codec: update to use std-future (#1214)

Strategy was to

- copy the old codec code that was temporarily being stashed in `tokio-io`
- modify all the type signatures to use Pin, as literal a translation as possible
- fix up the tests likewise

This is intended just to get things compiling and passing tests. Beyond that there is surely
lots of refactoring that can be done to make things more idiomatic. The docs are unchanged.

Closes #1189

tokio-codec/src/lines_codec.rs

@@ -1,6 +1,7 @@
+use crate::decoder::Decoder;
+use crate::encoder::Encoder;
 use bytes::{BufMut, BytesMut};
-use std::{cmp, io, str, usize};
-use tokio_io::_tokio_codec::{Decoder, Encoder};
+use std::{cmp, fmt, io, str, usize};
 
 /// A simple `Codec` implementation that splits up data into lines.
 #[derive(Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
@@ -117,11 +118,9 @@ fn without_carriage_return(s: &[u8]) -> &[u8] {
 
 impl Decoder for LinesCodec {
     type Item = String;
-    // TODO: in the next breaking change, this should be changed to a custom
-    // error type that indicates the "max length exceeded" condition better.
-    type Error = io::Error;
+    type Error = LinesCodecError;
 
-    fn decode(&mut self, buf: &mut BytesMut) -> Result<Option<String>, io::Error> {
+    fn decode(&mut self, buf: &mut BytesMut) -> Result<Option<String>, LinesCodecError> {
         loop {
             // Determine how far into the buffer we'll search for a newline. If
             // there's no max_length set, we'll read to the end of the buffer.
@@ -149,10 +148,7 @@ impl Decoder for LinesCodec {
                     // newline, return an error and start discarding on the
                     // next call.
                     self.is_discarding = true;
-                    Err(io::Error::new(
-                        io::ErrorKind::Other,
-                        "line length limit exceeded",
-                    ))
+                    Err(LinesCodecError::MaxLineLengthExceeded)
                 } else {
                     // We didn't find a line or reach the length limit, so the next
                     // call will resume searching at the current offset.
@@ -163,7 +159,7 @@ impl Decoder for LinesCodec {
         }
     }
 
-    fn decode_eof(&mut self, buf: &mut BytesMut) -> Result<Option<String>, io::Error> {
+    fn decode_eof(&mut self, buf: &mut BytesMut) -> Result<Option<String>, LinesCodecError> {
         Ok(match self.decode(buf)? {
             Some(frame) => Some(frame),
             None => {
@@ -184,12 +180,35 @@ impl Decoder for LinesCodec {
 
 impl Encoder for LinesCodec {
     type Item = String;
-    type Error = io::Error;
+    type Error = LinesCodecError;
 
-    fn encode(&mut self, line: String, buf: &mut BytesMut) -> Result<(), io::Error> {
+    fn encode(&mut self, line: String, buf: &mut BytesMut) -> Result<(), LinesCodecError> {
         buf.reserve(line.len() + 1);
         buf.put(line);
         buf.put_u8(b'\n');
         Ok(())
     }
 }
+
+#[derive(Debug)]
+pub enum LinesCodecError {
+    MaxLineLengthExceeded,
+    Io(io::Error),
+}
+
+impl fmt::Display for LinesCodecError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            LinesCodecError::MaxLineLengthExceeded => write!(f, "max line length exceeded"),
+            LinesCodecError::Io(e) => write!(f, "{}", e),
+        }
+    }
+}
+
+impl From<io::Error> for LinesCodecError {
+    fn from(e: io::Error) -> LinesCodecError {
+        LinesCodecError::Io(e)
+    }
+}
+
+impl std::error::Error for LinesCodecError {}