fmt: add frame info to Errorf and support %w

Partly implements proposal Issue #29934.

Change-Id: Ibcf12f383158dcfbc313ab29c417a710571d1acb
Reviewed-on: https://go-review.googlesource.com/c/163559
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, 46 insertions, 0 deletions

diff --git a/src/fmt/format_example_test.go b/src/fmt/format_example_test.go
new file mode 100644
index 0000000000..386f10ef23
--- /dev/null
+++ b/src/fmt/format_example_test.go
@@ -0,0 +1,46 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fmt_test
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+	"regexp"
+)
+
+func baz() error { return errors.New("baz flopped") }
+func bar() error { return fmt.Errorf("bar(nameserver 139): %v", baz()) }
+func foo() error { return fmt.Errorf("foo: %s", bar()) }
+
+func Example_formatting() {
+	err := foo()
+	fmt.Println("Error:")
+	fmt.Printf("%v\n", err)
+	fmt.Println()
+	fmt.Println("Detailed error:")
+	fmt.Println(stripPath(fmt.Sprintf("%+v\n", err)))
+	// Output:
+	// Error:
+	// foo: bar(nameserver 139): baz flopped
+	//
+	// Detailed error:
+	// foo:
+	//     fmt_test.foo
+	//         fmt/format_example_test.go:16
+	//   - bar(nameserver 139):
+	//     fmt_test.bar
+	//         fmt/format_example_test.go:15
+	//   - baz flopped:
+	//     fmt_test.baz
+	//         fmt/format_example_test.go:14
+}
+
+func stripPath(s string) string {
+	rePath := regexp.MustCompile(`( [^ ]*)fmt`)
+	s = rePath.ReplaceAllString(s, " fmt")
+	s = filepath.ToSlash(s)
+	return s
+}