lint.go

// Package staticcheck contains a linter for Go source code.
package staticcheck // import "honnef.co/go/tools/staticcheck"

import (
	"fmt"
	"go/ast"
	"go/build"
	"go/constant"
	"go/token"
	"go/types"
	htmltemplate "html/template"
	"net/http"
	"runtime"
	"strconv"
	"strings"
	"sync"
	texttemplate "text/template"

	"honnef.co/go/tools/functions"
	"honnef.co/go/tools/gcsizes"
	"honnef.co/go/tools/lint"
	"honnef.co/go/tools/ssa"
	"honnef.co/go/tools/staticcheck/vrp"

	"golang.org/x/tools/go/ast/astutil"
)

func validRegexp(call *Call) {
	arg := call.Args[0]
	err := ValidateRegexp(arg.Value)
	if err != nil {
		arg.Invalid(err.Error())
	}
}

func utf8Cutset(call *Call) {
	arg := call.Args[1]
	if InvalidUTF8(arg.Value) {
		arg.Invalid(MsgInvalidUTF8)
	}
}

func unmarshalPointer(name string, arg int) CallCheck {
	return func(call *Call) {
		if !Pointer(call.Args[arg].Value) {
			call.Args[arg].Invalid(fmt.Sprintf("%s expects to unmarshal into a pointer, but the provided value is not a pointer", name))
		}
	}
}

func pointlessIntMath(call *Call) {
	if ConvertedFromInt(call.Args[0].Value) {
		call.Invalid(fmt.Sprintf("calling %s on a converted integer is pointless", lint.CallName(call.Instr.Common())))
	}
}

func checkValidHostPort(arg int) CallCheck {
	return func(call *Call) {
		if !ValidHostPort(call.Args[arg].Value) {
			call.Args[arg].Invalid(MsgInvalidHostPort)
		}
	}
}

var (
	checkRegexpRules = map[string]CallCheck{
		"regexp.MustCompile": validRegexp,
		"regexp.Compile":     validRegexp,
	}

	checkTimeParseRules = map[string]CallCheck{
		"time.Parse": func(call *Call) {
			arg := call.Args[0]
			err := ValidateTimeLayout(arg.Value)
			if err != nil {
				arg.Invalid(err.Error())
			}
		},
	}

	checkEncodingBinaryRules = map[string]CallCheck{
		"encoding/binary.Write": func(call *Call) {
			arg := call.Args[2]
			if !CanBinaryMarshal(call.Job, arg.Value) {
				arg.Invalid(fmt.Sprintf("value of type %s cannot be used with binary.Write", arg.Value.Value.Type()))
			}
		},
	}

	checkURLsRules = map[string]CallCheck{
		"net/url.Parse": func(call *Call) {
			arg := call.Args[0]
			err := ValidateURL(arg.Value)
			if err != nil {
				arg.Invalid(err.Error())
			}
		},
	}

	checkDubiousSyncPoolSizeRules = map[string]CallCheck{
		"(*sync.Pool).Put": func(call *Call) {
			// TODO(dh): allow users to pass in a custom build environment
			sizes := gcsizes.ForArch(build.Default.GOARCH)
			arg := call.Args[0]
			typ := arg.Value.Value.Type()
			if !types.IsInterface(typ) && sizes.Sizeof(typ) > sizes.WordSize {
				arg.Invalid("argument should be one word large or less to avoid allocations")
			}
		},
	}

	checkRegexpFindAllRules = map[string]CallCheck{
		"(*regexp.Regexp).FindAll":                    RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllIndex":               RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllString":              RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllStringIndex":         RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllStringSubmatch":      RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllStringSubmatchIndex": RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllSubmatch":            RepeatZeroTimes("a FindAll method", 1),
		"(*regexp.Regexp).FindAllSubmatchIndex":       RepeatZeroTimes("a FindAll method", 1),
	}

	checkUTF8CutsetRules = map[string]CallCheck{
		"strings.IndexAny":     utf8Cutset,
		"strings.LastIndexAny": utf8Cutset,
		"strings.ContainsAny":  utf8Cutset,
		"strings.Trim":         utf8Cutset,
		"strings.TrimLeft":     utf8Cutset,
		"strings.TrimRight":    utf8Cutset,
	}

