Adds a plugin for empty hash formatting

README.md

@@ -711,6 +711,7 @@ To register plugins, define a file somewhere in your load path named `syntax_tre
 * `plugin/single_quotes` - This will change all of your string literals to use single quotes instead of the default double quotes.
 * `plugin/trailing_comma` - This will put trailing commas into multiline array literals, hash literals, and method calls that can support trailing commas.
 * `plugin/disable_auto_ternary` - This will prevent the automatic conversion of `if ... else` to ternary expressions.
+* `plugin/compact_empty_hash` - This will prevent the splitting of empty hashes `{}` over multiple lines.
 
 If you're using Syntax Tree as a library, you can require those files directly or manually pass those options to the formatter initializer through the `SyntaxTree::Formatter::Options` class.
 

lib/syntax_tree/formatter.rb

@@ -24,12 +24,14 @@ class Options
       attr_reader :quote,
                   :trailing_comma,
                   :disable_auto_ternary,
+                  :compact_empty_hash,
                   :target_ruby_version
 
       def initialize(
         quote: :default,
         trailing_comma: :default,
         disable_auto_ternary: :default,
+        compact_empty_hash: :default,
         target_ruby_version: :default
       )
         @quote =
@@ -65,6 +67,17 @@ def initialize(
             disable_auto_ternary
           end
 
+        @compact_empty_hash =
+          if compact_empty_hash == :default
+            # We ship with a compact empty hash plugin that will define this
+            # constant. That constant is responsible for determining the default
+            # compact empty hash value. If it's defined, then we default to true.
+            # Otherwise we default to false.
+            defined?(COMPACT_EMPTY_HASH)
+          else
+            compact_empty_hash
+          end
+
         @target_ruby_version =
           if target_ruby_version == :default
             # The default target Ruby version is the current version of Ruby.
@@ -87,10 +100,12 @@ def initialize(
     attr_reader :quote,
                 :trailing_comma,
                 :disable_auto_ternary,
+                :compact_empty_hash,
                 :target_ruby_version
 
     alias trailing_comma? trailing_comma
     alias disable_auto_ternary? disable_auto_ternary
+    alias compact_empty_hash? compact_empty_hash
 
     def initialize(source, *args, options: Options.new)
       super(*args)
@@ -102,6 +117,7 @@ def initialize(source, *args, options: Options.new)
       @quote = options.quote
       @trailing_comma = options.trailing_comma
       @disable_auto_ternary = options.disable_auto_ternary
+      @compact_empty_hash = options.compact_empty_hash
       @target_ruby_version = options.target_ruby_version
     end
 

lib/syntax_tree/node.rb

@@ -5766,7 +5766,7 @@ def format_contents(q)
       q.format(lbrace)
 
       if assocs.empty?
-        q.breakable_empty
+        q.breakable_empty unless q.compact_empty_hash?
       else
         q.indent do
           q.breakable_space

lib/syntax_tree/plugin/compact_empty_hash.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+module SyntaxTree
+  class Formatter
+    COMPACT_EMPTY_HASH = true
+  end
+end

test/plugin/compact_empty_hash_test.rb

@@ -0,0 +1,79 @@
+# frozen_string_literal: true
+
+require_relative "../test_helper"
+
+module SyntaxTree
+  class CompactEmptyHashTest < Minitest::Test
+    def test_empty_hash
+      assert_format("{}\n", "{}")
+    end
+
+    def test_empty_hash_with_spaces
+      assert_format("{}\n", "{ }")
+    end
+
+    def test_empty_hash_with_newlines
+      assert_format("{}\n", "{\n}")
+    end
+
+    def test_empty_hash_in_assignment
+      assert_format("x = {}\n", "x = {}")
+    end
+
+    def test_empty_hash_in_method_call
+      assert_format("method({})\n", "method({})")
+    end
+
+    def test_empty_hash_in_array
+      assert_format("[{}]\n", "[{}]")
+    end
+
+    def test_long_assignment_with_empty_hash
+      source = "this_is_a_very_long_variable_name_that_might_cause_line_breaks_when_assigned_an_empty_hash = {}"
+      expected = "this_is_a_very_long_variable_name_that_might_cause_line_breaks_when_assigned_an_empty_hash = {}\n"
+      assert_format(expected, source)
+    end
+
+    def test_empty_hash_values_in_multiline_hash
+      source = "{ very_long_key_name_that_might_cause_issues: {}, another_very_long_key_name: {}, yet_another_key: {} }"
+      expected = <<~RUBY
+        {
+          very_long_key_name_that_might_cause_issues: {},
+          another_very_long_key_name: {},
+          yet_another_key: {}
+        }
+      RUBY
+      assert_format(expected, source)
+    end
+
+    def test_non_empty_hash_still_works
+      source = "{ key: value }"
+      expected = "{ key: value }\n"
+      assert_format(expected, source)
+    end
+
+    def test_without_plugin_allows_multiline_empty_hash
+      source = "this_is_a_very_long_variable_name_that_might_cause_line_breaks_when_assigned_an_empty_hash = {}"
+
+      # Format without the compact_empty_hash option
+      options = Formatter::Options.new(compact_empty_hash: false)
+      formatter = Formatter.new(source, [], options: options)
+      SyntaxTree.parse(source).format(formatter)
+      formatter.flush
+      result = formatter.output.join
+
+      # Should allow the hash to break across lines
+      assert(result.include?("= {\n}"), "Expected empty hash to break across lines when plugin is disabled")
+    end
+
+    private
+
+    def assert_format(expected, source = expected.chomp)
+      options = Formatter::Options.new(compact_empty_hash: true)
+      formatter = Formatter.new(source, [], options: options)
+      SyntaxTree.parse(source).format(formatter)
+      formatter.flush
+      assert_equal(expected, formatter.output.join)
+    end
+  end
+end