Enum DiscriminantStrategy 

pub enum DiscriminantStrategy {
    Separate {
        tag_offset: u32,
        tag_width: u8,
        payload_offset: u32,
    },
    Niche {
        unit_variant: u32,
        data_variant: u32,
        niche_offset: u32,
        niche_width: u8,
        niche_value: u128,
    },
}

How an enum encodes its discriminant within its storage.

Returned by Layout::discriminant_strategy for enum types. The constructor and match-dispatch in codegen consult this to decide where to read/write the tag.

Variants

Separate

Standard tagged union: discriminant lives in its own slot at tag_offset, with payload following at payload_offset.

In LLVM this is the { discrim, [N x i8] } struct shape used today (tag_offset = 0, payload_offset = tag_width).

Fields

tag_offset: u32

Byte offset of the discriminant tag within the enum’s storage.

tag_width: u8

Width of the discriminant tag in bytes.

payload_offset: u32

Byte offset where variant payload bytes start.

Niche

Niche-encoded: no separate discriminant slot. The unit variant is identified by reading niche_width bytes at niche_offset and seeing niche_value; any other bit pattern means the data variant.

Reserved for Phase 5+ of ADR-0069.

Fields

unit_variant: u32

Index of the (single) unit variant.

data_variant: u32

Index of the (single) data variant.

niche_offset: u32

Byte offset of the niche bytes within the enum’s storage.

niche_width: u8

Niche width in bytes.

niche_value: u128

Bit pattern (little-endian) that encodes the unit variant.

Trait Implementations

impl Clone for DiscriminantStrategy

fn clone(&self) -> DiscriminantStrategy

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for DiscriminantStrategy

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl PartialEq for DiscriminantStrategy

fn eq(&self, other: &DiscriminantStrategy) -> bool

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

Tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

impl Copy for DiscriminantStrategy

impl Eq for DiscriminantStrategy

impl StructuralPartialEq for DiscriminantStrategy