// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package printf defines an Analyzer that checks consistency
// of Printf format strings and arguments.
package printf

import (
	"bytes"
	"fmt"
	"go/ast"
	"go/constant"
	"go/token"
	"go/types"
	"reflect"
	"regexp"
	"sort"
	"strconv"
	"strings"
	"unicode/utf8"

	"golang.org/x/tools/go/analysis"
	"golang.org/x/tools/go/analysis/passes/inspect"
	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
	"golang.org/x/tools/go/ast/inspector"
	"golang.org/x/tools/go/types/typeutil"
	"golang.org/x/tools/internal/typeparams"
)

func init() {
	Analyzer.Flags.Var(isPrint, "funcs", "comma-separated list of print function names to check")
}

var Analyzer = &analysis.Analyzer{
	Name:       "printf",
	Doc:        Doc,
	URL:        "https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/printf",
	Requires:   []*analysis.Analyzer{inspect.Analyzer},
	Run:        run,
	ResultType: reflect.TypeOf((*Result)(nil)),
	FactTypes:  []analysis.Fact{new(isWrapper)},
}

const Doc = `check consistency of Printf format strings and arguments

The check applies to known functions (for example, those in package fmt)
as well as any detected wrappers of known functions.

To enable printf checking on a function that is not found by this
analyzer's heuristics (for example, because control is obscured by
dynamic method calls), insert a bogus call:

	func MyPrintf(format string, args ...any) {
		if false {
			_ = fmt.Sprintf(format, args...) // enable printf checker
		}
		...
	}

The -funcs flag specifies a comma-separated list of names of additional
known formatting functions or methods. If the name contains a period,
it must denote a specific function using one of the following forms:

	dir/pkg.Function
	dir/pkg.Type.Method
	(*dir/pkg.Type).Method

Otherwise the name is interpreted as a case-insensitive unqualified
identifier such as "errorf". Either way, if a listed name ends in f, the
function is assumed to be Printf-like, taking a format string before the
argument list. Otherwise it is assumed to be Print-like, taking a list
of arguments with no format string.
`

// Kind is a kind of fmt function behavior.
type Kind int

const (
	KindNone   Kind = iota // not a fmt wrapper function
	KindPrint              // function behaves like fmt.Print
	KindPrintf             // function behaves like fmt.Printf
	KindErrorf             // function behaves like fmt.Errorf
)

func (kind Kind) String() string {
	switch kind {
	case KindPrint:
		return "print"
	case KindPrintf:
		return "printf"
	case KindErrorf:
		return "errorf"
	}
	return ""
}

// Result is the printf analyzer's result type. Clients may query the result
// to learn whether a function behaves like fmt.Print or fmt.Printf.
type Result struct {
	funcs map[*types.Func]Kind
}

// Kind reports whether fn behaves like fmt.Print or fmt.Printf.
func (r *Result) Kind(fn *types.Func) Kind {
	_, ok := isPrint[fn.FullName()]
	if !ok {
		// Next look up just "printf", for use with -printf.funcs.
		_, ok = isPrint[strings.ToLower(fn.Name())]
	}
	if ok {
		if strings.HasSuffix(fn.Name(), "f") {
			return KindPrintf
		} else {
			return KindPrint
		}
	}

	return r.funcs[fn]
}

// isWrapper is a fact indicating that a function is a print or printf wrapper.
type isWrapper struct{ Kind Kind }

func (f *isWrapper) AFact() {}

func (f *isWrapper) String() string {
	switch f.Kind {
	case KindPrintf:
		return "printfWrapper"
	case KindPrint:
		return "printWrapper"
	case KindErrorf:
		return "errorfWrapper"
	default:
		return "unknownWrapper"
	}
}

func run(pass *analysis.Pass) (interface{}, error) {
	res := &Result{
		funcs: make(map[*types.Func]Kind),
	}
	findPrintfLike(pass, res)
	checkCall(pass)
	return res, nil
}

type printfWrapper struct {
	obj     *types.Func
	fdecl   *ast.FuncDecl
	format  *types.Var
	args    *types.Var
	callers []printfCaller
	failed  bool // if true, not a printf wrapper
}

type printfCaller struct {
	w    *printfWrapper
	call *ast.CallExpr
}

// maybePrintfWrapper decides whether decl (a declared function) may be a wrapper
// around a fmt.Printf or fmt.Print function. If so it returns a printfWrapper
// function describing the declaration. Later processing will analyze the
// graph of potential printf wrappers to pick out the ones that are true wrappers.
// A function may be a Printf or Print wrapper if its last argument is ...interface{}.
// If the next-to-last argument is a string, then this may be a Printf wrapper.
// Otherwise it may be a Print wrapper.
func maybePrintfWrapper(info *types.Info, decl ast.Decl) *printfWrapper {
	// Look for functions with final argument type ...interface{}.
	fdecl, ok := decl.(*ast.FuncDecl)
	if !ok || fdecl.Body == nil {
		return nil
	}
	fn, ok := info.Defs[fdecl.Name].(*types.Func)
	// Type information may be incomplete.
	if !ok {
		return nil
	}

	sig := fn.Type().(*types.Signature)
	if !sig.Variadic() {
		return nil // not variadic
	}

	params := sig.Params()
	nparams := params.Len() // variadic => nonzero

	args := params.At(nparams - 1)
	iface, ok := args.Type().(*types.Slice).Elem().(*types.Interface)
	if !ok || !iface.Empty() {
		return nil // final (args) param is not ...interface{}
	}

