Re: [whatwg/streams] ReadableStream.from(asyncIterable) (#1083)

@MattiasBuelens commented on this pull request.



> + 1. Let |startAlgorithm| be the following steps:
+  1. Set |iteratorRecord| to ? [$GetIterator$](|asyncIterable|, async).
+ 1. Let |pullAlgorithm| be the following steps:
+  1. Let |nextResult| be [$IteratorNext$](|iteratorRecord|).
+  1. If |nextResult| is an abrupt completion, return [=a promise rejected with=]
+     |nextResult|.\[[Value]].
+  1. Let |nextPromise| be [=a promise resolved with=] |nextResult|.\[[Value]].
+  1. Return the result of [=reacting=] to |nextPromise| with the following fulfillment steps,
+     given the argument |iterResult|:
+   1. If [$Type$](|iterResult|) is not Object, throw a {{TypeError}}.
+   1. Let |done| be ? [$IteratorComplete$](|iterResult|).
+   1. If |done| is true:
+    1. Perform ! [$ReadableStreamDefaultControllerClose$](stream.[=ReadableStream/[[controller]]=]).
+   1. Otherwise:
+    1. Let |value| be ? [$IteratorValue$](|iterResult|).
+    1. Perform ! [$ReadableStreamDefaultControllerEnqueue$](stream.[=ReadableStream/[[controller]]=],

One possible use case would be if you had an (slow) async iterable, and you wanted to quickly add buffering around it:
```javascript
const bufferedReadable = ReadableStream
  .from(createSlowIterable(), new CountQueuingStrategy({ highWaterMark: 10 }));
```
That said, you can also achieve this with an intermediate `TransformStream`, which is already explained in [this example](https://streams.spec.whatwg.org/#example-transform-identity):
```javascript
const slowReadable = ReadableStream.from(createSlowIterable());
const bufferedReadable = slowReadable
  .pipeThrough(new TransformStream(undefined, new CountQueuingStrategy({ highWaterMark: 10 })));
```
(Note that this assumes that `ReadableStream.from` creates a stream with HWM = 0, [see other thread](https://github.com/whatwg/streams/pull/1083#discussion_r549040797).)

I don't know how compelling this use case is though... 🤷

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