errors: add Frame and Formatter/Printer interfaces

errors.New now implements Formatter and includes Frame
information that is reported when detail is requested.

Partly implements proposal Issue #29934.

Change-Id: Id76888d246d7d862595b5e92d517b9c03f23a7a6
Reviewed-on: https://go-review.googlesource.com/c/163557
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>

1 files changed, 12 insertions, 2 deletions

diff --git a/src/errors/errors.go b/src/errors/errors.go
index b8a46921be..f23a96c43e 100644
--- a/src/errors/errors.go
+++ b/src/errors/errors.go
@@ -6,15 +6,25 @@
 package errors
 
 // New returns an error that formats as the given text.
+//
+// The returned error contains a Frame set to the caller's location and
+// implements Formatter to show this information when printed with details.
 func New(text string) error {
-	return &errorString{text}
+	return &errorString{text, Caller(1)}
 }
 
 // errorString is a trivial implementation of error.
 type errorString struct {
-	s string
+	s     string
+	frame Frame
 }
 
 func (e *errorString) Error() string {
 	return e.s
 }
+
+func (e *errorString) FormatError(p Printer) (next error) {
+	p.Print(e.s)
+	e.frame.Format(p)
+	return nil
+}