[Fix #13556] Fix false positives for Style/FileNull

Fixes #13556.

This PR fixes false positives for `Style/FileNull` when using `'nul'` string.

This PR ensures only files that use the string `'/dev/null'` are targeted for detection.
This is because the string `'nul'` is not limited to the null device.
This behavior results in false negatives when the `'/dev/null'` string is not used,
but it is a trade-off to avoid false positives.

changelog/fix_false_positives_for_style_file_null.md

@@ -0,0 +1 @@
+* [#13556](https://github.com/rubocop/rubocop/issues/13556): Fix false positives for `Style/FileNull` when using `'nul'` string. ([@koic][])

lib/rubocop/cop/style/file_null.rb

@@ -8,6 +8,12 @@ module Style
       # Only looks for full string matches, substrings within a longer string are not
       # considered.
       #
+      # However, only files that use the string `'/dev/null'` are targeted for detection.
+      # This is because the string `'NUL'` is not limited to the null device.
+      # This behavior results in false negatives when the `'/dev/null'` string is not used,
+      # but it is a trade-off to avoid false positives. `NULL:`
+      # Unlike `'NUL'`, `'NUL:'` is regarded as something like `C:` and is always detected.
+      #
       # NOTE: Uses inside arrays and hashes are ignored.
       #
       # @safety
@@ -42,11 +48,21 @@ class FileNull < Base
         REGEXP = %r{\A(/dev/null|NUL:?)\z}i.freeze
         MSG = 'Use `File::NULL` instead of `%<source>s`.'
 
+        def on_new_investigation
+          return unless (ast = processed_source.ast)
+
+          @contain_dev_null_string_in_file = ast.each_descendant(:str).any? do |str|
+            content = str.str_content
+
+            valid_string?(content) && content.downcase == '/dev/null' # rubocop:disable Style/FileNull
+          end
+        end
+
         def on_str(node)
           value = node.value
-
-          return if invalid_string?(value)
+          return unless valid_string?(value)
           return if acceptable?(node)
+          return if value.downcase == 'nul' && !@contain_dev_null_string_in_file # rubocop:disable Style/FileNull
           return unless REGEXP.match?(value)
 
           add_offense(node, message: format(MSG, source: value)) do |corrector|
@@ -56,8 +72,8 @@ def on_str(node)
 
         private
 
-        def invalid_string?(value)
-          value.empty? || !value.valid_encoding?
+        def valid_string?(value)
+          !value.empty? && value.valid_encoding?
         end
 
         def acceptable?(node)

spec/rubocop/cop/style/file_null_spec.rb

@@ -24,18 +24,40 @@
     RUBY
   end
 
-  it 'registers an offense and corrects when the entire string is `NUL`' do
+  it "registers an offense and corrects when the entire string is `NUL` or '/dev/null'" do
     expect_offense(<<~RUBY)
+      CONST = '/dev/null'
+              ^^^^^^^^^^^ Use `File::NULL` instead of `/dev/null`.
       path = 'NUL'
              ^^^^^ Use `File::NULL` instead of `NUL`.
     RUBY
 
     expect_correction(<<~RUBY)
+      CONST = File::NULL
       path = File::NULL
     RUBY
   end
 
-  it 'registers an offense and corrects when the entire string is `NUL:`' do
+  it "does not register an offense when the entire string is `NUL` without '/dev/null'" do
+    expect_no_offenses(<<~RUBY)
+      path = 'NUL'
+    RUBY
+  end
+
+  it "registers an offense and corrects when the entire string is `NUL:` or '/dev/null'" do
+    expect_offense(<<~RUBY)
+      path = cond ? '/dev/null' : 'NUL:'
+                    ^^^^^^^^^^^ Use `File::NULL` instead of `/dev/null`.
+                                  ^^^^^^ Use `File::NULL` instead of `NUL:`.
+    RUBY
+
+    # Different cops will detect duplication of the branch bodies.
+    expect_correction(<<~RUBY)
+      path = cond ? File::NULL : File::NULL
+    RUBY
+  end
+
+  it "registers an offense when the entire string is `NUL:` without '/dev/null'" do
     expect_offense(<<~RUBY)
       path = 'NUL:'
              ^^^^^^ Use `File::NULL` instead of `NUL:`.