message: add Errorf and %w support to formatter

internal/format/errors_test.go

@@ -0,0 +1,42 @@
+// Copyright 2024 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 format_test
+
+import (
+	"errors"
+	"slices"
+	"testing"
+
+	"golang.org/x/text/internal/format"
+)
+
+func TestErrorf(t *testing.T) {
+	wrapped := errors.New("inner error")
+	for _, test := range []struct {
+		fmtStr      string
+		args        []any
+		wantWrapped []int
+	}{
+		0: {
+			fmtStr:      "%w",
+			args:        []any{wrapped},
+			wantWrapped: []int{0},
+		},
+		1: {
+			fmtStr:      "%w %v%w",
+			args:        []any{wrapped, 1, wrapped},
+			wantWrapped: []int{0, 2},
+		},
+	} {
+		p := format.Parser{}
+		p.Reset(test.args)
+		p.SetFormat(test.fmtStr)
+		for p.Scan() {
+		}
+		if slices.Compare(test.wantWrapped, p.WrappedErrs) != 0 {
+			t.Errorf("wrong wrapped: got=%v, want=%v", p.WrappedErrs, test.wantWrapped)
+		}
+	}
+}

internal/format/parser.go

@@ -43,6 +43,9 @@ type Parser struct {
 	// goodArgNum records whether the most recent reordering directive was valid.
 	goodArgNum bool
 
+	// WrappedErrs records the targets of the %w verb.
+	WrappedErrs []int
+
 	// position info
 	format   string
 	startPos int
@@ -55,6 +58,7 @@ func (p *Parser) Reset(args []interface{}) {
 	p.Args = args
 	p.ArgNum = 0
 	p.startPos = 0
+	p.WrappedErrs = p.WrappedErrs[:0]
 	p.Reordered = false
 }
 
@@ -148,7 +152,11 @@ simpleFormat:
 			// Fast path for common case of ascii lower case simple verbs
 			// without precision or width or argument indices.
 			if 'a' <= c && c <= 'z' && p.ArgNum < len(p.Args) {
-				if c == 'v' {
+				switch c {
+				case 'w':
+					p.WrappedErrs = append(p.WrappedErrs, p.ArgNum)
+					fallthrough
+				case 'v':
 					// Go syntax
 					p.SharpV = p.Sharp
 					p.Sharp = false
@@ -245,6 +253,9 @@ simpleFormat:
 	case p.ArgNum >= len(p.Args): // No argument left over to print for the current verb.
 		p.Status = StatusMissingArg
 		p.ArgNum++
+	case verb == 'w':
+		p.WrappedErrs = append(p.WrappedErrs, p.ArgNum)
+		fallthrough
 	case verb == 'v':
 		// Go syntax
 		p.SharpV = p.Sharp

message/errrors_test.go

@@ -0,0 +1,106 @@
+// Copyright 2024 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 message_test
+
+import (
+	"errors"
+	"reflect"
+	"testing"
+
+	"golang.org/x/text/language"
+	"golang.org/x/text/message"
+)
+
+func TestErorrf(t *testing.T) {
+	wrapped := errors.New("inner error")
+	p := message.NewPrinter(language.Und)
+	for _, test := range []struct {
+		err        error
+		wantText   string
+		wantUnwrap error
+		wantSplit  []error
+	}{{
+		err:        p.Errorf("%w", wrapped),
+		wantText:   "inner error",
+		wantUnwrap: wrapped,
+	}, {
+		err:        p.Errorf("added context: %w", wrapped),
+		wantText:   "added context: inner error",
+		wantUnwrap: wrapped,
+	}, {
+		err:        p.Errorf("%w with added context", wrapped),
+		wantText:   "inner error with added context",
+		wantUnwrap: wrapped,
+	}, {
+		err:        p.Errorf("%s %w %v", "prefix", wrapped, "suffix"),
+		wantText:   "prefix inner error suffix",
+		wantUnwrap: wrapped,
+	}, {
+		err:        p.Errorf("%[2]s: %[1]w", wrapped, "positional verb"),
+		wantText:   "positional verb: inner error",
+		wantUnwrap: wrapped,
+	}, {
+		err:      p.Errorf("%v", wrapped),
+		wantText: "inner error",
+	}, {
+		err:      p.Errorf("added context: %v", wrapped),
+		wantText: "added context: inner error",
+	}, {
+		err:      p.Errorf("%v with added context", wrapped),
+		wantText: "inner error with added context",
+	}, {
+		err:      p.Errorf("%w is not an error", "not-an-error"),
+		wantText: "%!w(string=not-an-error) is not an error",
+	}, {
+		err:       p.Errorf("wrapped two errors: %w %w", errString("1"), errString("2")),
+		wantText:  "wrapped two errors: 1 2",
+		wantSplit: []error{errString("1"), errString("2")},
+	}, {
+		err:       p.Errorf("wrapped three errors: %w %w %w", errString("1"), errString("2"), errString("3")),
+		wantText:  "wrapped three errors: 1 2 3",
+		wantSplit: []error{errString("1"), errString("2"), errString("3")},
+	}, {
+		err:       p.Errorf("wrapped nil error: %w %w %w", errString("1"), nil, errString("2")),
+		wantText:  "wrapped nil error: 1 %!w(<nil>) 2",
+		wantSplit: []error{errString("1"), errString("2")},
+	}, {
+		err:       p.Errorf("wrapped one non-error: %w %w %w", errString("1"), "not-an-error", errString("3")),
+		wantText:  "wrapped one non-error: 1 %!w(string=not-an-error) 3",
+		wantSplit: []error{errString("1"), errString("3")},
+	}, {
+		err:       p.Errorf("wrapped errors out of order: %[3]w %[2]w %[1]w", errString("1"), errString("2"), errString("3")),
+		wantText:  "wrapped errors out of order: 3 2 1",
+		wantSplit: []error{errString("1"), errString("2"), errString("3")},
+	}, {
+		err:       p.Errorf("wrapped several times: %[1]w %[1]w %[2]w %[1]w", errString("1"), errString("2")),
+		wantText:  "wrapped several times: 1 1 2 1",
+		wantSplit: []error{errString("1"), errString("2")},
+	}, {
+		err:        p.Errorf("%w", nil),
+		wantText:   "%!w(<nil>)",
+		wantUnwrap: nil, // still nil
+	}} {
+		if got, want := errors.Unwrap(test.err), test.wantUnwrap; got != want {
+			t.Errorf("Formatted error: %v\nerrors.Unwrap() = %v, want %v", test.err, got, want)
+		}
+		if got, want := splitErr(test.err), test.wantSplit; !reflect.DeepEqual(got, want) {
+			t.Errorf("Formatted error: %v\nUnwrap() []error = %v, want %v", test.err, got, want)
+		}
+		if got, want := test.err.Error(), test.wantText; got != want {
+			t.Errorf("err.Error() = %q, want %q", got, want)
+		}
+	}
+}
+
+func splitErr(err error) []error {
+	if e, ok := err.(interface{ Unwrap() []error }); ok {
+		return e.Unwrap()
+	}
+	return nil
+}
+
+type errString string
+
+func (e errString) Error() string { return string(e) }

