Enum miri::ConstValue

pub enum ConstValue<'tcx> {
    Scalar(Scalar<AllocId>),
    ZeroSized,
    Slice {
        data: ConstAllocation<'tcx>,
        start: usize,
        end: usize,
    },
    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<AllocId>)

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

Fields

data: ConstAllocation<'tcx>

start: usize

end: usize

Used for &[u8] and &str.

This is worth an optimized representation since Rust has literals of these types. Not having to indirect those through an AllocId (or two, if we used Indirect) has shown measurable performance improvements on stress tests.

Indirect

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

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.

Layout

Note: Most layout information is completely unstable and may even differ between compilations. The only exception is types with certain repr(...) attributes. Please see the Rust Reference’s “Type Layout” chapter for details on type layout guarantees.

Size:32 bytes

Size for each variant:

• Scalar: 24 bytes
• ZeroSized: 0 bytes
• Slice: 24 bytes
• Indirect: 16 bytes