feat(host): add object publication and dispatch

crates/wip-host/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "wip-host"
+version = "0.1.0"
+edition = "2024"
+description = "Host-side object publication and operation dispatch for WIP"
+license = "MIT OR Apache-2.0"
+
+[dependencies]
+wip-interface = { path = "../wip-interface" }
+wip-protocol = { path = "../wip-protocol" }
+
+[dev-dependencies]
+trybuild = "1"

crates/wip-host/src/lib.rs

@@ -0,0 +1,623 @@
+//! Host-side publication and operation dispatch for Web Interface Protocol objects.
+//!
+//! # Responsibility boundaries
+//!
+//! [`InterfaceRegistry::register`] validates and freezes a generated interface
+//! descriptor, assigns an opaque [`InterfaceRef`], and makes the same descriptor
+//! observation available to clients and dispatch. [`PublishedObject::expose`]
+//! explicitly combines a request-scoped Rust object with a registered interface;
+//! its [`InterfaceImplementation`] bound proves at compile time that the object
+//! implements that interface. Implementing a Rust trait alone never publishes it,
+//! and a publication with no interfaces is valid.
+//!
+//! [`PublishedObject::dispatch`] selects by `(InterfaceRef, operation name)`,
+//! checks object and interface validators, current membership, the frozen
+//! descriptor, and then invokes the matching generated typed dispatcher on the
+//! same object instance. Arguments and results cross the protocol [`Value`]
+//! boundary only through generated validation and codecs. Rust fields that are
+//! not returned by [`WipObject`] are host-private and are never serialized by
+//! this crate.
+//!
+//! The call method is deliberately an execution boundary rather than a global
+//! object registry: a provider may resolve and publish an object per request and
+//! drop it afterward. The provider must enter any backend-specific transaction,
+//! lock, stable-handle scope, or conditional-update primitive *before* creating
+//! or dispatching the publication and retain it until dispatch returns. This
+//! crate keeps validator checks, descriptor and implementation selection, and
+//! invocation in one borrow of that publication; it does not claim that an
+//! arbitrary backend stays stable, and it never reopens a path or resolves a
+//! second implementation snapshot after validation.
+
+#![deny(missing_docs)]
+
+use std::any::TypeId;
+use std::collections::{HashMap, HashSet};
+use std::error::Error;
+use std::fmt::{self, Display, Formatter};
+use std::marker::PhantomData;
+use std::sync::Arc;
+use std::sync::atomic::{AtomicU64, Ordering};
+
+use wip_interface::DispatchError;
+pub use wip_interface::{InterfaceDefinition, InterfaceImplementation};
+pub use wip_protocol;
+use wip_protocol::{
+    InterfaceDescriptor, NamedValue, Object, ObjectId, OperationCall, OperationTarget,
+    ProtocolError, ProtocolErrorCategory, ValidationError, Value,
+};
+
+/// Opaque bytes used as a conditional execution precondition.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct Validator(Vec<u8>);
+
+impl Validator {
+    /// Creates a validator without assigning meaning to its bytes.
+    #[must_use]
+    pub fn new(bytes: impl Into<Vec<u8>>) -> Self {
+        Self(bytes.into())
+    }
+
+    /// Returns the opaque bytes for transport encoding or comparison.
+    #[must_use]
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.0
+    }
+}
+
+/// Opaque host-assigned reference to one registered interface artifact.
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct InterfaceRef(String);
+
+impl InterfaceRef {
+    /// Returns the opaque reference for a protocol transport.
+    #[must_use]
+    pub fn as_str(&self) -> &str {
+        &self.0
+    }
+}
+
+/// An immutable observation returned for an opaque interface reference.
+#[derive(Debug, Clone, PartialEq)]
+pub struct InterfaceObservation {
+    /// Opaque reference selected by published objects and operation calls.
+    pub reference: InterfaceRef,
+    /// Optional validator. Its presence makes it a required call precondition.
+    pub validator: Option<Validator>,
+    /// Validated descriptor frozen when the interface was registered.
+    pub descriptor: InterfaceDescriptor,
+}
+
+#[derive(Debug)]
+struct InterfaceArtifact {
+    reference: InterfaceRef,
+    validator: Option<Validator>,
+    descriptor: InterfaceDescriptor,
+}
+
+/// A typed registration token for a generated interface definition.
+///
+/// Tokens can be reused by any number of request-scoped objects. They expose an
+/// opaque reference, not the Rust type name or an address.
+pub struct RegisteredInterface<D> {
+    artifact: Arc<InterfaceArtifact>,
+    definition: PhantomData<fn() -> D>,
+}
+
+impl<D> Clone for RegisteredInterface<D> {
+    fn clone(&self) -> Self {
+        Self {
+            artifact: Arc::clone(&self.artifact),
+            definition: PhantomData,
+        }
+    }
+}
+
+impl<D> RegisteredInterface<D> {
+    /// Returns the opaque protocol reference assigned by the registry.
