Add empty environment variable values

corpus/commands.txt

@@ -79,6 +79,19 @@ VAR1=a VAR2="ok" git diff --word-diff=color
     (argument)
     (argument)))
 
+===================================
+Empty environment variables
+===================================
+
+VAR1=
+VAR2= echo
+
+---
+
+(program
+  (environment_variable_assignment (variable_name))
+  (command (environment_variable_assignment (variable_name)) (command_name)))
+
 ===================================
 Pipelines
 ===================================

grammar.js

@@ -8,7 +8,8 @@ module.exports = grammar({
     $._heredoc_beginning,
     $._heredoc_middle,
     $._heredoc_end,
-    $.file_descriptor
+    $.file_descriptor,
+    $._empty_value
   ],
 
   extras: $ => [
@@ -149,7 +150,10 @@ module.exports = grammar({
     environment_variable_assignment: $ => seq(
       rename($.leading_word, 'variable_name'),
       '=',
-      $.value
+      choice(
+        $.value,
+        $._empty_value
+      )
     ),
 
     value: $ => choice(

src/grammar.json

@@ -546,9 +546,18 @@
           "type": "STRING",
           "value": "="
         },
+        {
+          "type": "CHOICE",
+          "members": [
             {
               "type": "SYMBOL",
               "name": "value"
+            },
+            {
+              "type": "SYMBOL",
+              "name": "_empty_value"
+            }
+          ]
         }
       ]
     },
@@ -927,6 +936,10 @@
     {
       "type": "SYMBOL",
       "name": "file_descriptor"
+    },
+    {
+      "type": "SYMBOL",
+      "name": "_empty_value"
     }
   ],
   "inline": [

src/scanner.cc

@@ -12,6 +12,7 @@ enum TokenType {
   HEREDOC_MIDDLE,
   HEREDOC_END,
   FILE_DESCRIPTOR,
+  EMPTY_VALUE,
 };
 
 struct Scanner {
@@ -75,7 +76,9 @@ struct Scanner {
   bool scan(TSLexer *lexer, const bool *valid_symbols) {
     if (valid_symbols[HEREDOC_MIDDLE]) {
       return scan_heredoc_content(lexer, HEREDOC_MIDDLE, HEREDOC_END);
-    } else if (valid_symbols[HEREDOC_BEGINNING]) {
+    }
+
+    if (valid_symbols[HEREDOC_BEGINNING]) {
       heredoc_identifier.clear();
       while (iswalpha(lexer->lookahead)) {
         heredoc_identifier += lexer->lookahead;
@@ -91,7 +94,9 @@ struct Scanner {
       }
 
       return scan_heredoc_content(lexer, HEREDOC_BEGINNING, SIMPLE_HEREDOC);
-    } else if (valid_symbols[FILE_DESCRIPTOR]) {
+    }
+
+    if (valid_symbols[FILE_DESCRIPTOR]) {
       while (lexer->lookahead == ' ' || lexer->lookahead == '\t') skip(lexer);
       if (iswdigit(lexer->lookahead)) {
         advance(lexer);
@@ -103,6 +108,13 @@ struct Scanner {
       }
     }
 
+    if (valid_symbols[EMPTY_VALUE]) {
+      if (iswspace(lexer->lookahead)) {
+        lexer->result_symbol = EMPTY_VALUE;
+        return true;
+      }
+    }
+
     return false;
   }