Implement Kernel#raise cause parameter

Author: bjfish

CHANGELOG.md

@@ -60,6 +60,7 @@ Compatibility:
 * Update random implementation layout to be more compatible (#2234).
 * Set `RbConfig::CONFIG['LIBPATHFLAG'/'RPATHFLAG']` like MRI to let `$LIBPATH` changes in `extconf.rb` work.
 * Access to path and mode via `rb_io_t` from C has been changed to improve compatibility for io-console.
+* Implemented `Kernel#raise` `cause` parameter.
 
 Performance:
 

spec/ruby/core/kernel/raise_spec.rb

@@ -27,6 +27,27 @@
 
     ScratchPad.recorded.should be_nil
   end
+
+  ruby_version_is "2.6" do
+    it "accepts a cause keyword argument that sets the cause" do
+      cause = StandardError.new
+      -> { raise("error", cause: cause) }.should raise_error(RuntimeError) { |e| e.cause.should == cause }
+    end
+
+    it "accepts a cause keyword argument that overrides the last exception" do
+      begin
+        raise "first raise"
+      rescue => e
+        cause = StandardError.new
+        -> { raise("error", cause: cause) }.should raise_error(RuntimeError) { |e| e.cause.should == cause }
+      end
+    end
+
+    it "raises an ArgumentError when only cause is given" do
+      cause = StandardError.new
+      -> { raise(cause: cause) }.should raise_error(ArgumentError)
+    end
+  end
 end
 
 describe "Kernel#raise" do

src/main/ruby/truffleruby/core/kernel.rb

@@ -683,16 +683,19 @@ def warn(*messages, uplevel: undefined)
   end
   module_function :warn
 
-  def raise(exc = undefined, msg = undefined, ctx = nil)
-    last = $!
-    if Primitive.undefined?(exc) and last
-      exc = last
+  def raise(exc = undefined, msg = undefined, ctx = nil, cause: undefined)
+    cause_given = !Primitive.undefined?(cause)
+    cause = cause_given ? cause : $!
+
+    if Primitive.undefined?(exc) and cause
+      raise ArgumentError, 'only cause is given with no arguments' if cause_given
+      exc = cause
     else
       exc = Truffle::ExceptionOperations.build_exception_for_raise(exc, msg)
 
       exc.set_backtrace(ctx) if ctx
       Primitive.exception_capture_backtrace(exc, 1) unless Truffle::ExceptionOperations.backtrace?(exc)
-      Primitive.exception_set_cause exc, last unless Primitive.object_equal(exc, last)
+      Primitive.exception_set_cause exc, cause unless Primitive.object_equal(exc, cause)
     end
 
     Truffle::ExceptionOperations.show_exception_for_debug(exc, 1) if $DEBUG