regex: better formatting for syntax errors

This commit adds an explicit Debug impl for regex's main Error type.
The purpose of this impl is to format parse errors in normal panic
messages more nicely. This is slightly idiosyncratic, but the default
Debug impl prints the full string anyway, we might as well format it
nicely.

See also: #450

Author: BurntSushi

src/error.rs

@@ -9,11 +9,12 @@
 // except according to those terms.
 
 use std::fmt;
+use std::iter::repeat;
 
 use syntax;
 
 /// An error that occurred during parsing or compiling a regular expression.
-#[derive(Clone, Debug, PartialEq)]
+#[derive(Clone, PartialEq)]
 pub enum Error {
     /// A syntax error.
     Syntax(String),
@@ -56,6 +57,34 @@ impl fmt::Display for Error {
     }
 }
 
+// We implement our own Debug implementation so that we show nicer syntax
+// errors when people use `Regex::new(...).unwrap()`. It's a little weird,
+// but the `Syntax` variant is already storing a `String` anyway, so we might
+// as well format it nicely.
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match *self {
+            Error::Syntax(ref err) => {
+                let hr: String = repeat('~').take(79).collect();
+                try!(writeln!(f, "Syntax("));
+                try!(writeln!(f, "{}", hr));
+                try!(writeln!(f, "{}", err));
+                try!(writeln!(f, "{}", hr));
+                try!(write!(f, ")"));
+                Ok(())
+            }
+            Error::CompiledTooBig(limit) => {
+                f.debug_tuple("CompiledTooBig")
+                    .field(&limit)
+                    .finish()
+            }
+            Error::__Nonexhaustive => {
+                f.debug_tuple("__Nonexhaustive").finish()
+            }
+        }
+    }
+}
+
 impl From<syntax::Error> for Error {
     fn from(err: syntax::Error) -> Error {
         Error::Syntax(err.to_string())