	checkUnmarshalPointerRules = map[string]CallCheck{
		"encoding/xml.Unmarshal":          unmarshalPointer("xml.Unmarshal", 1),
		"(*encoding/xml.Decoder).Decode":  unmarshalPointer("Decode", 0),
		"encoding/json.Unmarshal":         unmarshalPointer("json.Unmarshal", 1),
		"(*encoding/json.Decoder).Decode": unmarshalPointer("Decode", 0),
	}

	checkUnbufferedSignalChanRules = map[string]CallCheck{
		"os/signal.Notify": func(call *Call) {
			arg := call.Args[0]
			if UnbufferedChannel(arg.Value) {
				arg.Invalid("the channel used with signal.Notify should be buffered")
			}
		},
	}

	checkMathIntRules = map[string]CallCheck{
		"math.Ceil":  pointlessIntMath,
		"math.Floor": pointlessIntMath,
		"math.IsNaN": pointlessIntMath,
		"math.Trunc": pointlessIntMath,
		"math.IsInf": pointlessIntMath,
	}

	checkStringsReplaceZeroRules = map[string]CallCheck{
		"strings.Replace": RepeatZeroTimes("strings.Replace", 3),
		"bytes.Replace":   RepeatZeroTimes("bytes.Replace", 3),
	}

	checkListenAddressRules = map[string]CallCheck{
		"net/http.ListenAndServe":    checkValidHostPort(0),
		"net/http.ListenAndServeTLS": checkValidHostPort(0),
	}

	checkBytesEqualIPRules = map[string]CallCheck{
		"bytes.Equal": func(call *Call) {
			if ConvertedFrom(call.Args[0].Value, "net.IP") && ConvertedFrom(call.Args[1].Value, "net.IP") {
				call.Invalid("use net.IP.Equal to compare net.IPs, not bytes.Equal")
			}
		},
	}

	checkRegexpMatchLoopRules = map[string]CallCheck{
		"regexp.Match":       loopedRegexp("regexp.Match"),
		"regexp.MatchReader": loopedRegexp("regexp.MatchReader"),
		"regexp.MatchString": loopedRegexp("regexp.MatchString"),
	}
)

type Checker struct {
	CheckGenerated bool
	funcDescs      *functions.Descriptions
	deprecatedObjs map[types.Object]string
	nodeFns        map[ast.Node]*ssa.Function
}

func NewChecker() *Checker {
	return &Checker{}
}

func (c *Checker) Funcs() map[string]lint.Func {
	return map[string]lint.Func{
		"SA1000": c.callChecker(checkRegexpRules),
		"SA1001": c.CheckTemplate,
		"SA1002": c.callChecker(checkTimeParseRules),
		"SA1003": c.callChecker(checkEncodingBinaryRules),
		"SA1004": c.CheckTimeSleepConstant,
		"SA1005": c.CheckExec,
		"SA1006": c.CheckUnsafePrintf,
		"SA1007": c.callChecker(checkURLsRules),
		"SA1008": c.CheckCanonicalHeaderKey,
		"SA1009": nil,
		"SA1010": c.callChecker(checkRegexpFindAllRules),
		"SA1011": c.callChecker(checkUTF8CutsetRules),
		"SA1012": c.CheckNilContext,
		"SA1013": c.CheckSeeker,
		"SA1014": c.callChecker(checkUnmarshalPointerRules),
		"SA1015": c.CheckLeakyTimeTick,
		"SA1016": c.CheckUntrappableSignal,
		"SA1017": c.callChecker(checkUnbufferedSignalChanRules),
		"SA1018": c.callChecker(checkStringsReplaceZeroRules),
		"SA1019": c.CheckDeprecated,
		"SA1020": c.callChecker(checkListenAddressRules),
		"SA1021": c.callChecker(checkBytesEqualIPRules),
		"SA1022": c.CheckFlagUsage,
		"SA1023": c.CheckWriterBufferModified,

