// Find the field start and end indices.
	// Doing this in a separate pass (rather than slicing the string s
	// and collecting the result substrings right away) is significantly
	// more efficient, possibly due to cache effects.
	start := -1 // valid span start if >= 0
	for i := 0; i < len(s); {
		size := 1
		r := rune(s[i])
		if r >= utf8.RuneSelf {
			r, size = utf8.DecodeRune(s[i:])
		}
		if f(r) {

			// but for the golang.org/x repos it seems to be significantly (~2x)
			// faster than copying one file at a time (via filepath.WalkDir),
			// apparently due to high latency in 'adb' commands.
			if err := adb("push", modDir, deviceModDir); err != nil {
				return 0, err
			}
		} else {
			if err := adb("exec-out", "mkdir", "-p", deviceCwd); err != nil {
				return 0, err