Implement unit conversions in numeric subtraction

src/value/ops.rs

@@ -123,8 +123,17 @@ impl Sub for Value {
                     }
                     if unit == unit2 {
                         Value::Dimension(num - num2, unit)
+                    } else if unit == Unit::None {
+                        Value::Dimension(num - num2, unit2)
+                    } else if unit2 == Unit::None {
+                        Value::Dimension(num - num2, unit)
                     } else {
-                        todo!("unit conversions")
+                        Value::Dimension(
+                            num - num2
+                                * UNIT_CONVERSION_TABLE[&unit.to_string()][&unit2.to_string()]
+                                    .clone(),
+                            unit,
+                        )
                     }
                 }
                 _ => todo!(),

tests/units.rs

@@ -54,6 +54,16 @@ test!(
     "a {\n  color: 10 + 10px;\n}\n",
     "a {\n  color: 20px;\n}\n"
 );
+test!(
+    unit_minus_none,
+    "a {\n  color: 10px - 10;\n}\n",
+    "a {\n  color: 0px;\n}\n"
+);
+test!(
+    none_minus_unit,
+    "a {\n  color: 10 - 10px;\n}\n",
+    "a {\n  color: 0px;\n}\n"
+);
 
 macro_rules! test_unit_addition {
     ($u1:ident, $u2:ident, $out:literal) => {