fmt: float formatting should not permanently change width

formatFloat should restore the original f.wid value before
returning. Callers should not have to save and restore f.wid.

Fixes: #14642

Change-Id: I531dae15c7997fe8909e2ad1ef7c376654afb030
Reviewed-on: https://go-review.googlesource.com/20179
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -735,6 +735,13 @@ var fmtTests = []struct {
 	{"%0-5s", "abc", "abc  "},
 	{"%-05.1f", 1.0, "1.0  "},
 
+	// float and complex formatting should not change the padding width
+	// for other elements. See issue 14642.
+	{"%06v", []interface{}{+10.0, 10}, "[000010 000010]"},
+	{"%06v", []interface{}{-10.0, 10}, "[-00010 000010]"},
+	{"%06v", []interface{}{+10.0 + 10i, 10}, "[(000010+00010i) 000010]"},
+	{"%06v", []interface{}{-10.0 + 10i, 10}, "[(-00010+00010i) 000010]"},
+
 	// Complex fmt used to leave the plus flag set for future entries in the array
 	// causing +2+0i and +3+0i instead of 2+0i and 3+0i.
 	{"%v", []complex64{1, 2, 3}, "[(1+0i) (2+0i) (3+0i)]"},
@@ -1008,6 +1015,7 @@ func BenchmarkSprintfFloat(b *testing.B) {
 		}
 	})
 }
+
 func BenchmarkSprintfBoolean(b *testing.B) {
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {

src/fmt/format.go

@@ -448,7 +448,9 @@ func (f *fmt) formatFloat(v float64, verb byte, prec, n int) {
 		if f.zero && f.widPresent && f.wid > len(num) {
 			f.buf.WriteByte(num[0])
 			f.wid--
-			num = num[1:]
+			f.pad(num[1:])
+			f.wid++
+			return
 		}
 		f.pad(num)
 		return
@@ -512,7 +514,6 @@ func (f *fmt) fmt_complex(r, j float64, size int, verb rune) {
 	f.buf.WriteByte('(')
 	oldPlus := f.plus
 	oldSpace := f.space
-	oldWid := f.wid
 	for i := 0; ; i++ {
 		switch verb {
 		case 'b':
@@ -534,11 +535,9 @@ func (f *fmt) fmt_complex(r, j float64, size int, verb rune) {
 		// Imaginary part always has a sign.
 		f.plus = true
 		f.space = false
-		f.wid = oldWid
 		r = j
 	}
 	f.space = oldSpace
 	f.plus = oldPlus
-	f.wid = oldWid
 	f.buf.WriteString("i)")
 }