Module capability 

What is representable, per draft, per site, per stream — and why not.

One function answers that question — classify — and it has exactly two callers: Capabilities::supports / Capabilities::supports_on, which publish the table a caller reads before a run, and the engine’s executor, which decides what actually happens during one. That is the whole of the design: the table and the engine are the same code, so the table cannot become a documented lie about the engine. tests/action_matrix.rs asserts it against observed behaviour on all fourteen drafts.

Where each Refusal comes from

Not every refusal is a (site, kind) fact, so not every refusal is classify’s to produce:

• Classified here, from (site, kind) plus CapCtx: Refusal::WrongSite, Refusal::ControlStreamResetIllegal, Refusal::LengthChanged, Refusal::WouldRedefineSubgroupId, Refusal::WouldDestroyStatusObject, Refusal::ReservedHeaderMode and Refusal::PayloadNotDelimited.
• Produced by the executor, because they depend on the action’s payload or on session state rather than on the pair: Refusal::WrongComposition (what a Delay / Hold wrapped), Refusal::ErrorCodeOutOfRange (the numeric code) and Refusal::SessionAlreadyClosing (a close already in flight). They are reachable, and the sweep observes them; they are simply not decidable from a kind.
• Table-only: Refusal::StreamNotFramed and Refusal::ControlFrameNotDecodable. Both appear only inside Support::NotAttemptable and Support::Unreachable, where nothing is ever attempted, so neither is ever emitted as a ProxyEvent::ActionRefused.

tests/action_matrix.rs::every_declared_refusal_is_reachable_or_declared_table_only asserts that split per variant, in both directions.

The table answers for a build, not only for a draft

DraftVersion carries all fourteen variants under every feature set, so Capabilities::for_draft answers for drafts this binary cannot speak. A reduced-draft build — --no-default-features --features draft07, a shipped configuration and one of CI’s fourteen rows — cannot frame a byte of the twelve drafts it left out, and ProxySessionConfig::default().draft is Draft14 with nothing validating it against the compiled set. draft_is_compiled is therefore a fact classify reads, exactly like the draft number, and the object and control sites on an uncompiled draft are Support::Unreachable rather than Support::Yes. The two fail in different decoders and report different events, so they are two reads of the same fact rather than one; see Instead, which is where each names what a run emits in its place. In the default all-drafts build every row of the table is unchanged.

Structs

CapCtx

The facts classify needs.

Capabilities

What a draft can express, queryable before a run.

UnsupportedMatcherKey

A class rule keyed on something no unit it could claim ever carries.

Enums

ActionKind

A capability, named independently of whether Action can express it.

Instead

What a run reports in place of the action event a Support::Unreachable cell can never produce.

MatcherKey

One value key a Matcher can be built on.

NotAttemptable

Why a Support::NotAttemptable cell cannot be reached.

Precondition

A runtime fact a Support::Conditional verdict depends on.

Refusal

Why an action could not be executed.

Site

Where a decision was taken.

Support

Whether a capability is available.

Constants

ANY_KIND 🔒

Every kind a rule that names none of them may claim.

DEFAULT_DRAFT

The draft a session configuration takes when the caller names none.

DEFAULT_DRAFT_ORDER 🔒

Every draft this build could take as a default, in the order it would take them.

RESERVED_SUBGROUP_ID_MODE 🔒

The fourth value of the two-bit subgroup-ID mode, which no draft assigns.

Functions

classify

The single source of truth for what is executable.

classify_control 🔒

The control site, which is honoured on every draft.

classify_datagram 🔒

The datagram site.

classify_object 🔒

The object site, on a stream the framer can address: subgroup streams on every compiled draft, and fetch streams on the drafts whose objects this codec can read.

classify_stream_decision 🔒

The two stream-decision sites.

classify_stream_end 🔒

The stream-end site’s two columns: data streams and control streams.

datagram_replace_payload 🔒

The datagram site’s ReplacePayload rule: the payload’s start offset must be derivable, or there is nowhere to splice the replacement in.

default_draft 🔒

draft_is_compiled

Whether this build compiled a codec for draft.

fetch_group_order_is_needed 🔒

Whether a fetch stream on this draft can be read only by an endpoint that knows the Group Order the fetch was asked for.

filtered_earlier 🔒

The answer not_attemptable already gave, restated.

has_implicit_subgroup_id_mode 🔒

Whether this draft defines a subgroup ID is the first object’s ID stream type.

is_stream_decision 🔒

Whether this kind is one of the four StreamAction decisions.

keys_named 🔒

The keys a matcher names, in Matcher field order.

not_attemptable 🔒

Families 1-3 of Support::NotAttemptable, in the order classify documents: no constructor, then ReplaceObject’s single reading, then the return-type mismatch.

object_drop_elide 🔒

The object site’s DropElide rule, in guard order: facts about the stream before facts about the object.

object_framing_bypass 🔒

Why ObjectFramer cannot address objects on a stream of this shape, if it cannot.

object_replace_payload 🔒

The object site’s ReplacePayload rule: the replacement must be the declared payload length, and the object must not carry a status.

payload_not_delimited_detail 🔒

Which of Precondition::DatagramPayloadDelimited’s three cases failed.

site_returns_stream_action 🔒

Whether this site’s hook method returns StreamAction rather than Action.

subgroup_id_mode_must_be_consulted 🔒

Whether a header’s reserved subgroup-ID mode has to be told apart from mode 1 before an object behind it can be judged. Drafts 15-20.

supports_matcher

Whether a rule keyed on field can ever claim a unit arriving as kind, on draft, in this build.