		"SA2000": c.CheckWaitgroupAdd,
		"SA2001": c.CheckEmptyCriticalSection,
		"SA2002": c.CheckConcurrentTesting,
		"SA2003": c.CheckDeferLock,

		"SA3000": c.CheckTestMainExit,
		"SA3001": c.CheckBenchmarkN,

		"SA4000": c.CheckLhsRhsIdentical,
		"SA4001": c.CheckIneffectiveCopy,
		"SA4002": c.CheckDiffSizeComparison,
		"SA4003": c.CheckUnsignedComparison,
		"SA4004": c.CheckIneffectiveLoop,
		"SA4005": c.CheckIneffecitiveFieldAssignments,
		"SA4006": c.CheckUnreadVariableValues,
		// "SA4007": c.CheckPredeterminedBooleanExprs,
		"SA4007": nil,
		"SA4008": c.CheckLoopCondition,
		"SA4009": c.CheckArgOverwritten,
		"SA4010": nil,
		"SA4011": c.CheckScopedBreak,
		"SA4012": c.CheckNaNComparison,
		"SA4013": c.CheckDoubleNegation,
		"SA4014": c.CheckRepeatedIfElse,
		"SA4015": c.callChecker(checkMathIntRules),
		"SA4016": c.CheckSillyBitwiseOps,
		"SA4017": c.CheckPureFunctions,

		"SA5000": c.CheckNilMaps,
		"SA5001": c.CheckEarlyDefer,
		"SA5002": c.CheckInfiniteEmptyLoop,
		"SA5003": c.CheckDeferInInfiniteLoop,
		"SA5004": c.CheckLoopEmptyDefault,
		"SA5005": c.CheckCyclicFinalizer,
		// "SA5006": c.CheckSliceOutOfBounds,
		"SA5007": c.CheckInfiniteRecursion,

		"SA6000": c.callChecker(checkRegexpMatchLoopRules),

		"SA9000": c.callChecker(checkDubiousSyncPoolSizeRules),
		"SA9001": c.CheckDubiousDeferInChannelRangeLoop,
		"SA9002": c.CheckNonOctalFileMode,
		"SA9003": c.CheckEmptyBranch,
	}
}

func (c *Checker) filterGenerated(files []*ast.File) []*ast.File {
	if c.CheckGenerated {
		return files
	}
	var out []*ast.File
	for _, f := range files {
		if !lint.IsGenerated(f) {
			out = append(out, f)
		}
	}
	return out
}

func (c *Checker) Init(prog *lint.Program) {
	c.funcDescs = functions.NewDescriptions(prog.SSA)
	c.deprecatedObjs = map[types.Object]string{}
	c.nodeFns = map[ast.Node]*ssa.Function{}

	fns := prog.AllFunctions

	var pwg sync.WaitGroup
	var processFn func(*ssa.Function)
	processFn = func(fn *ssa.Function) {
		if fn.Blocks != nil {
			applyStdlibKnowledge(fn)
			ssa.OptimizeBlocks(fn)
		}
		pwg.Done()
	}
	for _, fn := range fns {
		pwg.Add(1)
		go processFn(fn)
	}
	pwg.Wait()

	c.nodeFns = lint.NodeFns(prog.Packages)

	chDeprecated := make(chan struct {
		obj types.Object
		msg string
	}, runtime.NumCPU()*2)
	wg := &sync.WaitGroup{}
	for _, pkginfo := range prog.Prog.AllPackages {
		pkginfo := pkginfo
		scope := pkginfo.Pkg.Scope()
		names := scope.Names()
		for _, name := range names {
			name := name
			wg.Add(1)
			go func() {
				obj := scope.Lookup(name)
				msg := c.deprecationMessage(pkginfo.Files, prog.SSA.Fset, obj)
				chDeprecated <- struct {
					obj types.Object
					msg string
				}{obj, msg}
				if typ, ok := obj.Type().Underlying().(*types.Struct); ok {
					n := typ.NumFields()
					for i := 0; i < n; i++ {
						// FIXME(dh): This code will not find deprecated
						// fields in anonymous structs.
						field := typ.Field(i)
						msg := c.deprecationMessage(pkginfo.Files, prog.SSA.Fset, field)
						chDeprecated <- struct {
							obj types.Object
							msg string
						}{field, msg}
					}
				}
				wg.Done()
			}()
		}
	}
	go func() {
		wg.Wait()
		close(chDeprecated)
	}()
	for dep := range chDeprecated {
		c.deprecatedObjs[dep.obj] = dep.msg
	}
}

