fmt: allow for space and plus flags when computing widths

Fixes #10770.
Fixes #10771.

This time maybe for sure?

Change-Id: I43d6e5fd6846cf58427fec183832d500a932df59
Reviewed-on: https://go-review.googlesource.com/9896Reviewed-by: Russ Cox <rsc@golang.org>

src/fmt/fmt_test.go

@@ -557,10 +557,13 @@ var fmtTests = []struct {
 	{"%0.100f", 1.0, zeroFill("1.", 100, "")},
 	{"%0.100f", -1.0, zeroFill("-1.", 100, "")},
 
-	// Used to panic: integer function didn't look at f.prec or f.unicode.
+	// Used to panic: integer function didn't look at f.prec or f.unicode or f.width.
 	{"%#.80x", 42, "0x0000000000000000000000000000000000000000000000000000000000000000000000000000002a"},
 	{"%.80U", 42, "U+0000000000000000000000000000000000000000000000000000000000000000000000000000002A"},
 	{"%#.80U", '日', "U+000000000000000000000000000000000000000000000000000000000000000000000000000065E5 '日'"},
+	{"%+.65d", 44, "+00000000000000000000000000000000000000000000000000000000000000044"},
+	{"% .65d", 44, " 00000000000000000000000000000000000000000000000000000000000000044"},
+	{"%  +.65d", 44, "+00000000000000000000000000000000000000000000000000000000000000044"},
 
 	// Comparison of padding rules with C printf.
 	/*

src/fmt/format.go

@@ -163,7 +163,7 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	}
 
 	var buf []byte = f.intbuf[0:]
-	if f.widPresent || f.precPresent {
+	if f.widPresent || f.precPresent || f.plus || f.space {
 		width := f.wid + f.prec // Only one will be set, both are positive; this provides the maximum.
 		if base == 16 && f.sharp {
 			// Also adds "0x".
@@ -177,6 +177,11 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 				width += 1 + 1 + utf8.UTFMax + 1
 			}
 		}
+		if f.plus {
+			width++
+		} else if f.space {
+			width++
+		}
 		if width > nByte {
 			// We're going to need a bigger boat.
 			buf = make([]byte, width)