de_singleton_inner() should no longer allocate a transform buffer when no transform is to be done.

Fortune for transfer's current commit: Blessing − 吉

src/enc.rs

@@ -75,6 +75,17 @@ pub struct SendOpt
 }
 ref_self!(SendOpt);
 
+impl SendOpt
+{
+    /// Does the binary data of this format require special handling?
+    ///
+    /// True if encryption and/or compression are specified.
+    fn is_spec(&self) -> bool
+    {
+	self.comp.is_some() || self.encrypt.is_some()
+    }
+}
+
 pub type RecvOpt = SendOpt;
 
 /// Default buffer size for encryption transform stream copying.
@@ -107,14 +118,19 @@ where F: AsyncRead + Unpin + ?Sized,
 }
 
 async fn de_singleton_inner<T: DeserializeOwned, B, F>(buf: F, mut from: &[u8], how: &RecvOpt) -> Result<T, TransformErrorKind>
-where B: AsRef<[u8]> + AsyncWrite + Unpin,
+where B: AsRef<[u8]> + AsyncWrite + Unpin + Default,
       F: FnOnce(&[u8]) -> B
 {
     // Decompressor
     // The output is written to this (through writer)
-    let mut is_spec = false; //TODO: Determine this before allocating `buf`.
-    let mut buf = buf(from);
-    //let mut buf = Vec::with_capacity(from.len()); // The `spec` output buffer. Used if there are transformations that need to be done to the data before deserialisation
+    let mut is_spec = false; // This is set later. The value will sometimes differ from `how.is_spec()` depending on combinations of options.
+    // The `spec` output buffer. Used if there are transformations that need to be done to the data before deserialisation
+    let mut buf = if how.is_spec() {
+	buf(&from)
+    } else {
+	Default::default()
+    };
+    //let mut buf = Vec::with_capacity(from.len()); 
     from = {
 	let mut b;
 	let writer: &mut (dyn AsyncWrite + Unpin) =
@@ -159,7 +175,6 @@ where B: AsRef<[u8]> + AsyncWrite + Unpin,
 	    from
 	}
     };
-    
     // Deserialise
     let v = match how.format {
 	SerialFormat::Text => serde_json::from_slice(&from[..])?,
@@ -283,6 +298,21 @@ pub enum TransformErrorKind
 #[derive(Debug)]
 pub struct RecvError(Box<(TransformErrorKind, RecvOpt)>);
 
+impl RecvError
+{
+    #[inline] pub fn kind(&self) -> &TransformErrorKind
+    {
+	&self.0.0
+    }
+}
+impl SendError
+{
+    #[inline] pub fn kind(&self) -> &TransformErrorKind
+    {
+	&self.0.0
+    }
+}
+
 impl error::Error for RecvError
 {
     fn source(&self) -> Option<&(dyn error::Error + 'static)> {