Reimplement fmt::Debug for Bytes and BytesMut (#84)

Standard `Debug` implementation for `[u8]` is comma separated list
of numbers. Since large amount of byte strings are in fact ASCII
strings or contain a lot of ASCII strings (e. g. HTTP), it is
convenient to print strings as ASCII when possible.

src/bytes.rs

@@ -1,5 +1,6 @@
 use {IntoBuf, Buf, BufMut};
 use buf::Iter;
+use debug;
 
 use std::{cmp, fmt, mem, hash, ops, slice, ptr, usize};
 use std::borrow::Borrow;
@@ -722,7 +723,7 @@ impl Eq for Bytes {
 
 impl fmt::Debug for Bytes {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        fmt::Debug::fmt(&self.inner.as_ref(), fmt)
+        fmt::Debug::fmt(&debug::BsDebug(&self.inner.as_ref()), fmt)
     }
 }
 
@@ -1260,7 +1261,7 @@ impl Eq for BytesMut {
 
 impl fmt::Debug for BytesMut {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
-        fmt::Debug::fmt(self.inner.as_ref(), fmt)
+        fmt::Debug::fmt(&debug::BsDebug(&self.inner.as_ref()), fmt)
     }
 }
 

src/debug.rs

@@ -0,0 +1,40 @@
+use std::fmt;
+
+/// Alternative implementation of `fmt::Debug` for byte slice.
+///
+/// Standard `Debug` implementation for `[u8]` is comma separated
+/// list of numbers. Since large amount of byte strings are in fact
+/// ASCII strings or contain a lot of ASCII strings (e. g. HTTP),
+/// it is convenient to print strings as ASCII when possible.
+///
+/// This struct wraps `&[u8]` just to override `fmt::Debug`.
+///
+/// `BsDebug` is not a part of public API of bytes crate.
+pub struct BsDebug<'a>(pub &'a [u8]);
+
+impl<'a> fmt::Debug for BsDebug<'a> {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        try!(write!(fmt, "b\""));
+        for &c in self.0 {
+            // https://doc.rust-lang.org/reference.html#byte-escapes
+            if c == b'\n' {
+                try!(write!(fmt, "\\n"));
+            } else if c == b'\r' {
+                try!(write!(fmt, "\\r"));
+            } else if c == b'\t' {
+                try!(write!(fmt, "\\t"));
+            } else if c == b'\\' || c == b'"' {
+                try!(write!(fmt, "\\{}", c as char));
+            } else if c == b'\0' {
+                try!(write!(fmt, "\\0"));
+            // ASCII printable except space
+            } else if c > 0x20 && c < 0x7f {
+                try!(write!(fmt, "{}", c as char));
+            } else {
+                try!(write!(fmt, "\\x{:02x}", c));
+            }
+        }
+        try!(write!(fmt, "\""));
+        Ok(())
+    }
+}

src/lib.rs

@@ -89,6 +89,7 @@ pub use buf::{
 };
 
 mod bytes;
+mod debug;
 pub use bytes::{Bytes, BytesMut};
 
 pub use byteorder::{ByteOrder, BigEndian, LittleEndian};