+    #[must_use]
+    pub fn reference(&self) -> &InterfaceRef {
+        &self.artifact.reference
+    }
+}
+
+/// Registry of validated, immutable interface artifacts.
+///
+/// This registry stores interface definitions, not object instances. Generated
+/// definitions are registered explicitly and receive process-local opaque
+/// references. A validator-free artifact is immutable for the lifetime of its
+/// reference; a validator-bearing artifact requires that exact validator on a
+/// call. Re-registering a definition is rejected instead of silently changing
+/// the descriptor associated with an existing reference.
+#[derive(Debug)]
+pub struct InterfaceRegistry {
+    namespace: u64,
+    next_reference: u64,
+    definitions: HashSet<TypeId>,
+    artifacts: HashMap<InterfaceRef, Arc<InterfaceArtifact>>,
+}
+
+static NEXT_REGISTRY_NAMESPACE: AtomicU64 = AtomicU64::new(1);
+
+impl Default for InterfaceRegistry {
+    fn default() -> Self {
+        Self {
+            namespace: NEXT_REGISTRY_NAMESPACE.fetch_add(1, Ordering::Relaxed),
+            next_reference: 0,
+            definitions: HashSet::new(),
+            artifacts: HashMap::new(),
+        }
+    }
+}
+
+impl InterfaceRegistry {
+    /// Creates an empty interface registry.
+    #[must_use]
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Registers and freezes one generated interface definition.
+    ///
+    /// `validator` should be present when the host intends the reference to
+    /// represent a versioned descriptor. Its presence makes the same value a
+    /// mandatory precondition for dispatch.
+    pub fn register<D>(
+        &mut self,
+        _definition: D,
+        validator: Option<Validator>,
+    ) -> Result<RegisteredInterface<D>, RegistrationError>
+    where
+        D: InterfaceDefinition + 'static,
+    {
+        let definition = TypeId::of::<D>();
+        if self.definitions.contains(&definition) {
+            return Err(RegistrationError::AlreadyRegistered);
+        }
+
+        let descriptor = D::descriptor();
+        descriptor
+            .validate()
+            .map_err(RegistrationError::InvalidDescriptor)?;
+
+        let reference = InterfaceRef(format!(
+            "wip-interface:{}:{}",
+            self.namespace, self.next_reference
+        ));
+        self.next_reference = self
+            .next_reference
+            .checked_add(1)
+            .ok_or(RegistrationError::ReferenceSpaceExhausted)?;
+        let artifact = Arc::new(InterfaceArtifact {
+            reference: reference.clone(),
+            validator,
+            descriptor,
+        });
+        self.definitions.insert(definition);
+        self.artifacts.insert(reference, Arc::clone(&artifact));
+
+        Ok(RegisteredInterface {
+            artifact,
+            definition: PhantomData,
+        })
+    }
+
+    /// Observes the descriptor and optional validator for an opaque reference.
+    #[must_use]
+    pub fn observe(&self, reference: &InterfaceRef) -> Option<InterfaceObservation> {
+        self.artifacts
+            .get(reference)
+            .map(|artifact| InterfaceObservation {
+                reference: artifact.reference.clone(),
+                validator: artifact.validator.clone(),
+                descriptor: artifact.descriptor.clone(),
+            })
+    }
+}
+
+/// Failure while registering a generated interface definition.
+#[derive(Debug)]
+pub enum RegistrationError {
+    /// The same generated definition was already assigned a reference.
+    AlreadyRegistered,
+    /// The generated descriptor is structurally invalid.
+    InvalidDescriptor(ValidationError),
+    /// The registry cannot assign another opaque reference.
+    ReferenceSpaceExhausted,
+}
+
+impl Display for RegistrationError {
+    fn fmt(&self, formatter: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::AlreadyRegistered => {
+                formatter.write_str("interface definition already registered")
+            }
+            Self::InvalidDescriptor(error) => {
+                write!(formatter, "invalid interface descriptor: {error}")
+            }
+            Self::ReferenceSpaceExhausted => {
+                formatter.write_str("interface reference space exhausted")
+            }
+        }
+    }
+}
+
+impl Error for RegistrationError {
+    fn source(&self) -> Option<&(dyn Error + 'static)> {
+        match self {
+            Self::InvalidDescriptor(error) => Some(error),
+            Self::AlreadyRegistered | Self::ReferenceSpaceExhausted => None,
+        }
+    }
+}
+
+/// Public metadata and conditional-execution policy for a host-private object.
+///
+/// Only values returned by this trait are projected into protocol metadata.
+/// Backend handles, filesystem paths, credentials, and other private fields on
+/// the implementing type are neither inspected nor serialized.
