core: Add Enumerator.produce

Add the signature for Enumerator.produce
https://bugs.ruby-lang.org/issues/14781

core/enumerator.rbs

@@ -124,6 +124,42 @@
 class Enumerator[unchecked out Elem, out Return] < Object
   include Enumerable[Elem]
 
+  # <!--
+  #   rdoc-file=enumerator.c
+  #   - Enumerator.produce(initial = nil) { |prev| block } -> enumerator
+  # -->
+  # Creates an infinite enumerator from any block, just called over and over.  The
+  # result of the previous iteration is passed to the next one. If `initial` is
+  # provided, it is passed to the first iteration, and becomes the first element
+  # of the enumerator; if it is not provided, the first iteration receives `nil`,
+  # and its result becomes the first element of the iterator.
+  #
+  # Raising StopIteration from the block stops an iteration.
+  #
+  #     Enumerator.produce(1, &:succ)   # => enumerator of 1, 2, 3, 4, ....
+  #
+  #     Enumerator.produce { rand(10) } # => infinite random number sequence
+  #
+  #     ancestors = Enumerator.produce(node) { |prev| node = prev.parent or raise StopIteration }
+  #     enclosing_section = ancestors.find { |n| n.type == :section }
+  #
+  # Using ::produce together with Enumerable methods like Enumerable#detect,
+  # Enumerable#slice_after, Enumerable#take_while can provide Enumerator-based
+  # alternatives for `while` and `until` cycles:
+  #
+  #     # Find next Tuesday
+  #     require "date"
+  #     Enumerator.produce(Date.today, &:succ).detect(&:tuesday?)
+  #
+  #     # Simple lexer:
+  #     require "strscan"
+  #     scanner = StringScanner.new("7+38/6")
+  #     PATTERN = %r{\d+|[-/+*]}
+  #     Enumerator.produce { scanner.scan(PATTERN) }.slice_after { scanner.eos? }.first
+  #     # => ["7", "+", "38", "/", "6"]
+  #
+  def self.produce: (?untyped initial) { (untyped prev) -> untyped } -> Producer[untyped]
+
   # <!--
   #   rdoc-file=enumerator.c
   #   - Enumerator.product(*enums) -> enumerator

test/stdlib/Enumerator_test.rb

@@ -25,6 +25,13 @@ class EnumeratorSingletonTest < Test::Unit::TestCase
 
   testing "singleton(::Enumerator)"
 
+  def test_produce
+    assert_send_type(
+      "(Integer initial) { (Integer) -> Integer } -> Enumerator[Integer, untyped]",
+      Enumerator, :produce, 1, &:succ
+    )
+  end
+
   def test_product
     assert_send_type(
       "(Array[String], Array[Integer]) -> Enumerator::Product[String | Integer]",