[Fix rubocop#313] Fix a false negative for `Performance/RedundantStri…

…ngChars`

Fixes rubocop#313.

This PR fixes a false negative for `Performance/RedundantStringChars`
when using `str.chars.last` without argument.

Report rubocop#249 has `str.chars.last(2)` argument. rubocop#250 mistakenly changed
even without arguments `str.chars.last`.

changelog/fix_a_false_negative_for_performance_redundant_string_chars.md

@@ -0,0 +1 @@
+* [#313](https://github.com/rubocop/rubocop-performance/issues/313): Fix a false negative for `Performance/RedundantStringChars` when using `str.chars.last` without argument. ([@koic][])

lib/rubocop/cop/performance/redundant_string_chars.rb

@@ -9,6 +9,7 @@ module Performance
       #   # bad
       #   str.chars[0..2]
       #   str.chars.slice(0..2)
+      #   str.chars.last
       #
       #   # good
       #   str[0..2].chars
@@ -20,6 +21,7 @@ module Performance
       #   # good
       #   str[0]
       #   str[0...2].chars
+      #   str[-1]
       #
       #   # bad
       #   str.chars.take(2)
@@ -33,9 +35,8 @@ module Performance
       #   str.size
       #   str.empty?
       #
-      #   # For example, if the receiver is a blank string, it will be incompatible.
+      #   # For example, if the receiver is an empty string, it will be incompatible.
       #   # If a negative value is specified for the receiver, `nil` is returned.
-      #   str.chars.last    # Incompatible with `str[-1]`.
       #   str.chars.last(2) # Incompatible with `str[-2..-1].chars`.
       #   str.chars.drop(2) # Incompatible with `str[2..-1].chars`.
       #
@@ -44,14 +45,15 @@ class RedundantStringChars < Base
         extend AutoCorrector
 
         MSG = 'Use `%<good_method>s` instead of `%<bad_method>s`.'
-        RESTRICT_ON_SEND = %i[[] slice first take length size empty?].freeze
+        RESTRICT_ON_SEND = %i[[] slice first last take length size empty?].freeze
 
         def_node_matcher :redundant_chars_call?, <<~PATTERN
           (send $(send _ :chars) $_ $...)
         PATTERN
 
         def on_send(node)
           return unless (receiver, method, args = redundant_chars_call?(node))
+          return if method == :last && !args.empty?
 
           range = offense_range(receiver, node)
           message = build_message(method, args)
@@ -86,6 +88,8 @@ def build_good_method(method, args)
             "[#{build_call_args(args)}].chars"
           when :[], :first
             build_good_method_for_brackets_or_first_method(method, args)
+          when :last
+            '[-1]'
           when :take
             "[0...#{args.first.source}].chars"
           else

spec/rubocop/cop/performance/redundant_string_chars_spec.rb

@@ -67,9 +67,14 @@
     RUBY
   end
 
-  it 'does not register an offense when using `str.chars.last`' do
-    expect_no_offenses(<<~RUBY)
+  it 'registers an offense and corrects when using `str.chars.last`' do
+    expect_offense(<<~RUBY)
       str.chars.last
+          ^^^^^^^^^^ Use `[-1]` instead of `chars.last`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      str[-1]
     RUBY
   end