+pub trait WipObject {
+    /// Returns the current logical identity of this object.
+    fn id(&self) -> ObjectId;
+
+    /// Returns the current opaque object validator, if any.
+    fn validator(&self) -> Option<&Validator> {
+        None
+    }
+
+    /// Whether a call must include the current object validator.
+    fn requires_validator(&self) -> bool {
+        false
+    }
+}
+
+type DispatchFn<T> = fn(&T, &str, Vec<NamedValue>) -> Result<Option<Value>, DispatchError>;
+
+struct BoundInterface<T> {
+    artifact: Arc<InterfaceArtifact>,
+    dispatch: DispatchFn<T>,
+}
+
+/// A host-private object instance combined with an explicit ordered interface set.
+///
+/// The instance is owned by this value and can therefore be request-scoped. No
+/// global or pre-populated object registry is required.
+pub struct PublishedObject<T: WipObject> {
+    implementation: T,
+    interfaces: Vec<BoundInterface<T>>,
+}
+
+impl<T: WipObject> PublishedObject<T> {
+    /// Starts a publication with no exposed interfaces.
+    #[must_use]
+    pub fn new(implementation: T) -> Self {
+        Self {
+            implementation,
+            interfaces: Vec::new(),
+        }
+    }
+
+    /// Explicitly exposes a registered generated interface in insertion order.
+    ///
+    /// The `InterfaceImplementation<T>` bound is generated only when `T`
+    /// implements the corresponding Rust interface trait. Duplicate opaque
+    /// references are rejected.
+    pub fn expose<D>(mut self, interface: &RegisteredInterface<D>) -> Result<Self, PublicationError>
+    where
+        D: InterfaceImplementation<T>,
+    {
+        if self
+            .interfaces
+            .iter()
+            .any(|bound| bound.artifact.reference == interface.artifact.reference)
+        {
+            return Err(PublicationError::DuplicateInterface(
+                interface.artifact.reference.clone(),
+            ));
+        }
+
+        self.interfaces.push(BoundInterface {
+            artifact: Arc::clone(&interface.artifact),
+            dispatch: D::dispatch,
+        });
+        Ok(self)
+    }
+
+    /// Returns the host-private implementation without projecting its fields.
+    #[must_use]
+    pub fn implementation(&self) -> &T {
+        &self.implementation
+    }
+
+    /// Builds the protocol object metadata with interface insertion order intact.
+    #[must_use]
+    pub fn protocol_object(&self) -> Object {
+        let object = Object {
+            id: self.implementation.id(),
+            interfaces: self
+                .interfaces
+                .iter()
+                .map(|bound| bound.artifact.reference.0.clone())
+                .collect(),
+        };
+        debug_assert!(object.validate().is_ok());
+        object
+    }
+
+    /// Dispatches one qualified operation against this current publication.
+    ///
+    /// Preconditions are checked before generated operation code runs. The
+    /// selected artifact and function pointer are borrowed once and are not
+    /// resolved again after validator checks.
+    pub fn dispatch(&self, request: CallRequest) -> Result<CallResult, CallError> {
+        if request.object != self.implementation.id() {
+            return Err(CallError::new(
+                CallErrorKind::ObjectNotFound,
+                "call target does not identify this published object",
+            ));
+        }
+
+        let current_object_validator = self.implementation.validator();
+        match request.object_validator.as_ref() {
+            Some(supplied) if Some(supplied) != current_object_validator => {
+                return Err(CallError::new(
+                    CallErrorKind::ValidatorMismatch,
+                    "object validator does not match current object state",
+                ));
+            }
+            None if self.implementation.requires_validator() => {
+                return Err(CallError::new(
+                    CallErrorKind::ValidatorRequired,
+                    "this object requires a validator",
+                ));
+            }
+            _ => {}
+        }
+
+        let Some(bound) = self
+            .interfaces
+            .iter()
+            .find(|bound| bound.artifact.reference == request.interface.reference)
+        else {
+            return Err(CallError::new(
+                CallErrorKind::InterfaceMismatch,
+                "interface is not a member of the current object publication",
+            ));
+        };
+
+        match (
+            bound.artifact.validator.as_ref(),
+            request.interface.validator.as_ref(),
+        ) {
+            (Some(_), None) => {
+                return Err(CallError::new(
+                    CallErrorKind::InterfaceValidatorRequired,
+                    "registered interface requires a validator",
+                ));
+            }
+            (expected, Some(actual)) if expected != Some(actual) => {
+                return Err(CallError::new(
+                    CallErrorKind::InterfaceValidatorMismatch,
+                    "interface validator does not match the registered descriptor",
+                ));
+            }
+            _ => {}
+        }
+
+        if !bound
+            .artifact
+            .descriptor
+            .operations
+            .iter()
+            .any(|operation| operation.name == request.operation)
+        {
+            return Err(CallError::new(
+                CallErrorKind::OperationNotFound,
+                format!("operation `{}` is not declared", request.operation),
+            ));
+        }
+
+        bound
+            .artifact
+            .descriptor
+            .validate_call(&OperationCall {
+                target: OperationTarget {
+                    object: request.object.clone(),
+                    interface: bound.artifact.descriptor.name.clone(),
+                    operation: request.operation.clone(),
+                },
+                arguments: request.arguments.clone(),
+            })
+            .map_err(|error| CallError::new(CallErrorKind::InvalidArguments, error.to_string()))?;
+
+        let value = (bound.dispatch)(&self.implementation, &request.operation, request.arguments)
+            .map_err(CallError::from_dispatch)?;
+
+        bound
+            .artifact
+            .descriptor
+            .validate_return(&request.operation, value.as_ref())
+            .map_err(|error| CallError::new(CallErrorKind::InvalidResult, error.to_string()))?;
+
+        Ok(CallResult {
+            value,
+            validator: self.implementation.validator().cloned(),
+        })
+    }
+
+    /// Consumes the publication and returns its host-private implementation.