	// Is second last param 'format string'?
	var format *types.Var
	if nparams >= 2 {
		if p := params.At(nparams - 2); p.Type() == types.Typ[types.String] {
			format = p
		}
	}

	return &printfWrapper{
		obj:    fn,
		fdecl:  fdecl,
		format: format,
		args:   args,
	}
}

// findPrintfLike scans the entire package to find printf-like functions.
func findPrintfLike(pass *analysis.Pass, res *Result) (interface{}, error) {
	// Gather potential wrappers and call graph between them.
	byObj := make(map[*types.Func]*printfWrapper)
	var wrappers []*printfWrapper
	for _, file := range pass.Files {
		for _, decl := range file.Decls {
			w := maybePrintfWrapper(pass.TypesInfo, decl)
			if w == nil {
				continue
			}
			byObj[w.obj] = w
			wrappers = append(wrappers, w)
		}
	}

	// Walk the graph to figure out which are really printf wrappers.
	for _, w := range wrappers {
		// Scan function for calls that could be to other printf-like functions.
		ast.Inspect(w.fdecl.Body, func(n ast.Node) bool {
			if w.failed {
				return false
			}

			// TODO: Relax these checks; issue 26555.
			if assign, ok := n.(*ast.AssignStmt); ok {
				for _, lhs := range assign.Lhs {
					if match(pass.TypesInfo, lhs, w.format) ||
						match(pass.TypesInfo, lhs, w.args) {
						// Modifies the format
						// string or args in
						// some way, so not a
						// simple wrapper.
						w.failed = true
						return false
					}
				}
			}
			if un, ok := n.(*ast.UnaryExpr); ok && un.Op == token.AND {
				if match(pass.TypesInfo, un.X, w.format) ||
					match(pass.TypesInfo, un.X, w.args) {
					// Taking the address of the
					// format string or args,
					// so not a simple wrapper.
					w.failed = true
					return false
				}
			}

			call, ok := n.(*ast.CallExpr)
			if !ok || len(call.Args) == 0 || !match(pass.TypesInfo, call.Args[len(call.Args)-1], w.args) {
				return true
			}

			fn, kind := printfNameAndKind(pass, call)
			if kind != 0 {
				checkPrintfFwd(pass, w, call, kind, res)
				return true
			}

			// If the call is to another function in this package,
			// maybe we will find out it is printf-like later.
			// Remember this call for later checking.
			if fn != nil && fn.Pkg() == pass.Pkg && byObj[fn] != nil {
				callee := byObj[fn]
				callee.callers = append(callee.callers, printfCaller{w, call})
			}

			return true
		})
	}
	return nil, nil
}

func match(info *types.Info, arg ast.Expr, param *types.Var) bool {
	id, ok := arg.(*ast.Ident)
	return ok && info.ObjectOf(id) == param
}

// checkPrintfFwd checks that a printf-forwarding wrapper is forwarding correctly.
// It diagnoses writing fmt.Printf(format, args) instead of fmt.Printf(format, args...).
func checkPrintfFwd(pass *analysis.Pass, w *printfWrapper, call *ast.CallExpr, kind Kind, res *Result) {
	matched := kind == KindPrint ||
		kind != KindNone && len(call.Args) >= 2 && match(pass.TypesInfo, call.Args[len(call.Args)-2], w.format)
	if !matched {
		return
	}

	if !call.Ellipsis.IsValid() {
		typ, ok := pass.TypesInfo.Types[call.Fun].Type.(*types.Signature)
		if !ok {
			return
		}
		if len(call.Args) > typ.Params().Len() {
			// If we're passing more arguments than what the
			// print/printf function can take, adding an ellipsis
			// would break the program. For example:
			//
			//   func foo(arg1 string, arg2 ...interface{} {
			//       fmt.Printf("%s %v", arg1, arg2)
			//   }
			return
		}
		desc := "printf"
		if kind == KindPrint {
			desc = "print"
		}
		pass.ReportRangef(call, "missing ... in args forwarded to %s-like function", desc)
		return
	}
	fn := w.obj
	var fact isWrapper
	if !pass.ImportObjectFact(fn, &fact) {
		fact.Kind = kind
		pass.ExportObjectFact(fn, &fact)
		res.funcs[fn] = kind
		for _, caller := range w.callers {
			checkPrintfFwd(pass, caller.w, caller.call, kind, res)
		}
	}
}