message/message.go

@@ -5,8 +5,10 @@
 package message // import "golang.org/x/text/message"
 
 import (
+	"errors"
 	"io"
 	"os"
+	"slices"
 
 	// Include features to facilitate generated catalogs.
 	_ "golang.org/x/text/feature/plural"
@@ -136,6 +138,65 @@ func (p *Printer) Printf(key Reference, a ...interface{}) (n int, err error) {
 	return n, err
 }
 
+// Errorf is like fmt.Errorf, but using language-specific formatting.
+func (p *Printer) Errorf(key Reference, a ...interface{}) error {
+	pp := newPrinter(p)
+	pp.wrapErrs = true
+	lookupAndFormat(pp, key, a)
+	s := pp.String()
+	var err error
+	switch len(pp.fmt.WrappedErrs) {
+	case 0:
+		err = errors.New(s)
+	case 1:
+		w := &wrapError{msg: s}
+		w.err, _ = a[pp.fmt.WrappedErrs[0]].(error)
+		err = w
+	default:
+		if pp.fmt.Reordered {
+			slices.Sort(pp.fmt.WrappedErrs)
+		}
+		var errs []error
+		for i, argNum := range pp.fmt.WrappedErrs {
+			if i > 0 && pp.fmt.WrappedErrs[i-1] == argNum {
+				continue
+			}
+			if e, ok := a[argNum].(error); ok {
+				errs = append(errs, e)
+			}
+		}
+		err = &wrapErrors{s, errs}
+	}
+	pp.free()
+	return err
+}
+
+type wrapError struct {
+	msg string
+	err error
+}
+
+func (e *wrapError) Error() string {
+	return e.msg
+}
+
+func (e *wrapError) Unwrap() error {
+	return e.err
+}
+
+type wrapErrors struct {
+	msg  string
+	errs []error
+}
+
+func (e *wrapErrors) Error() string {
+	return e.msg
+}
+
+func (e *wrapErrors) Unwrap() []error {
+	return e.errs
+}
+
 func lookupAndFormat(p *printer, r Reference, a []interface{}) {
 	p.fmt.Reset(a)
 	switch v := r.(type) {

message/print.go

@@ -60,6 +60,7 @@ func (p *printer) free() {
 	p.Buffer.Reset()
 	p.arg = nil
 	p.value = reflect.Value{}
+	p.fmt.WrappedErrs = p.fmt.WrappedErrs[:0]
 	printerPool.Put(p)
 }
 
@@ -82,6 +83,9 @@ type printer struct {
 	// fmt is used to format basic items such as integers or strings.
 	fmt formatInfo
 
+	// wrapErrs is set when the format string may contain a %w verb.
+	wrapErrs bool
+
 	// panicking is set by catchPanic to avoid infinite panic, recover, panic, ... recursion.
 	panicking bool
 	// erroring is set when printing an error string to guard against calling handleMethods.
@@ -594,6 +598,16 @@ func (p *printer) handleMethods(verb rune) (handled bool) {
 	if p.erroring {
 		return
 	}
+	if verb == 'w' {
+		// It is invalid to use %w other than with Errorf or with a non-error arg.
+		_, ok := p.arg.(error)
+		if !ok || !p.wrapErrs {
+			p.badVerb(verb)
+			return true
+		}
+		// If the arg is an error, pass 'v' as the verb to it.
+		verb = 'v'
+	}
 	// Is it a Formatter?
 	if formatter, ok := p.arg.(format.Formatter); ok {
 		handled = true