More powerful SlicingWithRange

Supports any expression as start index (only int previously).
Still correctly ignores startless range.

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 * [#7953](https://github.com/rubocop-hq/rubocop/issues/7953): Fix an error for `Lint/AmbiguousOperator` when a method with no arguments is used in advance. ([@koic][])
 * [#7962](https://github.com/rubocop-hq/rubocop/issues/7962): Fix a false positive for `Lint/ParenthesesAsGroupedExpression` when heredoc has a space between the same string as the method name and `(`. ([@koic][])
+* [#7967](https://github.com/rubocop-hq/rubocop/pull/7967): `Style/SlicingWithRange` cop now supports any expression as its first index. ([@zverok][])
 
 ### Changes
 

lib/rubocop/cop/style/slicing_with_range.rb

@@ -19,7 +19,7 @@ class SlicingWithRange < Cop
 
         MSG = 'Prefer ary[n..] over ary[n..-1].'
 
-        def_node_matcher :range_till_minus_one?, '(irange (int _) (int -1))'
+        def_node_matcher :range_till_minus_one?, '(irange !nil? (int -1))'
 
         def on_send(node)
           return unless node.method?(:[]) && node.arguments.count == 1

spec/rubocop/cop/style/slicing_with_range_spec.rb

@@ -23,6 +23,17 @@
       RUBY
     end
 
+    it 'reports an offense for slicing from expression to ..-1' do
+      expect_offense(<<~RUBY)
+        ary[fetch_start(true).first..-1]
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer ary[n..] over ary[n..-1].
+      RUBY
+
+      expect_correction(<<~RUBY)
+        ary[fetch_start(true).first..]
+      RUBY
+    end
+
     it 'reports no offense for excluding end' do
       expect_no_offenses(<<~RUBY)
         ary[1...-1]