Enum Tag 

pub enum Tag {
    AxisPresent {
        axis: TaxonomyAxis,
        key: String,
    },
    AxisValue {
        axis: TaxonomyAxis,
        key: String,
        value: String,
        separator: AxisSeparator,
    },
    Reserved {
        prefix: String,
        body: String,
    },
    Legacy(String),
}

Parsed capability tag.

Internal representation; the wire format is the canonical string returned by fmt::Display. Custom Serialize / Deserialize impls below render Tag → string and parse string → Tag, so a HashSet<Tag> rides over the wire as a JSON string array (["hardware.gpu", "scope:tenant:foo", ...]). The internal enum shape is an implementation detail callers don’t see.

Parser is permissive — see module docs for the parse (internal) vs. parse_user (application) split.

Variants

AxisPresent

Axis-prefixed presence tag with no value. Wire form: <axis>.<key> (e.g. hardware.gpu).

Fields

axis: TaxonomyAxis

Taxonomy axis (hardware / software / devices / dataforts).

key: String

Key portion after the <axis>. prefix (e.g. gpu).

AxisValue

Axis-prefixed keyed value. The separator is captured so the canonical Display round-trips byte-for-byte: some keys use = (e.g. hardware.gpu.vram_gb=80); the dataforts subset uses : (e.g. dataforts.has_chain:<hex>).

Fields

axis: TaxonomyAxis

Taxonomy axis.

key: String

Key portion (everything between <axis>. and the separator).

value: String

Value portion (everything after the separator).

separator: AxisSeparator

= or : — captured so wire form round-trips byte-for-byte.

Reserved

Reserved cross-axis prefix (causal:, fork-of:, heat:, scope:). Stored as (prefix, body) so the parser can route per-prefix at probe time without re-scanning.

Fields

prefix: String

One of RESERVED_PREFIXES (e.g. causal:).

body: String

Tag content after the reserved prefix.

Legacy(String)

Legacy pre-Warriors untyped tag. Parses with deprecation warning; one minor version of compatibility per CAPABILITY_SYSTEM_PLAN.md Locked decision 1.

Implementations

impl Tag

pub fn parse(s: &str) -> Result<Tag, CapabilityTagError>

Permissive parser: accepts every wire-form shape including reserved-prefix tags. Used by deserialization and by substrate code that has authority to emit reserved-prefix tags. Returns CapabilityTagError::Empty only on the empty string; otherwise produces some valid Tag variant (legacy if no shape recognized).

pub fn parse_user(s: &str) -> Result<Tag, CapabilityTagError>

Application-facing parser: rejects reserved-prefix tags with CapabilityTagError::ReservedPrefix. Use this in user-code builders (e.g. CapabilitySet::add_tag) so reserved-prefix emission is caught at the source rather than corrupting the substrate’s view of who’s authoritative for causal: etc.

pub fn axis_key(&self) -> Option<TagKey>

(axis, key) extraction for Predicate evaluation. Returns None for Reserved and Legacy tags (which don’t fit the axis taxonomy by construction).

Allocates: the returned TagKey owns its key string. Hot paths that iterate a tag set should prefer Self::axis_key_ref — see docs/misc/PERF_AUDIT_2026_05_28_CAPABILITY.md for the per-tag allocation cost this avoids.

pub fn axis_key_ref(&self) -> Option<(TaxonomyAxis, &str)>

Borrowing counterpart to Self::axis_key — returns (axis, &key) without cloning the key string. Prefer this in iteration / predicate hot paths; the cloning variant is only worth it when the caller actually needs an owned TagKey (e.g. collecting into a HashSet<TagKey> for diff).

pub fn value(&self) -> Option<&str>

Value half of a keyed axis tag. Returns None for presence tags, reserved tags, and legacy tags. (Reserved tags have a body accessible via Tag::reserved_body; semantically distinct from “axis value.”)

pub fn reserved_body(&self) -> Option<&str>

Body of a reserved-prefix tag (e.g. <hex> from causal:<hex>). None for non-reserved variants.

pub fn reserved_prefix(&self) -> Option<&str>

Reserved-prefix string (causal:, scope:, etc.) for reserved-variant tags. None otherwise.

pub fn is_legacy(&self) -> bool

true if this is a legacy untyped tag — useful for emitting the per-process deprecation log when the tag set is built.

pub fn axis(&self) -> Option<TaxonomyAxis>

Axis of an axis tag (AxisPresent / AxisValue). None for reserved + legacy tags.

pub fn to_wire(&self) -> String

Canonical wire string. Mirrors fmt::Display.

pub fn semantic_eq(&self, other: &Tag) -> bool

Semantic equality — like PartialEq but ignores the = vs : separator on AxisValue. Two tags that only differ in their wire-form separator describe the same (axis, key, value) and should compare equal for membership / require / diff purposes.

PartialEq itself stays separator-aware so the wire form round-trips byte-for-byte; callers comparing for meaning (rather than for bytes) should prefer this method. See CODE_REVIEW_2026_05_10_CAPABILITY_SYSTEM_2.md CR-1..CR-3 for the bug class this guards against.

Trait Implementations

impl Clone for Tag

fn clone(&self) -> Tag

Returns a duplicate of the value.

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

Performs copy-assignment from source.

impl Debug for Tag

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

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for Tag

fn deserialize<D>( deserializer: D, ) -> Result<Tag, <D as Deserializer<'de>>::Error>

where D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Display for Tag

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

Formats the value using the given formatter.

impl Eq for Tag

impl Hash for Tag

fn hash<__H>(&self, state: &mut __H)

where __H: Hasher,

Feeds this value into the given Hasher.

fn hash_slice<H>(data: &[Self], state: &mut H)

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl Ord for Tag

fn cmp(&self, other: &Tag) -> Ordering

This method returns an Ordering between self and other.

fn max(self, other: Self) -> Self

where Self: Sized,

Compares and returns the maximum of two values.

fn min(self, other: Self) -> Self

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl PartialEq for Tag

fn eq(&self, other: &Tag) -> 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 PartialOrd for Tag

fn partial_cmp(&self, other: &Tag) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

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

Tests less than (for self and other) and is used by the < operator.

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

Tests less than or equal to (for self and other) and is used by the <= operator.

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

Tests greater than (for self and other) and is used by the > operator.

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

Tests greater than or equal to (for self and other) and is used by the >= operator.

impl Serialize for Tag

fn serialize<S>( &self, serializer: S, ) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>

where S: Serializer,

Serialize this value into the given Serde serializer.

impl StructuralPartialEq for Tag