Independent SetLocation and WithType for errors.

- Added a standalone SetLocation func that will Set the location of any
  error. Doing this so we can start shedding our need to rely on the Err
  type in the package.

- Added WithType that allows for the annotation of errors to conform to
  Is of a ConstError.

errortypes.go

@@ -102,6 +102,24 @@ func makeWrappedConstError(err error, format string, args ...interface{}) error
 	return fmt.Errorf(strings.Join([]string{format, "%w"}, separator), append(args, err)...)
 }
 
+// WithType is responsible for annotating an already existing error so that it
+// also satisfies that of a ConstError. The resultant error returned should
+// satisfy Is(err, errType). If err is nil then a nil error will also be returned.
+//
+// Now with Go's Is, As and Unwrap support it no longer makes sense to Wrap()
+// 2 errors as both of those errors could be chains of errors in their own right.
+// WithType aims to solve some of the usefulness of Wrap with the ability to
+// make a pre-existing error also satisfy a ConstError type.
+func WithType(err error, errType ConstError) error {
+	if err == nil {
+		return nil
+	}
+	return &errWithType{
+		error:   err,
+		errType: errType,
+	}
+}
+
 // Timeoutf returns an error which satisfies Is(err, Timeout) and the Locationer
 // interface.
 func Timeoutf(format string, args ...interface{}) error {

errortypes_test.go

@@ -177,3 +177,19 @@ func (*errorTypeSuite) TestThatYouAlwaysGetError(c *gc.C) {
 		c.Assert(err.Error(), gc.Equals, "")
 	}
 }
+
+func (*errorTypeSuite) TestWithTypeNil(c *gc.C) {
+	myErr := errors.ConstError("do you feel lucky?")
+	c.Assert(errors.WithType(nil, myErr), gc.IsNil)
+}
+
+func (*errorTypeSuite) TestWithType(c *gc.C) {
+	myErr := errors.ConstError("do you feel lucky?")
+	myErr2 := errors.ConstError("i don't feel lucky")
+	err := errors.New("yes")
+
+	err = errors.WithType(err, myErr)
+	c.Assert(errors.Is(err, myErr), gc.Equals, true)
+	c.Assert(err.Error(), gc.Equals, "yes")
+	c.Assert(errors.Is(err, myErr2), gc.Equals, false)
+}

functions.go

@@ -355,3 +355,15 @@ func Is(err, target error) bool {
 func As(err error, target interface{}) bool {
 	return stderrors.As(err, target)
 }
+
+// SetLocation takes a given error and records where in the stack SetLocation
+// was called from and returns the wrapped error with the location information
+// set. The returned error implements the Locationer interface. If err is nil
+// then a nil error is returned.
+func SetLocation(err error, callDepth int) error {
+	if err == nil {
+		return nil
+	}
+
+	return newLocationError(err, callDepth)
+}

functions_test.go

@@ -373,3 +373,17 @@ func (*functionSuite) TestIs(c *gc.C) {
 		c.Check(val, gc.Equals, true)
 	}
 }
+
+func (*functionSuite) TestSetLocationWithNilError(c *gc.C) {
+	c.Assert(errors.SetLocation(nil, 1), gc.IsNil)
+}
+
+func (*functionSuite) TestSetLocation(c *gc.C) {
+	err := errors.New("test")
+	err = errors.SetLocation(err, 1)
+	stack := fmt.Sprintf("%s: test", errorLocationValue(c))
+	_, implements := err.(errors.Locationer)
+	c.Assert(implements, gc.Equals, true)
+
+	c.Check(errors.ErrorStack(err), gc.Equals, stack)
+}