errors: new package

The only function is errors.New, at least for now.

R=r, gustavo, adg, r
CC=golang-dev
https://golang.org/cl/5321061

src/pkg/Makefile

@@ -75,6 +75,7 @@ DIRS=\
 	encoding/git85\
 	encoding/hex\
 	encoding/pem\
+	errors\
 	exec\
 	exp/ebnf\
 	exp/ebnflint\

src/pkg/errors/Makefile

+# Copyright 2011 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.
+
+include ../../Make.inc
+
+TARG=errors
+GOFILES=\
+	errors.go\
+
+include ../../Make.pkg

src/pkg/errors/errors.go

+// Copyright 2011 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 errors implements functions to manipulate errors.
+package errors
+
+// New returns an error that formats as the given text.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}

src/pkg/errors/errors_test.go

+// Copyright 2011 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 errors_test
+
+import (
+	. "errors"
+	"testing"
+)
+
+func TestNewEqual(t *testing.T) {
+	// Different allocations should not be equal.
+	if New("abc") == New("abc") {
+		t.Errorf(`New("abc") == New("abc")`)
+	}
+	if New("abc") == New("xyz") {
+		t.Errorf(`New("abc") == New("xyz")`)
+	}
+
+	// Same allocation should be equal to itself (not crash).
+	err := New("jkl")
+	if err != err {
+		t.Errorf(`err != err`)
+	}
+}
+
+func TestErrorMethod(t *testing.T) {
+	err := New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}