Ensure that Error cannot be exhaustively matched on

This is going to be the "generic thing which can fail in a myriad of
ways" type. I want to be able to have specific useful cases which you
can recover from (e.g. no `Box<std::error::Error>`), but I don't want
people to exhaustively match on it.

Techincally adding a variant to this is still a violation of
[RFC #1105](https://github.com/rust-lang/rfcs/blob/master/text/1105-api-evolution.md),
but if you're actually matching against `__Nonexhaustive` and expected
it not to break eventually, you knew what was coming.

If some form of [RFC #757](rust-lang/rfcs#757)
ever lands, we'll switch to using that instead.

CHANGELOG.md

@@ -0,0 +1,18 @@
+# Change Log
+All user visible changes to this project will be documented in this file.
+This project adheres to [Semantic Versioning](http://semver.org/), as described
+for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/text/1105-api-evolution.md)
+
+
+## Unreleased
+
+### Changed
+
+* Added a hidden `__Nonexhaustive` variant to `result::Error`. This is not
+  intended to be something you can exhaustively match on, but I do want people
+  to be able to check for specific cases, so `Box<std::error::Error>` is
+  not an option.
+
+## [0.1.0] - 2015-11-29
+
+* Initial release

diesel/src/result.rs

@@ -7,6 +7,8 @@ use std::ffi::NulError;
 pub enum Error {
     InvalidCString(NulError),
     DatabaseError(String),
+    #[doc(hidden)]
+    __Nonexhaustive,
 }
 
 #[derive(Debug)]
@@ -57,6 +59,7 @@ impl Display for Error {
         match self {
             &Error::InvalidCString(ref nul_err) => nul_err.fmt(f),
             &Error::DatabaseError(ref s) => write!(f, "{}", &s),
+            &Error::__Nonexhaustive => unreachable!(),
         }
     }
 }
@@ -66,6 +69,7 @@ impl StdError for Error {
         match self {
             &Error::InvalidCString(ref nul_err) => nul_err.description(),
             &Error::DatabaseError(ref s) => &s,
+            &Error::__Nonexhaustive => unreachable!(),
         }
     }
 }