+    #[must_use]
+    pub fn into_inner(self) -> T {
+        self.implementation
+    }
+}
+
+/// Failure to construct an explicit object publication.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum PublicationError {
+    /// The same opaque interface reference was exposed more than once.
+    DuplicateInterface(InterfaceRef),
+}
+
+impl Display for PublicationError {
+    fn fmt(&self, formatter: &mut Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::DuplicateInterface(reference) => write!(
+                formatter,
+                "interface `{}` is already exposed by this object",
+                reference.as_str()
+            ),
+        }
+    }
+}
+
+impl Error for PublicationError {}
+
+/// Interface portion of a qualified operation target.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct InterfaceTarget {
+    /// Opaque reference selected from the current object's interface set.
+    pub reference: InterfaceRef,
+    /// Validator copied from the descriptor observation, when one was present.
+    pub validator: Option<Validator>,
+}
+
+/// One transport-independent request to a published object operation.
+#[derive(Debug, Clone, PartialEq)]
+pub struct CallRequest {
+    /// Current logical object identity.
+    pub object: ObjectId,
+    /// Optional current object-state precondition.
+    pub object_validator: Option<Validator>,
+    /// Qualified interface reference and descriptor precondition.
+    pub interface: InterfaceTarget,
+    /// Operation name within the selected interface descriptor.
+    pub operation: String,
+    /// Named protocol arguments.
+    pub arguments: Vec<NamedValue>,
+}
+
+/// Successful host dispatch result.
+#[derive(Debug, Clone, PartialEq)]
+pub struct CallResult {
+    /// Encoded application value, or `None` for a no-return operation.
+    pub value: Option<Value>,
+    /// Object validator observed after the implementation returned.
+    pub validator: Option<Validator>,
+}
+
+/// Stable host dispatch failure kind.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum CallErrorKind {
+    /// The call names a different or unavailable object.
+    ObjectNotFound,
+    /// The selected interface is not exposed by the current object.
+    InterfaceMismatch,
+    /// The operation is absent from the selected descriptor.
+    OperationNotFound,
+    /// Supplied named values do not satisfy the operation declaration.
+    InvalidArguments,
+    /// The encoded implementation result violates its declaration.
+    InvalidResult,
+    /// This object requires a validator and the call omitted it.
+    ValidatorRequired,
+    /// The supplied object validator is stale or belongs to another object.
+    ValidatorMismatch,
+    /// The selected mutable interface requires its observed validator.
+    InterfaceValidatorRequired,
+    /// The supplied interface validator does not identify the current descriptor.
+    InterfaceValidatorMismatch,
+    /// The typed implementation reported a host/backend failure.
+    Implementation,
+}
+
+/// A dispatch failure with a stable kind and non-normative diagnostic.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct CallError {
+    kind: CallErrorKind,
+    message: String,
+}
+
+impl CallError {
+    fn new(kind: CallErrorKind, message: impl Into<String>) -> Self {
+        Self {
+            kind,
+            message: message.into(),
+        }
+    }
+
+    fn from_dispatch(error: DispatchError) -> Self {
+        match error {
+            DispatchError::UnknownOperation(operation) => Self::new(
+                CallErrorKind::OperationNotFound,
+                format!("operation `{operation}` is not declared"),
+            ),
+            DispatchError::InvalidArguments(error) => {
+                Self::new(CallErrorKind::InvalidArguments, error.to_string())
+            }
+            DispatchError::Decode(error) => {
+                Self::new(CallErrorKind::InvalidArguments, error.to_string())
+            }
+            DispatchError::Host(error) => {
+                Self::new(CallErrorKind::Implementation, error.to_string())
+            }
+            DispatchError::Encode(error) => {
+                Self::new(CallErrorKind::InvalidResult, error.to_string())
+            }
+            DispatchError::InvalidResult(error) => {
+                Self::new(CallErrorKind::InvalidResult, error.to_string())
+            }
+        }
+    }
+
+    /// Returns the stable failure kind.