// isPrint records the print functions.
// If a key ends in 'f' then it is assumed to be a formatted print.
//
// Keys are either values returned by (*types.Func).FullName,
// or case-insensitive identifiers such as "errorf".
//
// The -funcs flag adds to this set.
//
// The set below includes facts for many important standard library
// functions, even though the analysis is capable of deducing that, for
// example, fmt.Printf forwards to fmt.Fprintf. We avoid relying on the
// driver applying analyzers to standard packages because "go vet" does
// not do so with gccgo, and nor do some other build systems.
// TODO(adonovan): eliminate the redundant facts once this restriction
// is lifted.
var isPrint = stringSet{
	"fmt.Errorf":   true,
	"fmt.Fprint":   true,
	"fmt.Fprintf":  true,
	"fmt.Fprintln": true,
	"fmt.Print":    true,
	"fmt.Printf":   true,
	"fmt.Println":  true,
	"fmt.Sprint":   true,
	"fmt.Sprintf":  true,
	"fmt.Sprintln": true,

	"runtime/trace.Logf": true,

	"log.Print":             true,
	"log.Printf":            true,
	"log.Println":           true,
	"log.Fatal":             true,
	"log.Fatalf":            true,
	"log.Fatalln":           true,
	"log.Panic":             true,
	"log.Panicf":            true,
	"log.Panicln":           true,
	"(*log.Logger).Fatal":   true,
	"(*log.Logger).Fatalf":  true,
	"(*log.Logger).Fatalln": true,
	"(*log.Logger).Panic":   true,
	"(*log.Logger).Panicf":  true,
	"(*log.Logger).Panicln": true,
	"(*log.Logger).Print":   true,
	"(*log.Logger).Printf":  true,
	"(*log.Logger).Println": true,

	"(*testing.common).Error":  true,
	"(*testing.common).Errorf": true,
	"(*testing.common).Fatal":  true,
	"(*testing.common).Fatalf": true,
	"(*testing.common).Log":    true,
	"(*testing.common).Logf":   true,
	"(*testing.common).Skip":   true,
	"(*testing.common).Skipf":  true,
	// *testing.T and B are detected by induction, but testing.TB is
	// an interface and the inference can't follow dynamic calls.
	"(testing.TB).Error":  true,
	"(testing.TB).Errorf": true,
	"(testing.TB).Fatal":  true,
	"(testing.TB).Fatalf": true,
	"(testing.TB).Log":    true,
	"(testing.TB).Logf":   true,
	"(testing.TB).Skip":   true,
	"(testing.TB).Skipf":  true,
}

// formatString returns the format string argument and its index within
// the given printf-like call expression.
//
// The last parameter before variadic arguments is assumed to be
// a format string.
//
// The first string literal or string constant is assumed to be a format string
// if the call's signature cannot be determined.
//
// If it cannot find any format string parameter, it returns ("", -1).
func formatString(pass *analysis.Pass, call *ast.CallExpr) (format string, idx int) {
	typ := pass.TypesInfo.Types[call.Fun].Type
	if typ != nil {
		if sig, ok := typ.(*types.Signature); ok {
			if !sig.Variadic() {
				// Skip checking non-variadic functions.
				return "", -1
			}
			idx := sig.Params().Len() - 2
			if idx < 0 {
				// Skip checking variadic functions without
				// fixed arguments.
				return "", -1
			}
			s, ok := stringConstantArg(pass, call, idx)
			if !ok {
				// The last argument before variadic args isn't a string.
				return "", -1
			}
			return s, idx
		}
	}

	// Cannot determine call's signature. Fall back to scanning for the first
	// string constant in the call.
	for idx := range call.Args {
		if s, ok := stringConstantArg(pass, call, idx); ok {
			return s, idx
		}
		if pass.TypesInfo.Types[call.Args[idx]].Type == types.Typ[types.String] {
			// Skip checking a call with a non-constant format
			// string argument, since its contents are unavailable
			// for validation.
			return "", -1
		}
	}
	return "", -1
}

// stringConstantArg returns call's string constant argument at the index idx.
//
// ("", false) is returned if call's argument at the index idx isn't a string
// constant.
func stringConstantArg(pass *analysis.Pass, call *ast.CallExpr, idx int) (string, bool) {
	if idx >= len(call.Args) {
		return "", false
	}
	return stringConstantExpr(pass, call.Args[idx])
}

