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 scenario author 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 thirteen 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 thirteen 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

DEFAULT_DRAFT

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

Functions

classify

The single source of truth for what is executable.

draft_is_compiled

Whether this build compiled a codec for draft.

supports_matcher

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