+    #[must_use]
+    pub fn kind(&self) -> CallErrorKind {
+        self.kind
+    }
+
+    /// Returns the human-readable diagnostic.
+    #[must_use]
+    pub fn message(&self) -> &str {
+        &self.message
+    }
+
+    /// Maps this host failure to the existing protocol error taxonomy.
+    ///
+    /// Validator details remain available through [`CallError::kind`], while
+    /// the current protocol model represents conditional-request failures as
+    /// `InvalidRequest`. Result/codec and implementation failures are internal,
+    /// because exposing malformed host output as an application failure would
+    /// be unsafe.
+    #[must_use]
+    pub fn protocol_error(&self) -> ProtocolError {
+        let category = match self.kind {
+            CallErrorKind::ObjectNotFound => ProtocolErrorCategory::ObjectNotFound,
+            CallErrorKind::InterfaceMismatch => ProtocolErrorCategory::InterfaceNotFound,
+            CallErrorKind::OperationNotFound => ProtocolErrorCategory::OperationNotFound,
+            CallErrorKind::InvalidArguments => ProtocolErrorCategory::InvalidArguments,
+            CallErrorKind::ValidatorRequired
+            | CallErrorKind::ValidatorMismatch
+            | CallErrorKind::InterfaceValidatorRequired
+            | CallErrorKind::InterfaceValidatorMismatch => ProtocolErrorCategory::InvalidRequest,
+            CallErrorKind::InvalidResult | CallErrorKind::Implementation => {
+                ProtocolErrorCategory::Internal
+            }
+        };
+        ProtocolError {
+            category,
+            message: self.message.clone(),
+        }
+    }
+}
+
+impl Display for CallError {
+    fn fmt(&self, formatter: &mut Formatter<'_>) -> fmt::Result {
+        write!(
+            formatter,
+            "host dispatch failed ({:?}): {}",
+            self.kind, self.message
+        )
+    }
+}
+
+impl Error for CallError {}

crates/wip-host/tests/compile_fail.rs

@@ -0,0 +1,5 @@
+#[test]
+fn rejects_exposing_an_unimplemented_interface() {
+    let tests = trybuild::TestCases::new();
+    tests.compile_fail("tests/ui/unimplemented_interface.rs");
+}

crates/wip-host/tests/host.rs

@@ -0,0 +1,488 @@
+use std::cell::{Cell, RefCell};
+
+use wip_host::{
+    CallErrorKind, CallRequest, InterfaceRegistry, InterfaceTarget, PublishedObject, Validator,
+    WipObject,
+};
+use wip_interface::{
+    CodecError, HostError, NamedValue, OperationResult, TypeDeclaration, TypeExpr, Value, WipType,
+    interface,
+};
+use wip_protocol::{ObjectId, ProtocolErrorCategory};
+
+#[interface]
+pub trait Reader {
+    /// Returns the reader-qualified label.
+    fn label(&self) -> OperationResult<String>;
+
+    /// Adds two integers.
+    fn add(&self, left: i64, right: i64) -> OperationResult<i64>;
+}
+
+#[interface]
+pub trait Writer {
+    /// Returns the writer-qualified label.
+    fn label(&self) -> OperationResult<String>;
+}
+
+#[interface]
+pub trait Hidden {
+    /// Returns a hidden value.
+    fn hidden(&self) -> OperationResult<String>;
+}
+
+#[interface]
+pub trait Broken {
+    /// Returns a deliberately invalid encoded value.