// stringConstantExpr returns expression's string constant value.
//
// ("", false) is returned if expression isn't a string
// constant.
func stringConstantExpr(pass *analysis.Pass, expr ast.Expr) (string, bool) {
	lit := pass.TypesInfo.Types[expr].Value
	if lit != nil && lit.Kind() == constant.String {
		return constant.StringVal(lit), true
	}
	return "", false
}

// checkCall triggers the print-specific checks if the call invokes a print function.
func checkCall(pass *analysis.Pass) {
	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
	nodeFilter := []ast.Node{
		(*ast.CallExpr)(nil),
	}
	inspect.Preorder(nodeFilter, func(n ast.Node) {
		call := n.(*ast.CallExpr)
		fn, kind := printfNameAndKind(pass, call)
		switch kind {
		case KindPrintf, KindErrorf:
			checkPrintf(pass, kind, call, fn)
		case KindPrint:
			checkPrint(pass, call, fn)
		}
	})
}

func printfNameAndKind(pass *analysis.Pass, call *ast.CallExpr) (fn *types.Func, kind Kind) {
	fn, _ = typeutil.Callee(pass.TypesInfo, call).(*types.Func)
	if fn == nil {
		return nil, 0
	}

	_, ok := isPrint[fn.FullName()]
	if !ok {
		// Next look up just "printf", for use with -printf.funcs.
		_, ok = isPrint[strings.ToLower(fn.Name())]
	}
	if ok {
		if fn.FullName() == "fmt.Errorf" {
			kind = KindErrorf
		} else if strings.HasSuffix(fn.Name(), "f") {
			kind = KindPrintf
		} else {
			kind = KindPrint
		}
		return fn, kind
	}

	var fact isWrapper
	if pass.ImportObjectFact(fn, &fact) {
		return fn, fact.Kind
	}

	return fn, KindNone
}

// isFormatter reports whether t could satisfy fmt.Formatter.
// The only interface method to look for is "Format(State, rune)".
func isFormatter(typ types.Type) bool {
	// If the type is an interface, the value it holds might satisfy fmt.Formatter.
	if _, ok := typ.Underlying().(*types.Interface); ok {
		// Don't assume type parameters could be formatters. With the greater
		// expressiveness of constraint interface syntax we expect more type safety
		// when using type parameters.
		if !typeparams.IsTypeParam(typ) {
			return true
		}
	}
	obj, _, _ := types.LookupFieldOrMethod(typ, false, nil, "Format")
	fn, ok := obj.(*types.Func)
	if !ok {
		return false
	}
	sig := fn.Type().(*types.Signature)
	return sig.Params().Len() == 2 &&
		sig.Results().Len() == 0 &&
		isNamed(sig.Params().At(0).Type(), "fmt", "State") &&
		types.Identical(sig.Params().At(1).Type(), types.Typ[types.Rune])
}

func isNamed(T types.Type, pkgpath, name string) bool {
	named, ok := T.(*types.Named)
	return ok && named.Obj().Pkg().Path() == pkgpath && named.Obj().Name() == name
}

// formatState holds the parsed representation of a printf directive such as "%3.*[4]d".
// It is constructed by parsePrintfVerb.
type formatState struct {
	verb     rune   // the format verb: 'd' for "%d"
	format   string // the full format directive from % through verb, "%.3d".
	name     string // Printf, Sprintf etc.
	flags    []byte // the list of # + etc.
	argNums  []int  // the successive argument numbers that are consumed, adjusted to refer to actual arg in call
	firstArg int    // Index of first argument after the format in the Printf call.
	// Used only during parse.
	pass         *analysis.Pass
	call         *ast.CallExpr
	argNum       int  // Which argument we're expecting to format now.
	hasIndex     bool // Whether the argument is indexed.
	indexPending bool // Whether we have an indexed argument that has not resolved.
	nbytes       int  // number of bytes of the format string consumed.
}

// checkPrintf checks a call to a formatted print routine such as Printf.
func checkPrintf(pass *analysis.Pass, kind Kind, call *ast.CallExpr, fn *types.Func) {
	format, idx := formatString(pass, call)
	if idx < 0 {
		if false {
			pass.Reportf(call.Lparen, "can't check non-constant format in call to %s", fn.FullName())
		}
		return
	}