// TODO(adonovan): make this a method: func (*token.File) Contains(token.Pos)
func tokenFileContainsPos(f *token.File, pos token.Pos) bool {
	p := int(pos)
	base := f.Base()
	return base <= p && p < base+f.Size()
}

func pathEnclosingInterval(files []*ast.File, fset *token.FileSet, start, end token.Pos) (path []ast.Node, exact bool) {
	for _, f := range files {
		if f.Pos() == token.NoPos {
			// This can happen if the parser saw
			// too many errors and bailed out.
			// (Use parser.AllErrors to prevent that.)
			continue
		}
		if !tokenFileContainsPos(fset.File(f.Pos()), start) {
			continue
		}
		if path, exact := astutil.PathEnclosingInterval(f, start, end); path != nil {
			return path, exact
		}
	}
	return nil, false
}

func (c *Checker) deprecationMessage(files []*ast.File, fset *token.FileSet, obj types.Object) (message string) {
	path, _ := pathEnclosingInterval(files, fset, obj.Pos(), obj.Pos())
	if len(path) <= 2 {
		return ""
	}
	var docs []*ast.CommentGroup
	switch n := path[1].(type) {
	case *ast.FuncDecl:
		docs = append(docs, n.Doc)
	case *ast.Field:
		docs = append(docs, n.Doc)
	case *ast.ValueSpec:
		docs = append(docs, n.Doc)
		if len(path) >= 3 {
			if n, ok := path[2].(*ast.GenDecl); ok {
				docs = append(docs, n.Doc)
			}
		}
	case *ast.TypeSpec:
		docs = append(docs, n.Doc)
		if len(path) >= 3 {
			if n, ok := path[2].(*ast.GenDecl); ok {
				docs = append(docs, n.Doc)
			}
		}
	default:
		return ""
	}

	for _, doc := range docs {
		if doc == nil {
			continue
		}
		parts := strings.Split(doc.Text(), "\n\n")
		last := parts[len(parts)-1]
		if !strings.HasPrefix(last, "Deprecated: ") {
			continue
		}
		alt := last[len("Deprecated: "):]
		alt = strings.Replace(alt, "\n", " ", -1)
		return alt
	}
	return ""
}

func (c *Checker) isInLoop(b *ssa.BasicBlock) bool {
	sets := c.funcDescs.Get(b.Parent()).Loops
	for _, set := range sets {
		if set[b] {
			return true
		}
	}
	return false
}

func applyStdlibKnowledge(fn *ssa.Function) {
	if len(fn.Blocks) == 0 {
		return
	}

	// comma-ok receiving from a time.Tick channel will never return
	// ok == false, so any branching on the value of ok can be
	// replaced with an unconditional jump. This will primarily match
	// `for range time.Tick(x)` loops, but it can also match
	// user-written code.
	for _, block := range fn.Blocks {
		if len(block.Instrs) < 3 {
			continue
		}
		if len(block.Succs) != 2 {
			continue
		}
		var instrs []*ssa.Instruction
		for i, ins := range block.Instrs {
			if _, ok := ins.(*ssa.DebugRef); ok {
				continue
			}
			instrs = append(instrs, &block.Instrs[i])
		}

		for i, ins := range instrs {
			unop, ok := (*ins).(*ssa.UnOp)
			if !ok || unop.Op != token.ARROW {
				continue
			}
			call, ok := unop.X.(*ssa.Call)
			if !ok {
				continue
			}
			if !lint.IsCallTo(call.Common(), "time.Tick") {
				continue
			}
			ex, ok := (*instrs[i+1]).(*ssa.Extract)
			if !ok || ex.Tuple != unop || ex.Index != 1 {
				continue
			}

			ifstmt, ok := (*instrs[i+2]).(*ssa.If)
			if !ok || ifstmt.Cond != ex {
				continue
			}

