Add error for unmatched double quotes.

src/error/display.rs

@@ -69,6 +69,7 @@ impl fmt::Display for EvalexprError {
             ),
             UnmatchedLBrace => write!(f, "Found an unmatched opening parenthesis '('."),
             UnmatchedRBrace => write!(f, "Found an unmatched closing parenthesis ')'."),
+            UnmatchedDoubleQuote => write!(f, "Found an unmatched double quote '\"'"),
             MissingOperatorOutsideOfBrace { .. } => write!(
                 f,
                 "Found an opening parenthesis that is preceded by something that does not take \

src/error/mod.rs

@@ -129,6 +129,9 @@ pub enum EvalexprError {
     /// A closing brace without a matching opening brace was found.
     UnmatchedRBrace,
 
+    /// A double quote without a matching second double quote was found.
+    UnmatchedDoubleQuote,
+
     /// Left of an opening brace or right of a closing brace is a token that does not expect the brace next to it.
     /// For example, writing `4(5)` would yield this error, as the `4` does not have any operands.
     MissingOperatorOutsideOfBrace,

src/token/mod.rs

@@ -251,13 +251,13 @@ fn parse_string_literal<Iter: Iterator<Item = char>>(
 
     while let Some(c) = iter.next() {
         match c {
-            '"' => break,
+            '"' => return Ok(PartialToken::Token(Token::String(result))),
             '\\' => result.push(parse_escape_sequence(&mut iter)?),
             c => result.push(c),
         }
     }
 
-    Ok(PartialToken::Token(Token::String(result)))
+    Err(EvalexprError::UnmatchedDoubleQuote)
 }
 
 /// Converts a string to a vector of partial tokens.

tests/integration.rs

@@ -2307,5 +2307,8 @@ fn test_hex() {
 
 #[test]
 fn test_broken_string() {
-    assert!(eval(r#""abc" == "broken string"#).is_err());
+    assert_eq!(
+        eval(r#""abc" == "broken string"#),
+        Err(EvalexprError::UnmatchedDoubleQuote)
+    );
 }