Add Indexable#rindex! method variant (#12759)

spec/std/indexable_spec.cr

@@ -133,6 +133,28 @@ describe Indexable do
     end
   end
 
+  describe "#rindex!" do
+    it "does rindex with big negative offset" do
+      indexable = SafeIndexable.new(3)
+      expect_raises Enumerable::NotFoundError do
+        indexable.rindex!(0, -100)
+      end
+    end
+
+    it "does rindex with big offset" do
+      indexable = SafeIndexable.new(3)
+      expect_raises Enumerable::NotFoundError do
+        indexable.rindex!(0, 100)
+      end
+    end
+
+    it "offset type" do
+      indexable = SafeIndexable.new(3)
+      indexable.rindex!(1, 2_i64).should eq 1
+      indexable.rindex!(1, 2_i64).should be_a(Int64)
+    end
+  end
+
   it "does each" do
     indexable = SafeIndexable.new(3)
     is = [] of Int32

src/indexable.cr

@@ -849,6 +849,13 @@ module Indexable(T)
     rindex(offset) { |elem| elem == value }
   end
 
+  # :ditto:
+  #
+  # Raises `Enumerable::NotFoundError` if *value* is not in `self`.
+  def rindex!(value, offset = size - 1)
+    rindex(value, offset) || raise Enumerable::NotFoundError.new
+  end
+
   # Returns the index of the first object in `self` for which the block
   # is truthy, starting from the last object, or `nil` if no match
   # is found.
@@ -872,6 +879,13 @@ module Indexable(T)
     nil
   end
 
+  # :ditto:
+  #
+  # Raises `Enumerable::NotFoundError` if no match is found.
+  def rindex!(offset = size - 1, & : T ->)
+    rindex(offset) { |e| yield e } || raise Enumerable::NotFoundError.new
+  end
+
   # Optimized version of `Enumerable#sample` that runs in O(1) time.
   #
   # ```