			*instrs[i+2] = ssa.NewJump(block)
			succ := block.Succs[1]
			block.Succs = block.Succs[0:1]
			succ.RemovePred(block)
		}
	}
}

func hasType(j *lint.Job, expr ast.Expr, name string) bool {
	return types.TypeString(j.Program.Info.TypeOf(expr), nil) == name
}

func (c *Checker) CheckUntrappableSignal(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		if !j.IsFunctionCallNameAny(call,
			"os/signal.Ignore", "os/signal.Notify", "os/signal.Reset") {
			return true
		}
		for _, arg := range call.Args {
			if conv, ok := arg.(*ast.CallExpr); ok && isName(j, conv.Fun, "os.Signal") {
				arg = conv.Args[0]
			}

			if isName(j, arg, "os.Kill") || isName(j, arg, "syscall.SIGKILL") {
				j.Errorf(arg, "%s cannot be trapped (did you mean syscall.SIGTERM?)", j.Render(arg))
			}
			if isName(j, arg, "syscall.SIGSTOP") {
				j.Errorf(arg, "%s signal cannot be trapped", j.Render(arg))
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckTemplate(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		var kind string
		if j.IsFunctionCallName(call, "(*text/template.Template).Parse") {
			kind = "text"
		} else if j.IsFunctionCallName(call, "(*html/template.Template).Parse") {
			kind = "html"
		} else {
			return true
		}
		sel := call.Fun.(*ast.SelectorExpr)
		if !j.IsFunctionCallName(sel.X, "text/template.New") &&
			!j.IsFunctionCallName(sel.X, "html/template.New") {
			// TODO(dh): this is a cheap workaround for templates with
			// different delims. A better solution with less false
			// negatives would use data flow analysis to see where the
			// template comes from and where it has been
			return true
		}
		s, ok := j.ExprToString(call.Args[0])
		if !ok {
			return true
		}
		var err error
		switch kind {
		case "text":
			_, err = texttemplate.New("").Parse(s)
		case "html":
			_, err = htmltemplate.New("").Parse(s)
		}
		if err != nil {
			// TODO(dominikh): whitelist other parse errors, if any
			if strings.Contains(err.Error(), "unexpected") {
				j.Errorf(call.Args[0], "%s", err)
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckTimeSleepConstant(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		if !j.IsFunctionCallName(call, "time.Sleep") {
			return true
		}
		lit, ok := call.Args[0].(*ast.BasicLit)
		if !ok {
			return true
		}
		n, err := strconv.Atoi(lit.Value)
		if err != nil {
			return true
		}
		if n == 0 || n > 120 {
			// time.Sleep(0) is a seldomly used pattern in concurrency
			// tests. >120 might be intentional. 120 was chosen
			// because the user could've meant 2 minutes.
			return true
		}
		recommendation := "time.Sleep(time.Nanosecond)"
		if n != 1 {
			recommendation = fmt.Sprintf("time.Sleep(%d * time.Nanosecond)", n)
		}
		j.Errorf(call.Args[0], "sleeping for %d nanoseconds is probably a bug. Be explicit if it isn't: %s", n, recommendation)
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckWaitgroupAdd(j *lint.Job) {
	fn := func(node ast.Node) bool {
		g, ok := node.(*ast.GoStmt)
		if !ok {
			return true
		}
		fun, ok := g.Call.Fun.(*ast.FuncLit)
		if !ok {
			return true
		}
		if len(fun.Body.List) == 0 {
			return true
		}
		stmt, ok := fun.Body.List[0].(*ast.ExprStmt)
		if !ok {
			return true
		}
		call, ok := stmt.X.(*ast.CallExpr)
		if !ok {
			return true
		}
		sel, ok := call.Fun.(*ast.SelectorExpr)
		if !ok {
			return true
		}
		fn, ok := j.Program.Info.ObjectOf(sel.Sel).(*types.Func)
		if !ok {
			return true
		}
		if fn.FullName() == "(*sync.WaitGroup).Add" {
			j.Errorf(sel, "should call %s before starting the goroutine to avoid a race",
				j.Render(stmt))
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckInfiniteEmptyLoop(j *lint.Job) {
	fn := func(node ast.Node) bool {
		loop, ok := node.(*ast.ForStmt)
		if !ok || len(loop.Body.List) != 0 || loop.Cond != nil || loop.Init != nil {
			return true
		}
		j.Errorf(loop, "should not use an infinite empty loop. It will spin. Consider select{} instead.")
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckDeferInInfiniteLoop(j *lint.Job) {
	fn := func(node ast.Node) bool {
		mightExit := false
		var defers []ast.Stmt
		loop, ok := node.(*ast.ForStmt)
		if !ok || loop.Cond != nil {
			return true
		}
		fn2 := func(node ast.Node) bool {
			switch stmt := node.(type) {
			case *ast.ReturnStmt:
				mightExit = true
			case *ast.BranchStmt:
				// TODO(dominikh): if this sees a break in a switch or
				// select, it doesn't check if it breaks the loop or
				// just the select/switch. This causes some false
				// negatives.
				if stmt.Tok == token.BREAK {
					mightExit = true
				}
			case *ast.DeferStmt:
				defers = append(defers, stmt)
			case *ast.FuncLit:
				// Don't look into function bodies
				return false
			}
			return true
		}
		ast.Inspect(loop.Body, fn2)
		if mightExit {
			return true
		}
		for _, stmt := range defers {
			j.Errorf(stmt, "defers in this infinite loop will never run")
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckDubiousDeferInChannelRangeLoop(j *lint.Job) {
	fn := func(node ast.Node) bool {
		loop, ok := node.(*ast.RangeStmt)
		if !ok {
			return true
		}
		typ := j.Program.Info.TypeOf(loop.X)
		_, ok = typ.Underlying().(*types.Chan)
		if !ok {
			return true
		}
		fn2 := func(node ast.Node) bool {
			switch stmt := node.(type) {
			case *ast.DeferStmt:
				j.Errorf(stmt, "defers in this range loop won't run unless the channel gets closed")
			case *ast.FuncLit:
				// Don't look into function bodies
				return false
			}
			return true
		}
		ast.Inspect(loop.Body, fn2)
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckTestMainExit(j *lint.Job) {
	fn := func(node ast.Node) bool {
		if !isTestMain(j, node) {
			return true
		}