	firstArg := idx + 1 // Arguments are immediately after format string.
	if !strings.Contains(format, "%") {
		if len(call.Args) > firstArg {
			pass.Reportf(call.Lparen, "%s call has arguments but no formatting directives", fn.FullName())
		}
		return
	}
	// Hard part: check formats against args.
	argNum := firstArg
	maxArgNum := firstArg
	anyIndex := false
	for i, w := 0, 0; i < len(format); i += w {
		w = 1
		if format[i] != '%' {
			continue
		}
		state := parsePrintfVerb(pass, call, fn.FullName(), format[i:], firstArg, argNum)
		if state == nil {
			return
		}
		w = len(state.format)
		if !okPrintfArg(pass, call, state) { // One error per format is enough.
			return
		}
		if state.hasIndex {
			anyIndex = true
		}
		if state.verb == 'w' {
			switch kind {
			case KindNone, KindPrint, KindPrintf:
				pass.Reportf(call.Pos(), "%s does not support error-wrapping directive %%w", state.name)
				return
			}
		}
		if len(state.argNums) > 0 {
			// Continue with the next sequential argument.
			argNum = state.argNums[len(state.argNums)-1] + 1
		}
		for _, n := range state.argNums {
			if n >= maxArgNum {
				maxArgNum = n + 1
			}
		}
	}
	// Dotdotdot is hard.
	if call.Ellipsis.IsValid() && maxArgNum >= len(call.Args)-1 {
		return
	}
	// If any formats are indexed, extra arguments are ignored.
	if anyIndex {
		return
	}
	// There should be no leftover arguments.
	if maxArgNum != len(call.Args) {
		expect := maxArgNum - firstArg
		numArgs := len(call.Args) - firstArg
		pass.ReportRangef(call, "%s call needs %v but has %v", fn.FullName(), count(expect, "arg"), count(numArgs, "arg"))
	}
}

// parseFlags accepts any printf flags.
func (s *formatState) parseFlags() {
	for s.nbytes < len(s.format) {
		switch c := s.format[s.nbytes]; c {
		case '#', '0', '+', '-', ' ':
			s.flags = append(s.flags, c)
			s.nbytes++
		default:
			return
		}
	}
}

// scanNum advances through a decimal number if present.
func (s *formatState) scanNum() {
	for ; s.nbytes < len(s.format); s.nbytes++ {
		c := s.format[s.nbytes]
		if c < '0' || '9' < c {
			return
		}
	}
}

// parseIndex scans an index expression. It returns false if there is a syntax error.
func (s *formatState) parseIndex() bool {
	if s.nbytes == len(s.format) || s.format[s.nbytes] != '[' {
		return true
	}
	// Argument index present.
	s.nbytes++ // skip '['
	start := s.nbytes
	s.scanNum()
	ok := true
	if s.nbytes == len(s.format) || s.nbytes == start || s.format[s.nbytes] != ']' {
		ok = false // syntax error is either missing "]" or invalid index.
		s.nbytes = strings.Index(s.format[start:], "]")
		if s.nbytes < 0 {
			s.pass.ReportRangef(s.call, "%s format %s is missing closing ]", s.name, s.format)
			return false
		}
		s.nbytes = s.nbytes + start
	}
	arg32, err := strconv.ParseInt(s.format[start:s.nbytes], 10, 32)
	if err != nil || !ok || arg32 <= 0 || arg32 > int64(len(s.call.Args)-s.firstArg) {
		s.pass.ReportRangef(s.call, "%s format has invalid argument index [%s]", s.name, s.format[start:s.nbytes])
		return false
	}
	s.nbytes++ // skip ']'
	arg := int(arg32)
	arg += s.firstArg - 1 // We want to zero-index the actual arguments.
	s.argNum = arg
	s.hasIndex = true
	s.indexPending = true
	return true
}

// parseNum scans a width or precision (or *). It returns false if there's a bad index expression.
func (s *formatState) parseNum() bool {
	if s.nbytes < len(s.format) && s.format[s.nbytes] == '*' {
		if s.indexPending { // Absorb it.
			s.indexPending = false
		}
		s.nbytes++
		s.argNums = append(s.argNums, s.argNum)
		s.argNum++
	} else {
		s.scanNum()
	}
	return true
}

// parsePrecision scans for a precision. It returns false if there's a bad index expression.
func (s *formatState) parsePrecision() bool {
	// If there's a period, there may be a precision.
	if s.nbytes < len(s.format) && s.format[s.nbytes] == '.' {
		s.flags = append(s.flags, '.') // Treat precision as a flag.
		s.nbytes++
		if !s.parseIndex() {
			return false
		}
		if !s.parseNum() {
			return false
		}
	}
	return true
}

// parsePrintfVerb looks the formatting directive that begins the format string
// and returns a formatState that encodes what the directive wants, without looking
// at the actual arguments present in the call. The result is nil if there is an error.
func parsePrintfVerb(pass *analysis.Pass, call *ast.CallExpr, name, format string, firstArg, argNum int) *formatState {
	state := &formatState{
		format:   format,
		name:     name,
		flags:    make([]byte, 0, 5),
		argNum:   argNum,
		argNums:  make([]int, 0, 1),
		nbytes:   1, // There's guaranteed to be a percent sign.
		firstArg: firstArg,
		pass:     pass,
		call:     call,
	}
	// There may be flags.
	state.parseFlags()
	// There may be an index.
	if !state.parseIndex() {
		return nil
	}
	// There may be a width.
	if !state.parseNum() {
		return nil
	}
	// There may be a precision.
	if !state.parsePrecision() {
		return nil
	}
	// Now a verb, possibly prefixed by an index (which we may already have).
	if !state.indexPending && !state.parseIndex() {
		return nil
	}
	if state.nbytes == len(state.format) {
		pass.ReportRangef(call.Fun, "%s format %s is missing verb at end of string", name, state.format)
		return nil
	}
	verb, w := utf8.DecodeRuneInString(state.format[state.nbytes:])
	state.verb = verb
	state.nbytes += w
	if verb != '%' {
		state.argNums = append(state.argNums, state.argNum)
	}
	state.format = state.format[:state.nbytes]
	return state
}

