: Size,
    },
}

Represents a constant value in Rust. Scalar and Slice are optimizations for array length computations, enum discriminants and the pattern matching logic.

Variants

Scalar(Scalar)

Used for types with layout::abi::Scalar ABI.

Not using the enum Value to encode that this must not be Uninit.

ZeroSized

Only for ZSTs.

Slice

Used for references to unsized types with slice tail.

This is worth an optimized representation since Rust has literals of type &str and &[u8]. Not having to indirect those through an AllocId (or two, if we used Indirect) has shown measurable performance improvements on stress tests. We then reuse this optimization for slice-tail types more generally during valtree-to-constval conversion.

Fields

data: ConstAllocation<'tcx>

The allocation storing the slice contents. This always points to the beginning of the allocation.

meta: u64

The metadata field of the reference. This is a “target usize”, so we use u64 as in the interpreter.

Indirect

A value not representable by the other variants; needs to be stored in-memory.

Must not be used for scalars or ZST, but having &str or other slices in this variant is fine.

Fields

alloc_id: AllocId

The backing memory of the value. May contain more memory than needed for just the value if this points into some other larger ConstValue.

We use an AllocId here instead of a ConstAllocation<'tcx> to make sure that when a raw constant (which is basically just an AllocId) is turned into a ConstValue and back, we can preserve the original AllocId.

offset: Size

Offset into alloc