		arg := j.Program.Info.ObjectOf(node.(*ast.FuncDecl).Type.Params.List[0].Names[0])
		callsRun := false
		fn2 := func(node ast.Node) bool {
			call, ok := node.(*ast.CallExpr)
			if !ok {
				return true
			}
			sel, ok := call.Fun.(*ast.SelectorExpr)
			if !ok {
				return true
			}
			ident, ok := sel.X.(*ast.Ident)
			if !ok {
				return true
			}
			if arg != j.Program.Info.ObjectOf(ident) {
				return true
			}
			if sel.Sel.Name == "Run" {
				callsRun = true
				return false
			}
			return true
		}
		ast.Inspect(node.(*ast.FuncDecl).Body, fn2)

		callsExit := false
		fn3 := func(node ast.Node) bool {
			if j.IsFunctionCallName(node, "os.Exit") {
				callsExit = true
				return false
			}
			return true
		}
		ast.Inspect(node.(*ast.FuncDecl).Body, fn3)
		if !callsExit && callsRun {
			j.Errorf(node, "TestMain should call os.Exit to set exit code")
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func isTestMain(j *lint.Job, node ast.Node) bool {
	decl, ok := node.(*ast.FuncDecl)
	if !ok {
		return false
	}
	if decl.Name.Name != "TestMain" {
		return false
	}
	if len(decl.Type.Params.List) != 1 {
		return false
	}
	arg := decl.Type.Params.List[0]
	if len(arg.Names) != 1 {
		return false
	}
	typ := j.Program.Info.TypeOf(arg.Type)
	return typ != nil && typ.String() == "*testing.M"
}