+    fn broken(&self) -> OperationResult<BadValue>;
+}
+
+#[derive(Debug)]
+pub struct BadValue;
+
+impl WipType for BadValue {
+    fn type_expr() -> TypeExpr {
+        TypeExpr::Integer
+    }
+
+    fn collect_declarations(_: &mut Vec<TypeDeclaration>) {}
+
+    fn encode(self) -> Result<Value, CodecError> {
+        Ok(Value::String("not an integer".into()))
+    }
+
+    fn decode(_: Value) -> Result<Self, CodecError> {
+        Ok(Self)
+    }
+}
+
+struct Document {
+    id: String,
+    validator: Option<Validator>,
+    require_validator: bool,
+    calls: Cell<usize>,
+    private_backend_handle: RefCell<String>,
+}
+
+impl Document {
+    fn new(id: &str) -> Self {
+        Self {
+            id: id.into(),
+            validator: None,
+            require_validator: false,
+            calls: Cell::new(0),
+            private_backend_handle: RefCell::new("/private/backend/path".into()),
+        }
+    }
+
+    fn strict(id: &str, validator: &[u8]) -> Self {
+        Self {
+            id: id.into(),
+            validator: Some(Validator::new(validator)),
+            require_validator: true,
+            calls: Cell::new(0),
+            private_backend_handle: RefCell::new("secret-handle".into()),
+        }
+    }
+
+    fn record_call(&self) {
+        self.calls.set(self.calls.get() + 1);
+    }
+}
+
+impl WipObject for Document {
+    fn id(&self) -> ObjectId {
+        ObjectId(self.id.clone())
+    }
+
+    fn validator(&self) -> Option<&Validator> {
+        self.validator.as_ref()
+    }
+
+    fn requires_validator(&self) -> bool {
+        self.require_validator
+    }
+}
+
+impl Reader for Document {
+    fn label(&self) -> OperationResult<String> {
+        self.record_call();
+        Ok("reader".into())
+    }
+
+    fn add(&self, left: i64, right: i64) -> OperationResult<i64> {
+        self.record_call();
+        Ok(left + right)
+    }
+}
+
+impl Writer for Document {
+    fn label(&self) -> OperationResult<String> {
+        self.record_call();
+        Ok("writer".into())
+    }
+}
+
+impl Hidden for Document {
+    fn hidden(&self) -> OperationResult<String> {
+        self.record_call();
+        Ok("hidden".into())
+    }
+}
+
+impl Broken for Document {
+    fn broken(&self) -> OperationResult<BadValue> {
+        self.record_call();
+        Ok(BadValue)
+    }
+}
+
+fn request(
+    object: &str,
+    object_validator: Option<&[u8]>,
+    reference: &wip_host::InterfaceRef,
+    interface_validator: Option<&[u8]>,
+    operation: &str,
+    arguments: Vec<NamedValue>,
+) -> CallRequest {
+    CallRequest {
+        object: ObjectId(object.into()),
+        object_validator: object_validator.map(Validator::new),
+        interface: InterfaceTarget {
+            reference: reference.clone(),
+            validator: interface_validator.map(Validator::new),
+        },
+        operation: operation.into(),
+        arguments,
+    }
+}
+
+#[test]
+fn registry_exposes_opaque_validated_descriptor_observations() {
+    let mut registry = InterfaceRegistry::new();
+    let reader = registry.register(ReaderDefinition, None).unwrap();
+
+    assert_ne!(reader.reference().as_str(), "Reader");
+    let observation = registry.observe(reader.reference()).unwrap();
+    assert_eq!(observation.reference, *reader.reference());
+    assert_eq!(observation.descriptor.name, "Reader");
+    assert_eq!(observation.validator, None);
+    observation.descriptor.validate().unwrap();
+
+    assert!(registry.register(ReaderDefinition, None).is_err());
+
+    let mut other_registry = InterfaceRegistry::new();
+    let other_reader = other_registry.register(ReaderDefinition, None).unwrap();
+    assert_ne!(reader.reference(), other_reader.reference());
+}
+
+#[test]
+fn publication_is_explicit_ordered_qualified_and_request_scoped() {
+    let mut registry = InterfaceRegistry::new();
+    let reader = registry.register(ReaderDefinition, None).unwrap();
+    let writer = registry.register(WriterDefinition, None).unwrap();
+    let _hidden = registry.register(HiddenDefinition, None).unwrap();
+
+    let publication = PublishedObject::new(Document::new("document-1"))
+        .expose(&writer)
+        .unwrap()
+        .expose(&reader)
+        .unwrap();
+
+    let object = publication.protocol_object();
+    assert_eq!(
+        object.interfaces,
+        vec![
+            writer.reference().as_str().to_owned(),
+            reader.reference().as_str().to_owned()
+        ]
+    );
+    assert!(!format!("{object:?}").contains("private/backend/path"));
+
+    let writer_result = publication
+        .dispatch(request(
+            "document-1",
+            None,
+            writer.reference(),
+            None,
+            "label",
+            vec![],
+        ))
+        .unwrap();
+    let reader_result = publication
+        .dispatch(request(
+            "document-1",
+            None,
+            reader.reference(),
+            None,
+            "label",
+            vec![],
+        ))
+        .unwrap();
+    assert_eq!(writer_result.value, Some(Value::String("writer".into())));
