Add support for passing a block to IO#readlines and IO.readlines.

io.c

@@ -4472,16 +4472,25 @@ rb_io_readlines(int argc, VALUE *argv, VALUE io)
 static VALUE
 io_readlines(const struct getline_arg *arg, VALUE io)
 {
-    VALUE line, ary;
+    VALUE line;
 
     if (arg->limit == 0)
         rb_raise(rb_eArgError, "invalid limit: 0 for readlines");
-    ary = rb_ary_new();
+
+    // If a block is provided, yield lines.
+    if (rb_block_given_p()) {
+        while (!NIL_P(line = rb_io_getline_1(arg->rs, arg->limit, arg->chomp, io))) {
+            rb_yield(line);
+        }
+        return Qnil;
+    } else {
+        VALUE ary = rb_ary_new();
         while (!NIL_P(line = rb_io_getline_1(arg->rs, arg->limit, arg->chomp, io))) {
             rb_ary_push(ary, line);
         }
         return ary;
     }
+}
 
 /*
  *  call-seq:

spec/ruby/core/io/readlines_spec.rb

@@ -20,6 +20,16 @@ describe "IO#readlines" do
     -> { @io.readlines }.should raise_error(IOError)
   end
 
+  ruby_version_is "3.3" do
+    describe "when invoked with a block" do
+      it "yields each line to the block" do
+        lines = []
+        @io.readlines { |line| lines << line }
+        lines.should == IOSpecs.lines
+      end
+    end
+  end
+
   describe "when passed no arguments" do
     before :each do
       suppress_warning {@sep, $/ = $/, " "}
@@ -214,6 +224,14 @@ describe "IO.readlines" do
         IO.readlines(cmd)
       }.should complain(/IO process creation with a leading '\|'/)
     end
+
+    describe "when invoked with a block" do
+      it "yields each line to the block" do
+        lines = []
+        IO.readlines(@name) { |line| lines << line }
+        lines.should == IOSpecs.lines
+      end
+    end
   end
 
   it_behaves_like :io_readlines, :readlines