fmt: do not remove trailing zeros for %g and %G with #(sharp) flag

Fixes #36562

Change-Id: Id98ae9f7362cfb825b306c36649d505692d6d60e
GitHub-Last-Rev: 405d51b
GitHub-Pull-Request: #36588
Reviewed-on: https://go-review.googlesource.com/c/go/+/215001
Reviewed-by: Rob Pike <[email protected]>

Author: yah01

src/fmt/fmt_test.go

@@ -463,6 +463,15 @@ var fmtTests = []struct {
 	{"%#.4x", 1.0, "0x1.0000p+00"},
 	{"%#.4g", 1.0, "1.000"},
 	{"%#.4g", 100000.0, "1.000e+05"},
+	{"%#.4g", 1.234, "1.234"},
+	{"%#.4g", 0.1234, "0.1234"},
+	{"%#.4g", 1.23, "1.230"},
+	{"%#.4g", 0.123, "0.1230"},
+	{"%#.4g", 1.2, "1.200"},
+	{"%#.4g", 0.12, "0.1200"},
+	{"%#.4g", 10.2, "10.20"},
+	{"%#.4g", 0.0, "0.000"},
+	{"%#.4g", 0.012, "0.01200"},
 	{"%#.0f", 123.0, "123."},
 	{"%#.0e", 123.0, "1.e+02"},
 	{"%#.0x", 123.0, "0x1.p+07"},

src/fmt/format.go

@@ -536,6 +536,7 @@ func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int) {
 		tail := tailBuf[:0]
 
 		hasDecimalPoint := false
+		sawNonzeroDigit := false
 		// Starting from i = 1 to skip sign at num[0].
 		for i := 1; i < len(num); i++ {
 			switch num[i] {
@@ -552,10 +553,20 @@ func (f *fmt) fmtFloat(v float64, size int, verb rune, prec int) {
 				}
 				fallthrough
 			default:
-				digits--
+				if num[i] != '0' {
+					sawNonzeroDigit = true
+				}
+				// Count significant digits after the first non-zero digit.
+				if sawNonzeroDigit {
+					digits--
+				}
 			}
 		}
 		if !hasDecimalPoint {
+			// Leading digit 0 should contribute once to digits.
+			if len(num) == 2 && num[1] == '0' {
+				digits--
+			}
 			num = append(num, '.')
 		}
 		for digits > 0 {