func (c *Checker) CheckExec(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		if !j.IsFunctionCallName(call, "os/exec.Command") {
			return true
		}
		val, ok := j.ExprToString(call.Args[0])
		if !ok {
			return true
		}
		if !strings.Contains(val, " ") || strings.Contains(val, `\`) {
			return true
		}
		j.Errorf(call.Args[0], "first argument to exec.Command looks like a shell command, but a program name or path are expected")
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckLoopEmptyDefault(j *lint.Job) {
	fn := func(node ast.Node) bool {
		loop, ok := node.(*ast.ForStmt)
		if !ok || len(loop.Body.List) != 1 || loop.Cond != nil || loop.Init != nil {
			return true
		}
		sel, ok := loop.Body.List[0].(*ast.SelectStmt)
		if !ok {
			return true
		}
		for _, c := range sel.Body.List {
			if comm, ok := c.(*ast.CommClause); ok && comm.Comm == nil && len(comm.Body) == 0 {
				j.Errorf(comm, "should not have an empty default case in a for+select loop. The loop will spin.")
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckLhsRhsIdentical(j *lint.Job) {
	fn := func(node ast.Node) bool {
		op, ok := node.(*ast.BinaryExpr)
		if !ok {
			return true
		}
		switch op.Op {
		case token.EQL, token.NEQ:
			if basic, ok := j.Program.Info.TypeOf(op.X).(*types.Basic); ok {
				if kind := basic.Kind(); kind == types.Float32 || kind == types.Float64 {
					// f == f and f != f might be used to check for NaN
					return true
				}
			}
		case token.SUB, token.QUO, token.AND, token.REM, token.OR, token.XOR, token.AND_NOT,
			token.LAND, token.LOR, token.LSS, token.GTR, token.LEQ, token.GEQ:
		default:
			// For some ops, such as + and *, it can make sense to
			// have identical operands
			return true
		}

		if j.Render(op.X) != j.Render(op.Y) {
			return true
		}
		j.Errorf(op, "identical expressions on the left and right side of the '%s' operator", op.Op)
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckScopedBreak(j *lint.Job) {
	fn := func(node ast.Node) bool {
		loop, ok := node.(*ast.ForStmt)
		if !ok {
			return true
		}
		for _, stmt := range loop.Body.List {
			var blocks [][]ast.Stmt
			switch stmt := stmt.(type) {
			case *ast.SwitchStmt:
				for _, c := range stmt.Body.List {
					blocks = append(blocks, c.(*ast.CaseClause).Body)
				}
			case *ast.SelectStmt:
				for _, c := range stmt.Body.List {
					blocks = append(blocks, c.(*ast.CommClause).Body)
				}
			default:
				continue
			}

			for _, body := range blocks {
				if len(body) == 0 {
					continue
				}
				lasts := []ast.Stmt{body[len(body)-1]}
				// TODO(dh): unfold all levels of nested block
				// statements, not just a single level if statement
				if ifs, ok := lasts[0].(*ast.IfStmt); ok {
					if len(ifs.Body.List) == 0 {
						continue
					}
					lasts[0] = ifs.Body.List[len(ifs.Body.List)-1]

