Specify that text in streams is transmitted in UTF-8.

.github/workflows/main.yml

@@ -10,5 +10,5 @@ jobs:
     name: Check ABI files are up-to-date
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
-    - uses: WebAssembly/wit-abi-up-to-date@v16
+    - uses: actions/checkout@v4
+    - uses: WebAssembly/wit-abi-up-to-date@v17

imports.md

@@ -12,7 +12,21 @@
 <hr />
 <h3>Types</h3>
 <h4><a name="error"><code>resource error</code></a></h4>
-<hr />
+<p>A resource which represents some error information.</p>
+<p>The only method provided by this resource is <code>to-debug-string</code>,
+which provides some human-readable information about the error.</p>
+<p>In the <code>wasi:io</code> package, this resource is returned through the
+<code>wasi:io/streams/stream-error</code> type.</p>
+<p>To provide more specific error information, other interfaces may
+provide functions to further &quot;downcast&quot; this error into more specific
+error information. For example, <a href="#error"><code>error</code></a>s returned in streams derived
+from filesystem types to be described using the filesystem's own
+error-code type, using the function
+<code>wasi:filesystem/types/filesystem-error-code</code>, which takes a parameter
+<code>borrow&lt;error&gt;</code> and returns
+<code>option&lt;wasi:filesystem/types/error-code&gt;</code>.</p>
+<h2>The set of functions which can &quot;downcast&quot; an <a href="#error"><code>error</code></a> into a more
+concrete type is open.</h2>
 <h3>Functions</h3>
 <h4><a name="method_error.to_debug_string"><code>[method]error.to-debug-string: func</code></a></h4>
 <p>Returns a string that is suitable to assist humans in debugging
@@ -35,7 +49,7 @@ at once.</p>
 <hr />
 <h3>Types</h3>
 <h4><a name="pollable"><code>resource pollable</code></a></h4>
-<hr />
+<h2><a href="#pollable"><code>pollable</code></a> represents a single I/O event which may be ready, or not.</h2>
 <h3>Functions</h3>
 <h4><a name="method_pollable.ready"><code>[method]pollable.ready: func</code></a></h4>
 <p>Return the readiness of a pollable. This function never blocks.</p>
@@ -109,11 +123,28 @@ future operations.
 </li>
 </ul>
 <h4><a name="input_stream"><code>resource input-stream</code></a></h4>
+<p>An input bytestream.</p>
+<p><a href="#input_stream"><code>input-stream</code></a>s are <em>non-blocking</em> to the extent practical on underlying
+platforms. I/O operations always return promptly; if fewer bytes are
+promptly available than requested, they return the number of bytes promptly
+available, which could even be zero. To wait for data to be available,
+use the <code>subscribe</code> function to obtain a <a href="#pollable"><code>pollable</code></a> which can be polled
+for using <code>wasi:io/poll</code>.</p>
 <h4><a name="output_stream"><code>resource output-stream</code></a></h4>
-<hr />
+<p>An output bytestream.</p>
+<h2><a href="#output_stream"><code>output-stream</code></a>s are <em>non-blocking</em> to the extent practical on
+underlying platforms. Except where specified otherwise, I/O operations also
+always return promptly, after the number of bytes that can be written
+promptly, which could even be zero. To wait for the stream to be ready to
+accept data, the <code>subscribe</code> function to obtain a <a href="#pollable"><code>pollable</code></a> which can be
+polled for using <code>wasi:io/poll</code>.</h2>
 <h3>Functions</h3>
 <h4><a name="method_input_stream.read"><code>[method]input-stream.read: func</code></a></h4>
 <p>Perform a non-blocking read from the stream.</p>
+<p>When the source of a <code>read</code> is binary data, the bytes from the source
+are returned verbatim. When the source of a <code>read</code> is known to the
+implementation to be text, bytes containing the UTF-8 encoding of the
+text are returned.</p>
 <p>This function returns a list of bytes containing the read data,
 when successful. The returned list will contain up to <code>len</code> bytes;
 it may return fewer than requested, but not more. The list is
@@ -209,6 +240,11 @@ error.</p>
 </ul>
 <h4><a name="method_output_stream.write"><code>[method]output-stream.write: func</code></a></h4>
 <p>Perform a write. This function never blocks.</p>
+<p>When the destination of a <code>write</code> is binary data, the bytes from
+<code>contents</code> are written verbatim. When the destination of a <code>write</code> is
+known to the implementation to be text, the bytes of <code>contents</code> are
+transcoded from UTF-8 into the encoding of the destination and then
+written.</p>
 <p>Precondition: check-write gave permit of Ok(n) and contents has a
 length of less than or equal to n. Otherwise, this function will trap.</p>
 <p>returns Err(closed) without writing if the stream has closed since

wit/streams.wit

@@ -32,6 +32,11 @@ interface streams {
     resource input-stream {
         /// Perform a non-blocking read from the stream.
         ///
+        /// When the source of a `read` is binary data, the bytes from the source
+        /// are returned verbatim. When the source of a `read` is known to the
+        /// implementation to be text, bytes containing the UTF-8 encoding of the
+        /// text are returned.
+        ///
         /// This function returns a list of bytes containing the read data,
         /// when successful. The returned list will contain up to `len` bytes;
         /// it may return fewer than requested, but not more. The list is
@@ -111,6 +116,12 @@ interface streams {
 
         /// Perform a write. This function never blocks.
         ///
+        /// When the destination of a `write` is binary data, the bytes from
+        /// `contents` are written verbatim. When the destination of a `write` is
+        /// known to the implementation to be text, the bytes of `contents` are
+        /// transcoded from UTF-8 into the encoding of the destination and then
+        /// written.
+        ///
         /// Precondition: check-write gave permit of Ok(n) and contents has a
         /// length of less than or equal to n. Otherwise, this function will trap.
         ///