Add sibling selectors

corpus/selectors.txt

@@ -158,3 +158,24 @@ a {
       (rule_set (selectors (class_selector (nesting_selector) (class_name))) (block))
       (rule_set (selectors (descendant_selector (nesting_selector) (tag_name))) (block))
       (rule_set (selectors (child_selector (nesting_selector) (tag_name))) (block)))))
+
+===========================
+Sibling selectors
+===========================
+
+a.b ~ c.d {}
+.e.f + .g.h {}
+
+---
+
+(stylesheet
+  (rule_set
+    (selectors (sibling_selector
+      (class_selector (tag_name) (class_name))
+      (class_selector (tag_name) (class_name))))
+    (block))
+  (rule_set
+    (selectors (adjacent_sibling_selector
+      (class_selector (class_selector (class_name)) (class_name))
+      (class_selector (class_selector (class_name)) (class_name))))
+    (block)))

grammar.js

@@ -118,18 +118,20 @@ module.exports = grammar({
       $.attribute_selector,
       $.string_value,
       $.child_selector,
-      $.descendant_selector
+      $.descendant_selector,
+      $.sibling_selector,
+      $.adjacent_sibling_selector
     ),
 
     nesting_selector: $ => '&',
 
     universal_selector: $ => '*',
 
-    class_selector: $ => seq(
+    class_selector: $ => prec(1, seq(
       optional($._selector),
       '.',
       alias($.identifier, $.class_name),
-    ),
+    )),
 
     pseudo_class_selector: $ => seq(
       optional($._selector),
@@ -165,6 +167,10 @@ module.exports = grammar({
 
     descendant_selector: $ => prec.left(seq($._selector, $._descendant_operator, $._selector)),
 
+    sibling_selector: $ => prec.left(seq($._selector, '~', $._selector)),
+
+    adjacent_sibling_selector: $ => prec.left(seq($._selector, '+', $._selector)),
+
     // Declarations
 
     declaration: $ => prec(1, seq(

src/grammar.json

@@ -463,6 +463,14 @@
         {
           "type": "SYMBOL",
           "name": "descendant_selector"
+        },
+        {
+          "type": "SYMBOL",
+          "name": "sibling_selector"
+        },
+        {
+          "type": "SYMBOL",
+          "name": "adjacent_sibling_selector"
         }
       ]
     },
@@ -475,6 +483,9 @@
       "value": "*"
     },
     "class_selector": {
+      "type": "PREC",
+      "value": 1,
+      "content": {
         "type": "SEQ",
         "members": [
           {
@@ -503,6 +514,7 @@
             "value": "class_name"
           }
         ]
+      }
     },
     "pseudo_class_selector": {
       "type": "SEQ",
@@ -728,6 +740,48 @@
         ]
       }
     },
+    "sibling_selector": {
+      "type": "PREC_LEFT",
+      "value": 0,
+      "content": {
+        "type": "SEQ",
+        "members": [
+          {
+            "type": "SYMBOL",
+            "name": "_selector"
+          },
+          {
+            "type": "STRING",
+            "value": "~"
+          },
+          {
+            "type": "SYMBOL",
+            "name": "_selector"
+          }
+        ]
+      }
+    },
+    "adjacent_sibling_selector": {
+      "type": "PREC_LEFT",
+      "value": 0,
+      "content": {
+        "type": "SEQ",
+        "members": [
+          {
+            "type": "SYMBOL",
+            "name": "_selector"
+          },
+          {
+            "type": "STRING",
+            "value": "+"
+          },
+          {
+            "type": "SYMBOL",
+            "name": "_selector"
+          }
+        ]
+      }
+    },
     "declaration": {
       "type": "PREC",
       "value": 1,