// printfArgType encodes the types of expressions a printf verb accepts. It is a bitmask.
type printfArgType int

const (
	argBool printfArgType = 1 << iota
	argInt
	argRune
	argString
	argFloat
	argComplex
	argPointer
	argError
	anyType printfArgType = ^0
)

type printVerb struct {
	verb  rune   // User may provide verb through Formatter; could be a rune.
	flags string // known flags are all ASCII
	typ   printfArgType
}

// Common flag sets for printf verbs.
const (
	noFlag       = ""
	numFlag      = " -+.0"
	sharpNumFlag = " -+.0#"
	allFlags     = " -+.0#"
)

// printVerbs identifies which flags are known to printf for each verb.
var printVerbs = []printVerb{
	// '-' is a width modifier, always valid.
	// '.' is a precision for float, max width for strings.
	// '+' is required sign for numbers, Go format for %v.
	// '#' is alternate format for several verbs.
	// ' ' is spacer for numbers
	{'%', noFlag, 0},
	{'b', sharpNumFlag, argInt | argFloat | argComplex | argPointer},
	{'c', "-", argRune | argInt},
	{'d', numFlag, argInt | argPointer},
	{'e', sharpNumFlag, argFloat | argComplex},
	{'E', sharpNumFlag, argFloat | argComplex},
	{'f', sharpNumFlag, argFloat | argComplex},
	{'F', sharpNumFlag, argFloat | argComplex},
	{'g', sharpNumFlag, argFloat | argComplex},
	{'G', sharpNumFlag, argFloat | argComplex},
	{'o', sharpNumFlag, argInt | argPointer},
	{'O', sharpNumFlag, argInt | argPointer},
	{'p', "-#", argPointer},
	{'q', " -+.0#", argRune | argInt | argString},
	{'s', " -+.0", argString},
	{'t', "-", argBool},
	{'T', "-", anyType},
	{'U', "-#", argRune | argInt},
	{'v', allFlags, anyType},
	{'w', allFlags, argError},
	{'x', sharpNumFlag, argRune | argInt | argString | argPointer | argFloat | argComplex},
	{'X', sharpNumFlag, argRune | argInt | argString | argPointer | argFloat | argComplex},
}

// okPrintfArg compares the formatState to the arguments actually present,
// reporting any discrepancies it can discern. If the final argument is ellipsissed,
// there's little it can do for that.
func okPrintfArg(pass *analysis.Pass, call *ast.CallExpr, state *formatState) (ok bool) {
	var v printVerb
	found := false
	// Linear scan is fast enough for a small list.
	for _, v = range printVerbs {
		if v.verb == state.verb {
			found = true
			break
		}
	}

	// Could current arg implement fmt.Formatter?
	// Skip check for the %w verb, which requires an error.
	formatter := false
	if v.typ != argError && state.argNum < len(call.Args) {
		if tv, ok := pass.TypesInfo.Types[call.Args[state.argNum]]; ok {
			formatter = isFormatter(tv.Type)
		}
	}

	if !formatter {
		if !found {
			pass.ReportRangef(call, "%s format %s has unknown verb %c", state.name, state.format, state.verb)
			return false
		}
		for _, flag := range state.flags {
			// TODO: Disable complaint about '0' for Go 1.10. To be fixed properly in 1.11.
			// See issues 23598 and 23605.
			if flag == '0' {
				continue
			}
			if !strings.ContainsRune(v.flags, rune(flag)) {
				pass.ReportRangef(call, "%s format %s has unrecognized flag %c", state.name, state.format, flag)
				return false
			}
		}
	}
	// Verb is good. If len(state.argNums)>trueArgs, we have something like %.*s and all
	// but the final arg must be an integer.
	trueArgs := 1
	if state.verb == '%' {
		trueArgs = 0
	}
	nargs := len(state.argNums)
	for i := 0; i < nargs-trueArgs; i++ {
		argNum := state.argNums[i]
		if !argCanBeChecked(pass, call, i, state) {
			return
		}
		arg := call.Args[argNum]
		if reason, ok := matchArgType(pass, argInt, arg); !ok {
			details := ""
			if reason != "" {
				details = " (" + reason + ")"
			}
			pass.ReportRangef(call, "%s format %s uses non-int %s%s as argument of *", state.name, state.format, analysisutil.Format(pass.Fset, arg), details)
			return false
		}
	}