+    assert_eq!(reader_result.value, Some(Value::String("reader".into())));
+    assert_eq!(publication.implementation().calls.get(), 2);
+    assert_eq!(
+        publication
+            .implementation()
+            .private_backend_handle
+            .borrow()
+            .as_str(),
+        "/private/backend/path"
+    );
+}
+
+#[test]
+fn implemented_but_unexposed_interface_is_not_a_member() {
+    let mut registry = InterfaceRegistry::new();
+    let reader = registry.register(ReaderDefinition, None).unwrap();
+    let hidden = registry.register(HiddenDefinition, None).unwrap();
+    let publication = PublishedObject::new(Document::new("document-2"))
+        .expose(&reader)
+        .unwrap();
+
+    let error = publication
+        .dispatch(request(
+            "document-2",
+            None,
+            hidden.reference(),
+            None,
+            "hidden",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(error.kind(), CallErrorKind::InterfaceMismatch);
+    assert_eq!(
+        error.protocol_error().category,
+        ProtocolErrorCategory::InterfaceNotFound
+    );
+    assert_eq!(publication.implementation().calls.get(), 0);
+}
+
+#[test]
+fn zero_interface_objects_are_valid() {
+    let publication = PublishedObject::new(Document::new("empty"));
+    let object = publication.protocol_object();
+    assert_eq!(object.id, ObjectId("empty".into()));
+    assert!(object.interfaces.is_empty());
+    object.validate().unwrap();
+}
+
+#[test]
+fn duplicate_exposure_is_rejected_without_reordering() {
+    let mut registry = InterfaceRegistry::new();
+    let reader = registry.register(ReaderDefinition, None).unwrap();
+    let publication = PublishedObject::new(Document::new("duplicate"))
+        .expose(&reader)
+        .unwrap();
+    let error = match publication.expose(&reader) {
+        Ok(_) => panic!("duplicate exposure unexpectedly succeeded"),
+        Err(error) => error,
+    };
+    assert!(error.to_string().contains(reader.reference().as_str()));
+}
+
+#[test]
+fn operation_and_argument_failures_are_mapped_without_execution() {
+    let mut registry = InterfaceRegistry::new();
+    let reader = registry.register(ReaderDefinition, None).unwrap();
+    let publication = PublishedObject::new(Document::new("validation"))
+        .expose(&reader)
+        .unwrap();
+
+    let missing = publication
+        .dispatch(request(
+            "validation",
+            None,
+            reader.reference(),
+            None,
+            "missing",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(missing.kind(), CallErrorKind::OperationNotFound);
+    assert_eq!(
+        missing.protocol_error().category,
+        ProtocolErrorCategory::OperationNotFound
+    );
+
+    let invalid = publication
+        .dispatch(request(
+            "validation",
+            None,
+            reader.reference(),
+            None,
+            "add",
+            vec![NamedValue {
+                name: "left".into(),
+                value: Value::String("wrong".into()),
+            }],
+        ))
+        .unwrap_err();
+    assert_eq!(invalid.kind(), CallErrorKind::InvalidArguments);
+    assert_eq!(
+        invalid.protocol_error().category,
+        ProtocolErrorCategory::InvalidArguments
+    );
+    assert_eq!(publication.implementation().calls.get(), 0);
+}
+
+#[test]
+fn invalid_implementation_result_is_internal_and_not_returned() {
+    let mut registry = InterfaceRegistry::new();
+    let broken = registry.register(BrokenDefinition, None).unwrap();
+    let publication = PublishedObject::new(Document::new("broken"))
+        .expose(&broken)
+        .unwrap();
+
+    let error = publication
+        .dispatch(request(
+            "broken",
+            None,
+            broken.reference(),
+            None,
+            "broken",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(error.kind(), CallErrorKind::InvalidResult);
+    assert_eq!(
+        error.protocol_error().category,
+        ProtocolErrorCategory::Internal
+    );
+    assert_eq!(publication.implementation().calls.get(), 1);
+}
+
+#[test]
+fn object_validator_missing_and_mismatch_are_distinct_and_prevent_execution() {
+    let mut registry = InterfaceRegistry::new();
+    let reader = registry.register(ReaderDefinition, None).unwrap();
+    let publication = PublishedObject::new(Document::strict("strict", b"object-v1"))
+        .expose(&reader)
+        .unwrap();
+
+    let missing = publication
+        .dispatch(request(
+            "strict",
+            None,
+            reader.reference(),
+            None,
+            "label",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(missing.kind(), CallErrorKind::ValidatorRequired);
+
+    let mismatch = publication
+        .dispatch(request(
+            "strict",
+            Some(b"stale"),
+            reader.reference(),
+            None,
+            "label",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(mismatch.kind(), CallErrorKind::ValidatorMismatch);
+    assert_eq!(publication.implementation().calls.get(), 0);
+
+    let result = publication
+        .dispatch(request(
+            "strict",
