`Proc` made by `Symbol#to_proc` should be a lambda [Bug #16260]

NEWS.md

@@ -34,6 +34,13 @@ sufficient information, see the ChangeLog file or Redmine
         * Hash#transform_keys now accepts a hash that maps keys to new
           keys.  [[Feature #16274]]
 
+* Symbol
+
+    * Modified method
+
+        * Symbol#to_proc now returns a lambda Proc.
+          [[Feature #16260]]
+
 ## Stdlib updates (outstanding ones only)
 
 * Net::HTTP
@@ -71,6 +78,7 @@ sufficient information, see the ChangeLog file or Redmine
 [Feature #8948]:  https://bugs.ruby-lang.org/issues/8948
 [Feature #15575]: https://bugs.ruby-lang.org/issues/15575
 [Feature #16131]: https://bugs.ruby-lang.org/issues/16131
+[Feature #16260]: https://bugs.ruby-lang.org/issues/16260
 [Feature #16274]: https://bugs.ruby-lang.org/issues/16274
 [Feature #16377]: https://bugs.ruby-lang.org/issues/16377
 [Feature #16555]: https://bugs.ruby-lang.org/issues/16555

proc.c

@@ -697,6 +697,7 @@ sym_proc_new(VALUE klass, VALUE sym)
     GetProcPtr(procval, proc);
 
     vm_block_type_set(&proc->block, block_type_symbol);
+    proc->is_lambda = TRUE;
     RB_OBJ_WRITE(procval, &proc->block.as.symbol, sym);
     return procval;
 }

spec/ruby/core/proc/shared/to_s.rb

@@ -46,7 +46,7 @@ def hello; end
   describe "for a proc created with Symbol#to_proc" do
     it "returns a description including '(&:symbol)'" do
       proc = :foobar.to_proc
-      proc.send(@method).should =~ /^#<Proc:0x\h+\(&:foobar\)>$/
+      proc.send(@method).should.include?('(&:foobar)')
     end
 
     it "has a binary encoding" do