cmd/vet: make printf checking more precise when arguments are changed

[ianlancetaylor]

For #26486 https://golang.org/cl/125039 changed the printf checking to only warn about functions that do not modify the arguments before passing them to fmt.Printf (or whatever). The example in that issue, drawn from real code, is:

func dbg(s string, va ...interface{}) {
	if s == "" {
		s = strings.Repeat("%v ", len(va))
	}
	_, fn, fl, _ := runtime.Caller(1)
	fmt.Printf("dbg %s:%d: ", path.Base(fn), fl)
	fmt.Printf(s, va...)
	fmt.Println()
}

This can be called as dbg("", err).

The fix in CL 125039 means that we no longer issue printf warnings for functions like this:

func Prefix(s string, args ...interface{}) {
    s = "error: " + s
    fmt.Printf(s, args)
}

We should figure out a way to continue issuing warnings for Prefix without issuing them for dbg.

[gopherbot]

Change https://golang.org/cl/125039 mentions this issue: cmd/vet: if a function modifies its args, it's not a print wrapper

[mvdan]

It seems to me like this would require the use of more advanced tooling in vet, such as the use of x/tools/go/ssa.

[blgm]

I've seen an issue with the following code which now fails vet in v1.11:

func Say(expected string, args ...interface{}) *sayMatcher {
	formattedRegexp := expected
	if len(args) > 0 {
		formattedRegexp = fmt.Sprintf(expected, args...)
	}
	return &sayMatcher{
		re: regexp.MustCompile(formattedRegexp),
	}
}

Re-writing like this allows it to pass vet, but this seems like a very fragile "fix".

func Say(expected string, args ...interface{}) *sayMatcher {
	if len(args) > 0 {
		expected = fmt.Sprintf(expected, args...)
	}
	return &sayMatcher{
		re: regexp.MustCompile(expected),
	}
}

Ideally go vet would be able to tell that not all branches are a print wrapper, and ignore functions like this.