	if state.verb == '%' || formatter {
		return true
	}
	argNum := state.argNums[len(state.argNums)-1]
	if !argCanBeChecked(pass, call, len(state.argNums)-1, state) {
		return false
	}
	arg := call.Args[argNum]
	if isFunctionValue(pass, arg) && state.verb != 'p' && state.verb != 'T' {
		pass.ReportRangef(call, "%s format %s arg %s is a func value, not called", state.name, state.format, analysisutil.Format(pass.Fset, arg))
		return false
	}
	if reason, ok := matchArgType(pass, v.typ, arg); !ok {
		typeString := ""
		if typ := pass.TypesInfo.Types[arg].Type; typ != nil {
			typeString = typ.String()
		}
		details := ""
		if reason != "" {
			details = " (" + reason + ")"
		}
		pass.ReportRangef(call, "%s format %s has arg %s of wrong type %s%s", state.name, state.format, analysisutil.Format(pass.Fset, arg), typeString, details)
		return false
	}
	if v.typ&argString != 0 && v.verb != 'T' && !bytes.Contains(state.flags, []byte{'#'}) {
		if methodName, ok := recursiveStringer(pass, arg); ok {
			pass.ReportRangef(call, "%s format %s with arg %s causes recursive %s method call", state.name, state.format, analysisutil.Format(pass.Fset, arg), methodName)
			return false
		}
	}
	return true
}

// recursiveStringer reports whether the argument e is a potential
// recursive call to stringer or is an error, such as t and &t in these examples:
//
//	func (t *T) String() string { printf("%s",  t) }
//	func (t  T) Error() string { printf("%s",  t) }
//	func (t  T) String() string { printf("%s", &t) }
func recursiveStringer(pass *analysis.Pass, e ast.Expr) (string, bool) {
	typ := pass.TypesInfo.Types[e].Type

	// It's unlikely to be a recursive stringer if it has a Format method.
	if isFormatter(typ) {
		return "", false
	}

	// Does e allow e.String() or e.Error()?
	strObj, _, _ := types.LookupFieldOrMethod(typ, false, pass.Pkg, "String")
	strMethod, strOk := strObj.(*types.Func)
	errObj, _, _ := types.LookupFieldOrMethod(typ, false, pass.Pkg, "Error")
	errMethod, errOk := errObj.(*types.Func)
	if !strOk && !errOk {
		return "", false
	}

	// inScope returns true if e is in the scope of f.
	inScope := func(e ast.Expr, f *types.Func) bool {
		return f.Scope() != nil && f.Scope().Contains(e.Pos())
	}

	// Is the expression e within the body of that String or Error method?
	var method *types.Func
	if strOk && strMethod.Pkg() == pass.Pkg && inScope(e, strMethod) {
		method = strMethod
	} else if errOk && errMethod.Pkg() == pass.Pkg && inScope(e, errMethod) {
		method = errMethod
	} else {
		return "", false
	}

	sig := method.Type().(*types.Signature)
	if !isStringer(sig) {
		return "", false
	}

	// Is it the receiver r, or &r?
	if u, ok := e.(*ast.UnaryExpr); ok && u.Op == token.AND {
		e = u.X // strip off & from &r
	}
	if id, ok := e.(*ast.Ident); ok {
		if pass.TypesInfo.Uses[id] == sig.Recv() {
			return method.FullName(), true
		}
	}
	return "", false
}

// isStringer reports whether the method signature matches the String() definition in fmt.Stringer.
func isStringer(sig *types.Signature) bool {
	return sig.Params().Len() == 0 &&
		sig.Results().Len() == 1 &&
		sig.Results().At(0).Type() == types.Typ[types.String]
}

// isFunctionValue reports whether the expression is a function as opposed to a function call.
// It is almost always a mistake to print a function value.
func isFunctionValue(pass *analysis.Pass, e ast.Expr) bool {
	if typ := pass.TypesInfo.Types[e].Type; typ != nil {
		_, ok := typ.(*types.Signature)
		return ok
	}
	return false
}

