[errors] de-specialize Transient in favor of Tags.

common/config/validation/validation_test.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package validation
 
 import (
         "testing"
 
-        "github.com/luci/luci-go/common/errors"
         "github.com/luci/luci-go/common/logging"
 
         . "github.com/smartystreets/goconvey/convey"
 )
 
 func TestValidation(t *testing.T) {
         t.Parallel()
 
         Convey("No errors", t, func() {
                 v := Context{}
@@ skipping 12 matching lines @@
 
                 v.SetFile("file.cfg")
                 v.Enter("ctx %d", 123)
                 v.Error("blah %s", "zzz")
                 err := v.Finalize()
                 So(err, ShouldHaveSameTypeAs, &Error{})
                 So(err.Error(), ShouldEqual, `in "file.cfg" (ctx 123): blah zzz`)
 
                 singleErr := err.(*Error).Errors[0]
                 So(singleErr.Error(), ShouldEqual, `in "file.cfg" (ctx 123): blah zzz`)
-                So(errors.ExtractData(singleErr, "file"), ShouldEqual, "file.cfg")
-                So(errors.ExtractData(singleErr, "element"), ShouldResemble, []string{"ctx 123"})
+                d, ok := fileTag.ValueIn(singleErr)
+                So(ok, ShouldBeTrue)
+                So(d.(string), ShouldEqual, "file.cfg")
+                d, ok = element0Tag.ValueIn(singleErr)
+                So(ok, ShouldBeTrue)
+                So(d.(string), ShouldResemble, "ctx 123")
         })
 
         Convey("Regular usage", t, func() {
                 v := Context{}
 
                 v.Error("top %d", 1)
                 v.Error("top %d", 2)
 
                 v.SetFile("file_1.cfg")
                 v.Error("f1")
@@ skipping 24 matching lines @@
                         `in <unspecified file>: top 2`,
                         `in "file_1.cfg": f1`,
                         `in "file_1.cfg": f2`,
                         `in "file_1.cfg" (p 1): zzz 1`,
                         `in "file_1.cfg" (p 1 / p 2): zzz 2`,
                         `in "file_1.cfg" (p 1): zzz 3`,
                         `in "file_2.cfg": zzz 4`,
                 })
         })
 }