error: Print errors using Display impl

I have been running into Debug-printed error messages too often and
needing to look up in the source code each level of nested errors to
find out from the comment on the error variant what the short name of
the variant means in human terms. Worse, many errors (like the one shown
below) already had error strings written but were being printed from the
calling code in the less helpful Debug representation anyway.

Before:
    [ERROR:src/main.rs:705] The architecture failed to build the vm: NoVarEmpty

After:
    [ERROR:src/main.rs:705] The architecture failed to build the vm: /var/empty doesn't exist, can't jail devices.

TEST=cargo check --all-features
TEST=FEATURES=test emerge-amd64-generic crosvm

Change-Id: I77122c7d6861b2d610de2fff718896918ab21e10
Reviewed-on: https://chromium-review.googlesource.com/1469225
Commit-Ready: David Tolnay <dtolnay@chromium.org>
Tested-by: David Tolnay <dtolnay@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/msg_socket/src/msg_on_socket.rs b/msg_socket/src/msg_on_socket.rs
index c6e4413..493c57c 100644
--- a/msg_socket/src/msg_on_socket.rs
+++ b/msg_socket/src/msg_on_socket.rs
@@ -4,6 +4,7 @@
 
 use data_model::*;
 use std;
+use std::fmt::{self, Display};
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use std::result;
 use sys_util::{Error as SysError, EventFd};
@@ -38,6 +39,23 @@ pub enum MsgError {
 
 pub type MsgResult<T> = result::Result<T, MsgError>;
 
+impl Display for MsgError {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        use self::MsgError::*;
+
+        match self {
+            Send(e) => write!(f, "failed to send request or response: {}", e),
+            Recv(e) => write!(f, "failed to receive request or response: {}", e),
+            InvalidType => write!(f, "invalid type"),
+            BadRecvSize(n) => write!(f, "wrong amount of data received; expected {} bytes", n),
+            ExpectFd => write!(f, "missing associated file descriptor for request"),
+            NotExpectFd => write!(f, "unexpected file descriptor is unused"),
+            WrongFdBufferSize => write!(f, "fd buffer size too small"),
+            WrongMsgBufferSize => write!(f, "msg buffer size too small"),
+        }
+    }
+}
+
 /// A msg that could be serialized to and deserialize from array in little endian.
 ///
 /// For structs, we always have fixed size of bytes and fixed count of fds.