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

 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

+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};

tests/test_bytes.rs

@@ -43,7 +43,7 @@ fn from_slice() {
 #[test]
 fn fmt() {
     let a = format!("{:?}", Bytes::from(&b"abcdefg"[..]));
-    let b = format!("{:?}", b"abcdefg");
+    let b = "b\"abcdefg\"";
 
     assert_eq!(a, b);
 

tests/test_debug.rs

+extern crate bytes;
+
+use bytes::Bytes;
+
+#[test]
+fn fmt() {
+    let vec: Vec<_> = (0..0x100).map(|b| b as u8).collect();
+
+    let expected = "b\"\
+        \\0\\x01\\x02\\x03\\x04\\x05\\x06\\x07\
+        \\x08\\t\\n\\x0b\\x0c\\r\\x0e\\x0f\
+        \\x10\\x11\\x12\\x13\\x14\\x15\\x16\\x17\
+        \\x18\\x19\\x1a\\x1b\\x1c\\x1d\\x1e\\x1f\
+        \\x20!\\\"#$%&'()*+,-./0123456789:;<=>?\
+        @ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\\\]^_\
+        `abcdefghijklmnopqrstuvwxyz{|}~\\x7f\
+        \\x80\\x81\\x82\\x83\\x84\\x85\\x86\\x87\
+        \\x88\\x89\\x8a\\x8b\\x8c\\x8d\\x8e\\x8f\
+        \\x90\\x91\\x92\\x93\\x94\\x95\\x96\\x97\
+        \\x98\\x99\\x9a\\x9b\\x9c\\x9d\\x9e\\x9f\
+        \\xa0\\xa1\\xa2\\xa3\\xa4\\xa5\\xa6\\xa7\
+        \\xa8\\xa9\\xaa\\xab\\xac\\xad\\xae\\xaf\
+        \\xb0\\xb1\\xb2\\xb3\\xb4\\xb5\\xb6\\xb7\
+        \\xb8\\xb9\\xba\\xbb\\xbc\\xbd\\xbe\\xbf\
+        \\xc0\\xc1\\xc2\\xc3\\xc4\\xc5\\xc6\\xc7\
+        \\xc8\\xc9\\xca\\xcb\\xcc\\xcd\\xce\\xcf\
+        \\xd0\\xd1\\xd2\\xd3\\xd4\\xd5\\xd6\\xd7\
+        \\xd8\\xd9\\xda\\xdb\\xdc\\xdd\\xde\\xdf\
+        \\xe0\\xe1\\xe2\\xe3\\xe4\\xe5\\xe6\\xe7\
+        \\xe8\\xe9\\xea\\xeb\\xec\\xed\\xee\\xef\
+        \\xf0\\xf1\\xf2\\xf3\\xf4\\xf5\\xf6\\xf7\
+        \\xf8\\xf9\\xfa\\xfb\\xfc\\xfd\\xfe\\xff\"";
+
+    assert_eq!(expected, format!("{:?}", Bytes::from(vec)));
+}