Fix crash encountering --

src/lexer.rs

@@ -59,6 +59,7 @@ impl<'a> Iterator for Lexer<'a> {
                             s.push_str(&kw.to_string());
                             TokenKind::Ident(s)
                         }
+                        TokenKind::Symbol(Symbol::Minus) => TokenKind::Ident(String::from("--")),
                         _ => unsafe { std::hint::unreachable_unchecked() },
                     },
                     _ => TokenKind::Symbol(Symbol::Minus),

tests/interpolation.rs

@@ -18,3 +18,8 @@ test!(
     "a {\n  color: a#{foo}1;\n}\n",
     "a {\n  color: afoo1;\n}\n"
 );
+test!(
+    double_hyphen_before_interpolation,
+    "a {\n  --#{foo}: red;\n}\n",
+    "a {\n  --foo: red;\n}\n"
+);