ws protocol: don't heap-allocate in serialize_20_bytes

TODO.md

@@ -43,11 +43,6 @@
       * move additional request sending to for each received response, maybe
         with probability 0.2
 
-* aquatic_ws
-  * large amount of temporary allocations in serialize_20_bytes, pretty many in deserialize_20_bytes
-    * 20 byte parsing: consider using something like ArrayString<80> to avoid
-      heap allocations
-
 # Not important
 
 * aquatic_http:

crates/ws_protocol/src/common.rs

@@ -84,9 +84,20 @@ fn serialize_20_bytes<S>(data: &[u8; 20], serializer: S) -> Result<S::Ok, S::Err
 where
     S: Serializer,
 {
-    let text: String = data.iter().map(|byte| char::from(*byte)).collect();
+    // Length of 40 is enough since each char created from a byte will
+    // utf-8-encode to max 2 bytes
+    let mut str_buffer = [0u8; 40];
+    let mut offset = 0;
 
-    serializer.serialize_str(&text)
+    for byte in data {
+        offset += char::from(*byte)
+            .encode_utf8(&mut str_buffer[offset..])
+            .len();
+    }
+
+    let text = ::std::str::from_utf8(&str_buffer[..offset]).unwrap();
+
+    serializer.serialize_str(text)
 }
 
 struct TwentyByteVisitor;