more robustly handle - as start of identifier

README.md

@@ -30,6 +30,7 @@ a special parser for plain css
 @forward (~400 tests)
 @keyframes (~30 tests)
 @supports (~128 tests)
+aribtrary control flow in @function and @mixin
 ```
 
 To run the official test suite,

src/value/parse.rs

@@ -245,12 +245,6 @@ fn eat_op<I: Iterator<Item = IntermediateValue>>(
         Op::Minus => {
             if devour_whitespace(iter) || !last_was_whitespace {
                 let right = single_value(iter, scope, super_selector, op.span)?;
-                if !last_was_whitespace && right.node == Value::Null {
-                    space_separated.push(
-                        right.map_node(|_| Value::Ident("-null".to_string(), QuoteKind::None)),
-                    );
-                    return Ok(());
-                }
                 if let Some(left) = space_separated.pop() {
                     space_separated.push(Spanned {
                         node: Value::BinaryOp(Box::new(left.node), op.node, Box::new(right.node)),
@@ -261,12 +255,6 @@ fn eat_op<I: Iterator<Item = IntermediateValue>>(
                 }
             } else {
                 let right = single_value(iter, scope, super_selector, op.span)?;
-                if right.node == Value::Null {
-                    space_separated.push(
-                        right.map_node(|_| Value::Ident("-null".to_string(), QuoteKind::None)),
-                    );
-                    return Ok(());
-                }
                 space_separated.push(right.map_node(|n| Value::UnaryOp(op.node, Box::new(n))));
             }
         }
@@ -601,6 +589,11 @@ impl Value {
             Some(v) => (v.kind, v.pos()),
             None => panic!("unexpected eof"),
         };
+
+        let next_is_hypen = |toks: &mut PeekMoreIterator<I>| {
+            toks.peek_forward(1).is_some()
+                && matches!(toks.peek().unwrap().kind, '-' | '_' | 'a'..='z' | 'A'..='Z')
+        };
         match kind {
             ',' => {
                 toks.next();
@@ -689,7 +682,8 @@ impl Value {
             _ if kind.is_ascii_alphabetic()
                 || kind == '_'
                 || kind == '\\'
-                || (!kind.is_ascii() && !kind.is_control()) =>
+                || (!kind.is_ascii() && !kind.is_control())
+                || (kind == '-' && next_is_hypen(toks)) =>
             {
                 Self::ident(toks, scope, super_selector)
             }

tests/null.rs

@@ -28,3 +28,8 @@ test!(
     "a {\n  color: [null null null];\n}\n",
     ""
 );
+test!(
+    negative_null_in_var,
+    "a {\n  $x: null;\n  color: -$x;\n}\n",
+    "a {\n  color: -;\n}\n"
+);