+            Some(b"object-v1"),
+            reader.reference(),
+            None,
+            "label",
+            vec![],
+        ))
+        .unwrap();
+    assert_eq!(result.validator, Some(Validator::new(b"object-v1")));
+    assert_eq!(publication.implementation().calls.get(), 1);
+}
+
+#[test]
+fn interface_validator_missing_and_mismatch_are_distinct_and_prevent_execution() {
+    let mut registry = InterfaceRegistry::new();
+    let writer = registry
+        .register(WriterDefinition, Some(Validator::new(b"schema-v1")))
+        .unwrap();
+    let publication = PublishedObject::new(Document::new("mutable-interface"))
+        .expose(&writer)
+        .unwrap();
+
+    let observation = registry.observe(writer.reference()).unwrap();
+    assert_eq!(observation.validator, Some(Validator::new(b"schema-v1")));
+
+    let missing = publication
+        .dispatch(request(
+            "mutable-interface",
+            None,
+            writer.reference(),
+            None,
+            "label",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(missing.kind(), CallErrorKind::InterfaceValidatorRequired);
+
+    let mismatch = publication
+        .dispatch(request(
+            "mutable-interface",
+            None,
+            writer.reference(),
+            Some(b"schema-v0"),
+            "label",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(mismatch.kind(), CallErrorKind::InterfaceValidatorMismatch);
+    assert_eq!(publication.implementation().calls.get(), 0);
+
+    let result = publication
+        .dispatch(request(
+            "mutable-interface",
+            None,
+            writer.reference(),
+            Some(b"schema-v1"),
+            "label",
+            vec![],
+        ))
+        .unwrap();
+    assert_eq!(result.value, Some(Value::String("writer".into())));
+}
+
+#[test]
+fn host_failures_map_to_internal_without_becoming_domain_values() {
+    #[interface]
+    pub trait Failing {
+        /// Always fails in the host implementation.
+        fn fail(&self) -> OperationResult<String>;
+    }
+
+    impl Failing for Document {
+        fn fail(&self) -> OperationResult<String> {
+            Err(HostError::new("backend unavailable"))
+        }
+    }
+
+    let mut registry = InterfaceRegistry::new();
+    let failing = registry.register(FailingDefinition, None).unwrap();
+    let publication = PublishedObject::new(Document::new("failure"))
+        .expose(&failing)
+        .unwrap();
+    let error = publication
+        .dispatch(request(
+            "failure",
+            None,
+            failing.reference(),
+            None,
+            "fail",
+            vec![],
+        ))
+        .unwrap_err();
+    assert_eq!(error.kind(), CallErrorKind::Implementation);
+    assert_eq!(
+        error.protocol_error().category,
+        ProtocolErrorCategory::Internal
+    );
+}

crates/wip-host/tests/ui/unimplemented_interface.rs

@@ -0,0 +1,22 @@
+use wip_host::{InterfaceRegistry, PublishedObject, WipObject};
+use wip_interface::{OperationResult, interface};
+use wip_protocol::ObjectId;
+
+#[interface]
+pub trait Search {
+    fn search(&self) -> OperationResult<String>;
+}
+
+struct Object;
+
+impl WipObject for Object {
+    fn id(&self) -> ObjectId {
+        ObjectId("object".into())
+    }
+}
+
+fn main() {
+    let mut registry = InterfaceRegistry::new();
+    let search = registry.register(SearchDefinition, None).unwrap();
+    let _published = PublishedObject::new(Object).expose(&search).unwrap();
+}

crates/wip-host/tests/ui/unimplemented_interface.stderr

@@ -0,0 +1,34 @@
+error[E0277]: the trait bound `Object: Search` is not satisfied
+  --> tests/ui/unimplemented_interface.rs:21:58
+   |
+21 |     let _published = PublishedObject::new(Object).expose(&search).unwrap();
+   |                                                   ------ ^^^^^^^ unsatisfied trait bound
+   |                                                   |
+   |                                                   required by a bound introduced by this call
+   |
+help: the trait `Search` is not implemented for `Object`
+  --> tests/ui/unimplemented_interface.rs:10:1
+   |
+10 | struct Object;
+   | ^^^^^^^^^^^^^
+help: this trait has no implementations, consider adding one
+  --> tests/ui/unimplemented_interface.rs:6:1
+   |
+ 6 | pub trait Search {
+   | ^^^^^^^^^^^^^^^^
+note: required for `SearchDefinition` to implement `InterfaceImplementation<Object>`
+  --> tests/ui/unimplemented_interface.rs:5:1
+   |
+ 5 | #[interface]
+   | ^^^^^^^^^^^^
+ 6 | pub trait Search {
+   |           ^^^^^^
+note: required by a bound in `PublishedObject::<T>::expose`
+  --> src/lib.rs
+   |
+   |     pub fn expose<D>(mut self, interface: &RegisteredInterface<D>) -> Result<Self, PublicationError>
+   |            ------ required by a bound in this associated function
+   |     where
+   |         D: InterfaceImplementation<T>,
+   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `PublishedObject::<T>::expose`
+   = note: this error originates in the attribute macro `interface` (in Nightly builds, run with -Z macro-backtrace for more info)