syntax: better error messages for '[\d-a]'

This commit adds a new type of error message that is used whenever a
character class escape sequence is used as the start or end of a
character class range.

Fixes #461

regex-syntax/src/ast/mod.rs

@@ -89,6 +89,10 @@ pub enum ErrorKind {
     /// An invalid character class range was found. An invalid range is any
     /// range where the start is greater than the end.
     ClassRangeInvalid,
+    /// An invalid range boundary was found in a character class. Range
+    /// boundaries must be a single literal codepoint, but this error indicates
+    /// that something else was found, such as a nested class.
+    ClassRangeLiteral,
     /// An opening `[` was found with no corresponding closing `]`.
     ClassUnclosed,
     /// An empty decimal number was given where one was expected.
@@ -182,6 +186,7 @@ impl error::Error for Error {
             CaptureLimitExceeded => "capture group limit exceeded",
             ClassEscapeInvalid => "invalid escape sequence in character class",
             ClassRangeInvalid => "invalid character class range",
+            ClassRangeLiteral => "invalid range boundary, must be a literal",
             ClassUnclosed => "unclosed character class",
             DecimalEmpty => "empty decimal literal",
             DecimalInvalid => "invalid decimal literal",
@@ -233,6 +238,9 @@ impl fmt::Display for ErrorKind {
                 write!(f, "invalid character class range, \
                            the start must be <= the end")
             }
+            ClassRangeLiteral => {
+                write!(f, "invalid range boundary, must be a literal")
+            }
             ClassUnclosed => {
                 write!(f, "unclosed character class")
             }

regex-syntax/src/ast/parse.rs

@@ -95,7 +95,7 @@ impl Primitive {
 
         match self {
             Literal(lit) => Ok(lit),
-            x => Err(p.error(*x.span(), ast::ErrorKind::ClassEscapeInvalid)),
+            x => Err(p.error(*x.span(), ast::ErrorKind::ClassRangeLiteral)),
         }
     }
 }
@@ -4677,13 +4677,13 @@ bar
             parser(r"[\w-a]").parse().unwrap_err(),
             TestError {
                 span: span(1..3),
-                kind: ast::ErrorKind::ClassEscapeInvalid,
+                kind: ast::ErrorKind::ClassRangeLiteral,
             });
         assert_eq!(
             parser(r"[a-\w]").parse().unwrap_err(),
             TestError {
                 span: span(3..5),
-                kind: ast::ErrorKind::ClassEscapeInvalid,
+                kind: ast::ErrorKind::ClassRangeLiteral,
             });
         assert_eq!(
             parser(r"[z-a]").parse().unwrap_err(),