// argCanBeChecked reports whether the specified argument is statically present;
// it may be beyond the list of arguments or in a terminal slice... argument, which
// means we can't see it.
func argCanBeChecked(pass *analysis.Pass, call *ast.CallExpr, formatArg int, state *formatState) bool {
	argNum := state.argNums[formatArg]
	if argNum <= 0 {
		// Shouldn't happen, so catch it with prejudice.
		panic("negative arg num")
	}
	if argNum < len(call.Args)-1 {
		return true // Always OK.
	}
	if call.Ellipsis.IsValid() {
		return false // We just can't tell; there could be many more arguments.
	}
	if argNum < len(call.Args) {
		return true
	}
	// There are bad indexes in the format or there are fewer arguments than the format needs.
	// This is the argument number relative to the format: Printf("%s", "hi") will give 1 for the "hi".
	arg := argNum - state.firstArg + 1 // People think of arguments as 1-indexed.
	pass.ReportRangef(call, "%s format %s reads arg #%d, but call has %v", state.name, state.format, arg, count(len(call.Args)-state.firstArg, "arg"))
	return false
}

// printFormatRE is the regexp we match and report as a possible format string
// in the first argument to unformatted prints like fmt.Print.
// We exclude the space flag, so that printing a string like "x % y" is not reported as a format.
var printFormatRE = regexp.MustCompile(`%` + flagsRE + numOptRE + `\.?` + numOptRE + indexOptRE + verbRE)

const (
	flagsRE    = `[+\-#]*`
	indexOptRE = `(\[[0-9]+\])?`
	numOptRE   = `([0-9]+|` + indexOptRE + `\*)?`
	verbRE     = `[bcdefgopqstvxEFGTUX]`
)

// checkPrint checks a call to an unformatted print routine such as Println.
func checkPrint(pass *analysis.Pass, call *ast.CallExpr, fn *types.Func) {
	firstArg := 0
	typ := pass.TypesInfo.Types[call.Fun].Type
	if typ == nil {
		// Skip checking functions with unknown type.
		return
	}
	if sig, ok := typ.(*types.Signature); ok {
		if !sig.Variadic() {
			// Skip checking non-variadic functions.
			return
		}
		params := sig.Params()
		firstArg = params.Len() - 1

		typ := params.At(firstArg).Type()
		typ = typ.(*types.Slice).Elem()
		it, ok := typ.(*types.Interface)
		if !ok || !it.Empty() {
			// Skip variadic functions accepting non-interface{} args.
			return
		}
	}
	args := call.Args
	if len(args) <= firstArg {
		// Skip calls without variadic args.
		return
	}
	args = args[firstArg:]

	if firstArg == 0 {
		if sel, ok := call.Args[0].(*ast.SelectorExpr); ok {
			if x, ok := sel.X.(*ast.Ident); ok {
				if x.Name == "os" && strings.HasPrefix(sel.Sel.Name, "Std") {
					pass.ReportRangef(call, "%s does not take io.Writer but has first arg %s", fn.FullName(), analysisutil.Format(pass.Fset, call.Args[0]))
				}
			}
		}
	}

	arg := args[0]
	if s, ok := stringConstantExpr(pass, arg); ok {
		// Ignore trailing % character
		// The % in "abc 0.0%" couldn't be a formatting directive.
		s = strings.TrimSuffix(s, "%")
		if strings.Contains(s, "%") {
			m := printFormatRE.FindStringSubmatch(s)
			if m != nil {
				pass.ReportRangef(call, "%s call has possible formatting directive %s", fn.FullName(), m[0])
			}
		}
	}
	if strings.HasSuffix(fn.Name(), "ln") {
		// The last item, if a string, should not have a newline.
		arg = args[len(args)-1]
		if s, ok := stringConstantExpr(pass, arg); ok {
			if strings.HasSuffix(s, "\n") {
				pass.ReportRangef(call, "%s arg list ends with redundant newline", fn.FullName())
			}
		}
	}
	for _, arg := range args {
		if isFunctionValue(pass, arg) {
			pass.ReportRangef(call, "%s arg %s is a func value, not called", fn.FullName(), analysisutil.Format(pass.Fset, arg))
		}
		if methodName, ok := recursiveStringer(pass, arg); ok {
			pass.ReportRangef(call, "%s arg %s causes recursive call to %s method", fn.FullName(), analysisutil.Format(pass.Fset, arg), methodName)
		}
	}
}

// count(n, what) returns "1 what" or "N whats"
// (assuming the plural of what is whats).
func count(n int, what string) string {
	if n == 1 {
		return "1 " + what
	}
	return fmt.Sprintf("%d %ss", n, what)
}

// stringSet is a set-of-nonempty-strings-valued flag.
// Note: elements without a '.' get lower-cased.
type stringSet map[string]bool

func (ss stringSet) String() string {
	var list []string
	for name := range ss {
		list = append(list, name)
	}
	sort.Strings(list)
	return strings.Join(list, ",")
}

func (ss stringSet) Set(flag string) error {
	for _, name := range strings.Split(flag, ",") {
		if len(name) == 0 {
			return fmt.Errorf("empty string")
		}
		if !strings.Contains(name, ".") {
			name = strings.ToLower(name)
		}
		ss[name] = true
	}
	return nil
}