syntax: remove '__Nonexhaustive' hack, use #[non_exhaustive]

This marks the various error types as '#[non_exhaustive]' instead of
using a __Nonexhaustive variant hack.

Closes #884

regex-syntax/src/ast/mod.rs

@@ -64,6 +64,10 @@ impl Error {
 }
 
 /// The type of an error that occurred while building an AST.
+///
+/// This error type is marked as `non_exhaustive`. This means that adding a
+/// new variant is not considered a breaking change.
+#[non_exhaustive]
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum ErrorKind {
     /// The capturing group limit was exceeded.
@@ -168,13 +172,6 @@ pub enum ErrorKind {
     /// `(?<!re)`. Note that all of these syntaxes are otherwise invalid; this
     /// error is used to improve the user experience.
     UnsupportedLookAround,
-    /// Hints that destructuring should not be exhaustive.
-    ///
-    /// This enum may grow additional variants, so this makes sure clients
-    /// don't count on exhaustive matching. (Otherwise, adding a new variant
-    /// could break existing code.)
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl std::error::Error for Error {}
@@ -270,7 +267,6 @@ impl fmt::Display for ErrorKind {
                 "look-around, including look-ahead and look-behind, \
                  is not supported"
             ),
-            _ => unreachable!(),
         }
     }
 }

regex-syntax/src/error.rs

@@ -9,6 +9,10 @@ use crate::hir;
 pub type Result<T> = result::Result<T, Error>;
 
 /// This error type encompasses any error that can be returned by this crate.
+///
+/// This error type is marked as `non_exhaustive`. This means that adding a
+/// new variant is not considered a breaking change.
+#[non_exhaustive]
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum Error {
     /// An error that occurred while translating concrete syntax into abstract
@@ -17,13 +21,6 @@ pub enum Error {
     /// An error that occurred while translating abstract syntax into a high
     /// level intermediate representation (HIR).
     Translate(hir::Error),
-    /// Hints that destructuring should not be exhaustive.
-    ///
-    /// This enum may grow additional variants, so this makes sure clients
-    /// don't count on exhaustive matching. (Otherwise, adding a new variant
-    /// could break existing code.)
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 impl From<ast::Error> for Error {
@@ -45,7 +42,6 @@ impl fmt::Display for Error {
         match *self {
             Error::Parse(ref x) => x.fmt(f),
             Error::Translate(ref x) => x.fmt(f),
-            _ => unreachable!(),
         }
     }
 }

regex-syntax/src/hir/mod.rs

@@ -52,6 +52,10 @@ impl Error {
 }
 
 /// The type of an error that occurred while building an `Hir`.
+///
+/// This error type is marked as `non_exhaustive`. This means that adding a
+/// new variant is not considered a breaking change.
+#[non_exhaustive]
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum ErrorKind {
     /// This error occurs when a Unicode feature is used when Unicode
@@ -80,13 +84,6 @@ pub enum ErrorKind {
     /// Note that this restriction in the translator may be removed in the
     /// future.
     EmptyClassNotAllowed,
-    /// Hints that destructuring should not be exhaustive.
-    ///
-    /// This enum may grow additional variants, so this makes sure clients
-    /// don't count on exhaustive matching. (Otherwise, adding a new variant
-    /// could break existing code.)
-    #[doc(hidden)]
-    __Nonexhaustive,
 }
 
 // BREADCRUMBS:
@@ -122,7 +119,6 @@ impl fmt::Display for ErrorKind {
                  (make sure the unicode-case feature is enabled)"
             }
             EmptyClassNotAllowed => "empty character classes are not allowed",
-            __Nonexhaustive => unreachable!(),
         };
         f.write_str(msg)
     }