fmt: document and fix the handling of precision for strings and byte slices

Previous behavior was undocumented and inconsistent. Now it is documented
and consistent and measures the input size, since that makes more sense
when talking about %q and %x. For %s the change has no effect.

Fixes #8151.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/144540044

src/fmt/doc.go

@@ -63,16 +63,20 @@
 		%9.2f  width 9, precision 2
 		%9.f   width 9, precision 0
 
-	Width and precision are measured in units of Unicode code points.
-	(This differs from C's printf where the units are numbers
-	of bytes.) Either or both of the flags may be replaced with the
-	character '*', causing their values to be obtained from the next
-	operand, which must be of type int.
+	Width and precision are measured in units of Unicode code points,
+	that is, runes. (This differs from C's printf where the
+	units are always measured in bytes.) Either or both of the flags
+	may be replaced with the character '*', causing their values to be
+	obtained from the next operand, which must be of type int.
 
-	For most values, width is the minimum number of characters to output,
+	For most values, width is the minimum number of runes to output,
 	padding the formatted form with spaces if necessary.
-	For strings, precision is the maximum number of characters to output,
-	truncating if necessary.
+
+	For strings, byte slices and byte arrays, however, precision
+	limits the length of the input to be formatted (not the size of
+	the output), truncating if necessary. Normally it is measured in
+	runes, but for these types when formatted with the %x or %X format
+	it is measured in bytes.
 
 	For floating-point values, width sets the minimum width of the field and
 	precision sets the number of places after the decimal, if appropriate,

src/fmt/fmt_test.go

@@ -194,8 +194,15 @@ var fmtTests = []struct {
 	{"%.5s", "日本語日本語", "日本語日本"},
 	{"%.5s", []byte("日本語日本語"), "日本語日本"},
 	{"%.5q", "abcdefghijklmnopqrstuvwxyz", `"abcde"`},
+	{"%.5x", "abcdefghijklmnopqrstuvwxyz", `6162636465`},
+	{"%.5q", []byte("abcdefghijklmnopqrstuvwxyz"), `"abcde"`},
+	{"%.5x", []byte("abcdefghijklmnopqrstuvwxyz"), `6162636465`},
 	{"%.3q", "日本語日本語", `"日本語"`},
 	{"%.3q", []byte("日本語日本語"), `"日本語"`},
+	{"%.1q", "日本語", `"日"`},
+	{"%.1q", []byte("日本語"), `"日"`},
+	{"%.1x", "日本語", `e6`},
+	{"%.1X", []byte("日本語"), `E6`},
 	{"%10.1q", "日本語日本語", `       "日"`},
 	{"%3c", '⌘', "  ⌘"},
 	{"%5q", '\u2026', `  '…'`},

src/fmt/format.go

@@ -340,11 +340,17 @@ func (f *fmt) fmt_sbx(s string, b []byte, digits string) {
 
 // fmt_sx formats a string as a hexadecimal encoding of its bytes.
 func (f *fmt) fmt_sx(s, digits string) {
+	if f.precPresent && f.prec < len(s) {
+		s = s[:f.prec]
+	}
 	f.fmt_sbx(s, nil, digits)
 }
 
 // fmt_bx formats a byte slice as a hexadecimal encoding of its bytes.
 func (f *fmt) fmt_bx(b []byte, digits string) {
+	if f.precPresent && f.prec < len(b) {
+		b = b[:f.prec]
+	}
 	f.fmt_sbx("", b, digits)
 }