From 97b7698c618d6779e98e1b1f81c30c9b5330276f Mon Sep 17 00:00:00 2001
From: Koichi ITO <koic.ito@gmail.com>
Date: Thu, 18 Nov 2021 11:22:19 +0900
Subject: [PATCH] Fix an incorrect autocorrect for
 `Performance/RedundantStringChars`

Follow up to https://github.com/rubocop/rubocop-performance/issues/249#issuecomment-972262774.

This PR fixes an incorrect autocorrect for `Performance/RedundantStringChars`
when using `str.chars[0]`.
---
 ..._for_performance_redundant_string_chars.md |  1 +
 .../cop/performance/redundant_string_chars.rb | 22 +++++++++++++------
 .../redundant_string_chars_spec.rb            | 11 ++++++++++
 3 files changed, 27 insertions(+), 7 deletions(-)
 create mode 100644 changelog/fix_incorrect_autocorrect_for_performance_redundant_string_chars.md

diff --git a/changelog/fix_incorrect_autocorrect_for_performance_redundant_string_chars.md b/changelog/fix_incorrect_autocorrect_for_performance_redundant_string_chars.md
new file mode 100644
index 0000000000..f29d3ab6eb
--- /dev/null
+++ b/changelog/fix_incorrect_autocorrect_for_performance_redundant_string_chars.md
@@ -0,0 +1 @@
+* [#273](https://github.com/rubocop/rubocop-performance/pull/273): Fix an incorrect autocorrect for `Performance/RedundantStringChars` when using `str.chars[0]`. ([@koic][])
diff --git a/lib/rubocop/cop/performance/redundant_string_chars.rb b/lib/rubocop/cop/performance/redundant_string_chars.rb
index 20d9bc0c0c..594b84a7e5 100644
--- a/lib/rubocop/cop/performance/redundant_string_chars.rb
+++ b/lib/rubocop/cop/performance/redundant_string_chars.rb
@@ -82,14 +82,10 @@ def build_message(method, args)
 
         def build_good_method(method, args)
           case method
-          when :[], :slice
+          when :slice
             "[#{build_call_args(args)}].chars"
-          when :first
-            if args.any?
-              "[0...#{args.first.source}].chars"
-            else
-              '[0]'
-            end
+          when :[], :first
+            build_good_method_for_brackets_or_first_method(args)
           when :take
             "[0...#{args.first.source}].chars"
           else
@@ -97,6 +93,18 @@ def build_good_method(method, args)
           end
         end
 
+        def build_good_method_for_brackets_or_first_method(args)
+          first_arg = args.first
+
+          if first_arg&.range_type?
+            "[#{build_call_args(args)}].chars"
+          elsif args.any? && first_arg.source != '0'
+            "[0...#{args.first.source}].chars"
+          else
+            '[0]'
+          end
+        end
+
         def build_bad_method(method, args)
           case method
           when :[]
diff --git a/spec/rubocop/cop/performance/redundant_string_chars_spec.rb b/spec/rubocop/cop/performance/redundant_string_chars_spec.rb
index df256631fe..93f7f17022 100644
--- a/spec/rubocop/cop/performance/redundant_string_chars_spec.rb
+++ b/spec/rubocop/cop/performance/redundant_string_chars_spec.rb
@@ -34,6 +34,17 @@
     RUBY
   end
 
+  it 'registers an offense and corrects when using `str.chars[0]`' do
+    expect_offense(<<~RUBY)
+      str.chars[0]
+          ^^^^^^^^ Use `[0]` instead of `chars[0]`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      str[0]
+    RUBY
+  end
+
   it 'registers an offense and corrects when using `str.chars.first(2)`' do
     expect_offense(<<~RUBY)
       str.chars.first(2)