Improve documentation for Lint/UselessSetterCall

CHANGELOG.md

@@ -28,6 +28,7 @@
 * [#7641](https://github.com/rubocop-hq/rubocop/issues/7641): **(Breaking)** Remove `Style/BracesAroundHashParameters` cop. ([@pocke][])
 * Add the method name to highlight area of `Layout/EmptyLineBetweenDefs` to help provide more context. ([@rrosenblum][])
 * [#7652](https://github.com/rubocop-hq/rubocop/pull/7652): Allow `pp` to allowed names of `Naming/MethodParameterName` cop in default config. ([@masarakki][])
+* [#7309](https://github.com/rubocop-hq/rubocop/pull/7309): Mark `Lint/UselessSetterCall` an "not safe" and improve documentation. ([@jonas054][])
 
 ## 0.79.0 (2020-01-06)
 

config/default.yml

@@ -1783,6 +1783,8 @@ Lint/UselessSetterCall:
   Description: 'Checks for useless setter call to a local variable.'
   Enabled: true
   VersionAdded: '0.13'
+  VersionChanged: '0.80'
+  Safe: false
 
 Lint/Void:
   Description: 'Possible use of operator/literal/variable in void context.'

lib/rubocop/cop/lint/useless_setter_call.rb

@@ -6,6 +6,10 @@ module Lint
       # This cop checks for setter call to local variable as the final
       # expression of a function definition.
       #
+      # Note: There are edge cases in which the local variable references a
+      # value that is also accessible outside the local scope. This is not
+      # detected by the cop, and it can yield a false positive.
+      #
       # @example
       #
       #   # bad

manual/cops_lint.md

@@ -2946,11 +2946,15 @@ end
 
 Enabled by default | Safe | Supports autocorrection | VersionAdded | VersionChanged
 --- | --- | --- | --- | ---
-Enabled | Yes | No | 0.13 | -
+Enabled | No | No | 0.13 | 0.80
 
 This cop checks for setter call to local variable as the final
 expression of a function definition.
 
+Note: There are edge cases in which the local variable references a
+value that is also accessible outside the local scope. This is not
+detected by the cop, and it can yield a false positive.
+
 ### Examples
 
 ```ruby