fix(parser/unchecked/attributes): Correctly enforce an identifier in `derive`

The ebnf grammar specifies an identifier (namely “Error” in the
`derive` attribute.), while the parser expected a string literal there.

src/parser/parsing/unchecked/mod.rs

@@ -193,14 +193,26 @@ impl Parser {
 
         let attribute = match keyword {
             AttributeKeyword::derive => {
-                let string_literal = self.parse_bracket_string_literal()?;
+                self.expect(token![CurvedBracketOpen])?;
-                match string_literal.content.as_str() {
+                let string_literal_token = self.expect(token![Identifier])?;
+                let string_literal =
+                    if let TokenKind::Identifier(ident) = string_literal_token.kind() {
+                        ident
+                    } else {
+                        unreachable! {"The token is a identifier, as checked by the `expect`"};
+                    };
+                self.expect(token![CurvedBracketClose])?;
+
+                match string_literal.as_str() {
                     "Error" => Ok(Attribute::derive {
                         value: DeriveValue::Error,
                         span,
                     }),
                     _ => Err(error::ParsingError::WrongDeriveValue {
-                        specified: string_literal,
+                        specified: StringLiteral {
+                            content: string_literal.to_owned(),
+                            span: string_literal_token.span,
+                        },
                     }),
                 }
             }