Add 'stream' type

design/mvp/Async.md

@@ -419,21 +419,25 @@ For now, this remains a [TODO](#todo) and validation will reject `async`-lifted
 
 ## TODO
 
-Native async support is being proposed in progressive chunks. The following
-features will be added in future chunks to complete "async" in Preview 3:
-* `future`/`stream`/`error`: add for use in function types for finer-grained
-  concurrency
-* `subtask.cancel`: allow a supertask to signal to a subtask that its result is
-  no longer wanted and to please wrap it up promptly
-* allow "tail-calling" a subtask so that the current wasm instance can be torn
-  down eagerly
-* `task.index`+`task.wake`: allow tasks in the same instance to wait on and
-  wake each other (async condvar-style)
+Native async support is being proposed incrementally. The following features
+will be added in future chunks roughly in the order list to complete the full
+"async" story:
+* add `future` type
+* add `error` type that can be included when closing a stream/future
 * `nonblocking` function type attribute: allow a function to declare in its
   type that it will not transitively do anything blocking
+* define what `async` means for `start` functions
+* `task.index`+`task.wake`: allow tasks in the same instance to wait on and
+  wake each other
+* `subtask.cancel`: allow a supertask to signal to a subtask that its result is
+  no longer wanted and to please wrap it up promptly
+* `stream.lull` built-in that says "no more elements are coming for a while"
 * `recursive` function type attribute: allow a function to be reentered
-  recursively (instead of trapping)
-* enable `async` `start` functions
+  recursively (instead of trapping) for the benefit of donut wrapping
+* built-in to "tail-call" a subtask so that the current wasm instance can be torn
+  down eagerly while preserving "structured concurrency"
+* allow pipelining multiple `stream.read`/`write` calls
+* allow chaining multiple async calls together ("promise pipelining")
 * integrate with `shared`: define how to lift and lower functions `async` *and*
   `shared`
 

design/mvp/CanonicalABI.md

@@ -2126,6 +2126,7 @@ where `$callee` has type `$ft`, validation specifies:
 * a `memory` is present if required by lifting and is a subtype of `(memory 1)`
 * a `realloc` is present if required by lifting and has type `(func (param i32 i32 i32 i32) (result i32))`
 * there is no `post-return` in `$opts`
+* if `contains_async($ft)`, then `$opts.async` must be set
 
 When instantiating component instance `$inst`:
 * Define `$f` to be the partially-bound closure: `canon_lower($opts, $ft, $callee)`

design/mvp/Explainer.md

@@ -1222,10 +1222,12 @@ freeing memory after lifting and thus `post-return` may be deprecated in the
 future.
 
 🔀 The `async` option specifies that the component wants to make (for imports)
-or support (for exports) multiple concurrent (asynchronous) calls. This option
-can be applied to any component-level function type and changes the derived
-Canonical ABI significantly. See the [async explainer](Async.md) for more
-details.
+or support (for exports) multiple concurrent (asynchronous) calls. This
+option can be applied to any component-level function type and changes the
+derived Canonical ABI significantly. See the [async explainer](Async.md) for
+more details. When a function signature contains a `future` or `stream`,
+validation requires the `async` option to be set (since a synchronous call to
+a function using these types is likely to deadlock).
 
 🔀 The `(callback ...)` option may only be present in `canon lift` when the
 `async` option has also been set and specifies a core function that is