Add human-readable message to string representations of Errno

src/errno.rs

@@ -1,9 +1,9 @@
 use std::error::Error;
-use std::fmt::{self, Display, Formatter};
+use std::fmt::{self, Debug, Display, Formatter};
 use std::io::Error as IoError;
 use std::os::raw::c_int;
 
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+#[derive(Copy, Clone, Eq, PartialEq)]
 /// linux errno wrap.
 pub struct Errno(c_int);
 
@@ -38,7 +38,22 @@ impl From<Errno> for IoError {
 
 impl Display for Errno {
     fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
-        write!(f, "errno is {}", self.0)
+        write!(f, "errno {} (", self.0)?;
+        match self.0 {
+            libc::ENOENT => write!(f, "does not exist"),
+            libc::EEXIST => write!(f, "already exists"),
+            libc::EISDIR => write!(f, "is a directory"),
+            libc::ENOTDIR => write!(f, "is not a directory"),
+            _ => write!(f, "unknown"),
+        }?;
+        write!(f, ")")?;
+        Ok(())
+    }
+}
+
+impl Debug for Errno {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        Display::fmt(self, f)
     }
 }