fmt: Printf("%02x") applied to []byte{} is equivalent to []byte{0}

[adonovan]

https://go.dev/play/p/yAGvguDS5Uc

	slice := []byte("hello")
	for i := range slice {
		fmt.Printf("%[1]q\t%02[1]x\n", slice[:len(slice)-1-i])
	}

"hell"	68656c6c
"hel"	68656c
"he"	6865
"h"		68
""		00

The fmt docs don't go into detail on exactly what Printf should do here, but the behavior in the zero case is surprisingly non-monotonic, and seems to me to be a bug.

[adonovan]

cc: @robpike

[thepudds]

This might be expected behavior?

Maybe one way to see the behavior is if I change the format to use %04 instead of %02 (playground):

		fmt.Printf("%[1]q\t%04[1]x\n", slice[:len(slice)-1-i])

That now outputs:

"hell"	68656c6c
"hel"	68656c
"he"	6865
"h"	0068
""	0000

The last two output lines are now padded with leading zeros to reach the requested width.

From the doc:

Width is specified by an optional decimal number immediately preceding the verb.

and

For most values, width is the minimum number of runes to output, padding the formatted form with spaces if necessary.

and

'0' pad with leading zeros rather than spaces;
for numbers, this moves the padding after the sign

[ianlancetaylor]

It does seem wrong to me. There is a special []byte rule for %x, and it says that it prints two characters per slice element. When there are no slice elements, it seems to me that it shouldn't print anything.

That said the current code handles explicitly in fmtSbx:

	} else { // The byte slice or string that should be encoded is empty.
		if f.widPresent {
			f.writePadding(f.wid)
		}
		return
	}

That is, for an empty slice, it pads to the width of a single element.

There are explicit tests for this behavior. I doubt we can change it.

[thepudds]

One other quick data point -- if we get rid of the original %02 and use a different string like this (playground):

	slice := []byte("\n1234")
	for i := range slice {
		fmt.Printf("%[1]q\t%[1]x\n", slice[:len(slice)-1-i])
	}

That outputs:

"\n123"	0a313233
"\n12"	0a3132
"\n1"	0a31
"\n"	0a
""	

[adonovan]

Another data point: the field padding usually applies to each element of a slice, not to the formatted slice as a whole:

https://go.dev/play/p/7srSUKCi34v

	fmt.Printf("[%10d]\n", 123)
	fmt.Printf("[%-10d]\n", 123)
	fmt.Printf("[%10d]\n", []int{123, 456})
	fmt.Printf("[%-10d]\n", []int{123, 456})

[       123]
[123       ]
[[       123        456]]
[[123        456       ]]