Support local variable declarations

corpus/expressions.txt

@@ -143,6 +143,24 @@ find "`dirname $file`" -name "$base"'*'
       (string (simple_expansion (variable_name)))
       (raw_string))))
 
+=========================================
+Local Variable Declaration
+=========================================
+
+function a {
+  local a=42
+  local b
+}
+
+---
+
+(program
+  (function_definition
+    (word)
+    (compound_statement
+      (local_variable_declaration (simple_variable_name) (word))
+      (local_variable_declaration (simple_variable_name)))))
+
 =========================================
 Arrays and array expansions
 =========================================

grammar.js

@@ -48,6 +48,9 @@ module.exports = grammar({
 
     _statement: $ => choice(
       $.environment_variable_assignment,
+      // Local variable are only allowed inside the body of a function, but to
+      // keep the grammar simple we'll ignore that requirements.
+      $.local_variable_declaration,
       $.command,
       $.bracket_command,
       $.for_statement,
@@ -182,6 +185,16 @@ module.exports = grammar({
         $.variable_name,
         $.subscript
       ),
+      $._assignment
+    ),
+
+    local_variable_declaration: $ => seq(
+      'local',
+      $.simple_variable_name,
+      optional($._assignment)
+    ),
+
+    _assignment: $ => seq(
       choice(
         '=',
         '+='

src/grammar.json

@@ -28,6 +28,10 @@
           "type": "SYMBOL",
           "name": "environment_variable_assignment"
         },
+        {
+          "type": "SYMBOL",
+          "name": "local_variable_declaration"
+        },
         {
           "type": "SYMBOL",
           "name": "command"
@@ -601,6 +605,40 @@
             }
           ]
         },
+        {
+          "type": "SYMBOL",
+          "name": "_assignment"
+        }
+      ]
+    },
+    "local_variable_declaration": {
+      "type": "SEQ",
+      "members": [
+        {
+          "type": "STRING",
+          "value": "local"
+        },
+        {
+          "type": "SYMBOL",
+          "name": "simple_variable_name"
+        },
+        {
+          "type": "CHOICE",
+          "members": [
+            {
+              "type": "SYMBOL",
+              "name": "_assignment"
+            },
+            {
+              "type": "BLANK"
+            }
+          ]
+        }
+      ]
+    },
+    "_assignment": {
+      "type": "SEQ",
+      "members": [
         {
           "type": "CHOICE",
           "members": [