Make Proc#parameters support lambda keyword for returning parameters as if lambda

This makes it easier to use Proc#parameters to build wrappers.

Implements [Feature #15357]

NEWS.md

@@ -69,6 +69,7 @@ Note: We're only listing outstanding class updates.
 
 * Proc
     * Proc#dup returns an instance of subclass. [[Bug #17545]]
+    * Proc#parameters now accepts lambda keyword. [[Feature #15357]]
 
 * Refinement
     * Refinement#refined_class has been added. [[Feature #12737]]
@@ -170,6 +171,7 @@ The following deprecated APIs are removed.
 [Feature #13110]: https://bugs.ruby-lang.org/issues/13110
 [Feature #14332]: https://bugs.ruby-lang.org/issues/14332
 [Feature #15231]: https://bugs.ruby-lang.org/issues/15231
+[Feature #15357]: https://bugs.ruby-lang.org/issues/15357
 [Bug #15928]:     https://bugs.ruby-lang.org/issues/15928
 [Feature #16131]: https://bugs.ruby-lang.org/issues/16131
 [Feature #16806]: https://bugs.ruby-lang.org/issues/16806

proc.c

@@ -1415,19 +1415,46 @@ rb_unnamed_parameters(int arity)
 
 /*
  * call-seq:
- *    prc.parameters  -> array
+ *    prc.parameters(lambda: nil)  -> array
  *
- * Returns the parameter information of this proc.
+ * Returns the parameter information of this proc.  If the lambda
+ * keyword is provided and not nil, treats the proc as a lambda if
+ * true and as a non-lambda if false.
  *
+ *    prc = proc{|x, y=42, *other|}
+ *    prc.parameters  #=> [[:opt, :x], [:opt, :y], [:rest, :other]]
  *    prc = lambda{|x, y=42, *other|}
  *    prc.parameters  #=> [[:req, :x], [:opt, :y], [:rest, :other]]
+ *    prc = proc{|x, y=42, *other|}
+ *    prc.parameters(lambda: true)  #=> [[:req, :x], [:opt, :y], [:rest, :other]]
+ *    prc = lambda{|x, y=42, *other|}
+ *    prc.parameters(lamdba: false) #=> [[:opt, :x], [:opt, :y], [:rest, :other]]
  */
 
 static VALUE
-rb_proc_parameters(VALUE self)
+rb_proc_parameters(int argc, VALUE *argv, VALUE self)
 {
-    int is_proc;
-    const rb_iseq_t *iseq = rb_proc_get_iseq(self, &is_proc);
+    static ID keyword_ids[1];
+    VALUE opt, lambda;
+    VALUE kwargs[1];
+    int is_proc ;
+    const rb_iseq_t *iseq;
+
+    iseq = rb_proc_get_iseq(self, &is_proc);
+
+    if (!keyword_ids[0]) {
+        CONST_ID(keyword_ids[0], "lambda");
+    }
+
+    rb_scan_args(argc, argv, "0:", &opt);
+    if (!NIL_P(opt)) {
+        rb_get_kwargs(opt, keyword_ids, 0, 1, kwargs);
+        lambda = kwargs[0];
+        if (!NIL_P(lambda)) {
+            is_proc = !RTEST(lambda);
+        }
+    }
+
     if (!iseq) {
 	return rb_unnamed_parameters(rb_proc_arity(self));
     }
@@ -4248,7 +4275,7 @@ Init_Proc(void)
     rb_define_method(rb_cProc, "==", proc_eq, 1);
     rb_define_method(rb_cProc, "eql?", proc_eq, 1);
     rb_define_method(rb_cProc, "source_location", rb_proc_location, 0);
-    rb_define_method(rb_cProc, "parameters", rb_proc_parameters, 0);
+    rb_define_method(rb_cProc, "parameters", rb_proc_parameters, -1);
     rb_define_method(rb_cProc, "ruby2_keywords", proc_ruby2_keywords, 0);
     // rb_define_method(rb_cProc, "isolate", rb_proc_isolate, 0); is not accepted.
 

spec/ruby/core/proc/parameters_spec.rb

@@ -20,6 +20,21 @@ describe "Proc#parameters" do
     proc {|x| }.parameters.first.first.should == :opt
   end
 
+  ruby_version_is "3.2" do
+    it "sets the first element of each sub-Array to :req if argument would be required if a lambda if lambda keyword used" do
+      proc {|x| }.parameters(lambda: true).first.first.should == :req
+      proc {|y,*x| }.parameters(lambda: true).first.first.should == :req
+    end
+
+    it "regards named parameters in procs as required if lambda keyword used" do
+      proc {|x| }.parameters(lambda: true).first.first.should == :req
+    end
+
+    it "regards named parameters in lambda as optional if lambda: false keyword used" do
+      lambda {|x| }.parameters(lambda: false).first.first.should == :opt
+    end
+  end
+
   it "regards optional keyword parameters in procs as optional" do
     proc {|x: :y| }.parameters.first.first.should == :key
   end

test/ruby/test_proc.rb

@@ -1234,6 +1234,28 @@ class TestProc < Test::Unit::TestCase
     assert_empty(pr.parameters.map{|_,n|n}.compact)
   end
 
+  def test_parameters_lambda
+    assert_equal([], proc {}.parameters(lambda: true))
+    assert_equal([], proc {||}.parameters(lambda: true))
+    assert_equal([[:req, :a]], proc {|a|}.parameters(lambda: true))
+    assert_equal([[:req, :a], [:req, :b]], proc {|a, b|}.parameters(lambda: true))
+    assert_equal([[:opt, :a], [:block, :b]], proc {|a=:a, &b|}.parameters(lambda: true))
+    assert_equal([[:req, :a], [:opt, :b]], proc {|a, b=:b|}.parameters(lambda: true))
+    assert_equal([[:rest, :a]], proc {|*a|}.parameters(lambda: true))
+    assert_equal([[:req, :a], [:rest, :b], [:block, :c]], proc {|a, *b, &c|}.parameters(lambda: true))
+    assert_equal([[:req, :a], [:rest, :b], [:req, :c]], proc {|a, *b, c|}.parameters(lambda: true))
+    assert_equal([[:req, :a], [:rest, :b], [:req, :c], [:block, :d]], proc {|a, *b, c, &d|}.parameters(lambda: true))
+    assert_equal([[:req, :a], [:opt, :b], [:rest, :c], [:req, :d], [:block, :e]], proc {|a, b=:b, *c, d, &e|}.parameters(lambda: true))
+    assert_equal([[:req], [:block, :b]], proc {|(a), &b|a}.parameters(lambda: true))
+    assert_equal([[:req, :a], [:req, :b], [:opt, :c], [:opt, :d], [:rest, :e], [:req, :f], [:req, :g], [:block, :h]], proc {|a,b,c=:c,d=:d,*e,f,g,&h|}.parameters(lambda: true))
+
+    pr = eval("proc{|"+"(_),"*30+"|}")
+    assert_empty(pr.parameters(lambda: true).map{|_,n|n}.compact)
+
+    assert_equal([[:opt, :a]], lambda {|a|}.parameters(lambda: false))
+    assert_equal([[:opt, :a], [:opt, :b], [:opt, :c], [:opt, :d], [:rest, :e], [:opt, :f], [:opt, :g], [:block, :h]], lambda {|a,b,c=:c,d=:d,*e,f,g,&h|}.parameters(lambda: false))
+  end
+
   def pm0() end
   def pm1(a) end
   def pm2(a, b) end