cm: initial support for Component Model async types

https://github.com/WebAssembly/component-model/blob/main/design/mvp/Async.md

cm/CHANGELOG.md

@@ -2,6 +2,12 @@
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- Initial support for Component Model [async](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Async.md) types `stream`, `future`, and `error-context`.
+
 ## [v0.1.0] — 2024-12-14
 
 Initial version, extracted into module [`go.bytecodealliance.org/cm`](https://pkg.go.dev/go.bytecodealliance.org/cm).

cm/empty.s

@@ -0,0 +1,3 @@
+// This file exists for testing this package without WebAssembly,
+// allowing empty function bodies with a //go:wasmimport directive.
+// See https://pkg.go.dev/cmd/compile for more information.

cm/error.go

@@ -0,0 +1,46 @@
+package cm
+
+// ErrorContext represents the Component Model [error-context] type,
+// an immutable, non-deterministic, host-defined value meant to aid in debugging.
+//
+// [error-context]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#error-context-type
+type ErrorContext struct {
+	_ HostLayout
+	errorContext
+}
+
+type errorContext uint32
+
+// Error implements the [error] interface. It returns the debug message associated with err.
+func (err errorContext) Error() string {
+	return err.DebugMessage()
+}
+
+// String implements [fmt.Stringer].
+func (err errorContext) String() string {
+	return err.DebugMessage()
+}
+
+// DebugMessage represents the Canonical ABI [error-context.debug-message] function.
+//
+// [error-context.debug-message]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#error-contextdebug-message
+func (err errorContext) DebugMessage() string {
+	var s string
+	errorContextDebugMessage(err, &s)
+	return s
+}
+
+//go:wasmimport canon error-context.debug-message
+//go:noescape
+func errorContextDebugMessage(err errorContext, msg *string)
+
+// Drop represents the Canonical ABI [error-context.drop] function.
+//
+// [error-context.drop]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#error-contextdrop
+func (err errorContext) Drop() {
+	errorContextDrop(err)
+}
+
+//go:wasmimport canon error-context.drop
+//go:noescape
+func errorContextDrop(err errorContext)

cm/future.go

@@ -0,0 +1,15 @@
+package cm
+
+// Future represents the Component Model [future] type.
+// A future is a special case of stream. In non-error cases,
+// a future delivers exactly one value before being automatically closed.
+//
+// [future]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#asynchronous-value-types
+type Future[T any] struct {
+	_ HostLayout
+	future[T]
+}
+
+type future[T any] uint32
+
+// TODO: implement methods on type future

cm/stream.go

@@ -0,0 +1,15 @@
+package cm
+
+// Stream represents the Component Model [stream] type.
+// A stream is a special case of stream. In non-error cases,
+// a stream delivers exactly one value before being automatically closed.
+//
+// [stream]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/Explainer.md#asynchronous-value-types
+type Stream[T any] struct {
+	_ HostLayout
+	stream[T]
+}
+
+type stream[T any] uint32
+
+// TODO: implement methods on type stream