					if block, ok := ifs.Else.(*ast.BlockStmt); ok {
						if len(block.List) != 0 {
							lasts = append(lasts, block.List[len(block.List)-1])
						}
					}
				}
				for _, last := range lasts {
					branch, ok := last.(*ast.BranchStmt)
					if !ok || branch.Tok != token.BREAK || branch.Label != nil {
						continue
					}
					j.Errorf(branch, "ineffective break statement. Did you mean to break out of the outer loop?")
				}
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckUnsafePrintf(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		if !j.IsFunctionCallNameAny(call, "fmt.Printf", "fmt.Sprintf", "log.Printf") {
			return true
		}
		if len(call.Args) != 1 {
			return true
		}
		switch call.Args[0].(type) {
		case *ast.CallExpr, *ast.Ident:
		default:
			return true
		}
		j.Errorf(call.Args[0], "printf-style function with dynamic first argument and no further arguments should use print-style function instead")
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckEarlyDefer(j *lint.Job) {
	fn := func(node ast.Node) bool {
		block, ok := node.(*ast.BlockStmt)
		if !ok {
			return true
		}
		if len(block.List) < 2 {
			return true
		}
		for i, stmt := range block.List {
			if i == len(block.List)-1 {
				break
			}
			assign, ok := stmt.(*ast.AssignStmt)
			if !ok {
				continue
			}
			if len(assign.Rhs) != 1 {
				continue
			}
			if len(assign.Lhs) < 2 {
				continue
			}
			if lhs, ok := assign.Lhs[len(assign.Lhs)-1].(*ast.Ident); ok && lhs.Name == "_" {
				continue
			}
			call, ok := assign.Rhs[0].(*ast.CallExpr)
			if !ok {
				continue
			}
			sig, ok := j.Program.Info.TypeOf(call.Fun).(*types.Signature)
			if !ok {
				continue
			}
			if sig.Results().Len() < 2 {
				continue
			}
			last := sig.Results().At(sig.Results().Len() - 1)
			// FIXME(dh): check that it's error from universe, not
			// another type of the same name
			if last.Type().String() != "error" {
				continue
			}
			lhs, ok := assign.Lhs[0].(*ast.Ident)
			if !ok {
				continue
			}
			def, ok := block.List[i+1].(*ast.DeferStmt)
			if !ok {
				continue
			}
			sel, ok := def.Call.Fun.(*ast.SelectorExpr)
			if !ok {
				continue
			}
			ident, ok := selectorX(sel).(*ast.Ident)
			if !ok {
				continue
			}
			if ident.Obj != lhs.Obj {
				continue
			}
			if sel.Sel.Name != "Close" {
				continue
			}
			j.Errorf(def, "should check returned error before deferring %s", j.Render(def.Call))
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func selectorX(sel *ast.SelectorExpr) ast.Node {
	switch x := sel.X.(type) {
	case *ast.SelectorExpr:
		return selectorX(x)
	default:
		return x
	}
}

func (c *Checker) CheckEmptyCriticalSection(j *lint.Job) {
	mutexParams := func(s ast.Stmt) (x ast.Expr, funcName string, ok bool) {
		expr, ok := s.(*ast.ExprStmt)
		if !ok {
			return nil, "", false
		}
		call, ok := expr.X.(*ast.CallExpr)
		if !ok {
			return nil, "", false
		}
		sel, ok := call.Fun.(*ast.SelectorExpr)
		if !ok {
			return nil, "", false
		}

		fn, ok := j.Program.Info.ObjectOf(sel.Sel).(*types.Func)
		if !ok {
			return nil, "", false
		}
		sig := fn.Type().(*types.Signature)
		if sig.Params().Len() != 0 || sig.Results().Len() != 0 {
			return nil, "", false
		}

		return sel.X, fn.Name(), true
	}

	fn := func(node ast.Node) bool {
		block, ok := node.(*ast.BlockStmt)
		if !ok {
			return true
		}
		if len(block.List) < 2 {
			return true
		}
		for i := range block.List[:len(block.List)-1] {
			sel1, method1, ok1 := mutexParams(block.List[i])
			sel2, method2, ok2 := mutexParams(block.List[i+1])

			if !ok1 || !ok2 || j.Render(sel1) != j.Render(sel2) {
				continue
			}
			if (method1 == "Lock" && method2 == "Unlock") ||
				(method1 == "RLock" && method2 == "RUnlock") {
				j.Errorf(block.List[i+1], "empty critical section")
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckIneffectiveCopy(j *lint.Job) {
	fn := func(node ast.Node) bool {
		if unary, ok := node.(*ast.UnaryExpr); ok {
			if _, ok := unary.X.(*ast.StarExpr); ok && unary.Op == token.AND {
				j.Errorf(unary, "&*x will be simplified to x. It will not copy x.")
			}
		}

		if star, ok := node.(*ast.StarExpr); ok {
			if unary, ok := star.X.(*ast.UnaryExpr); ok && unary.Op == token.AND {
				j.Errorf(star, "*&x will be simplified to x. It will not copy x.")
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckDiffSizeComparison(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, b := range ssafn.Blocks {
			for _, ins := range b.Instrs {
				binop, ok := ins.(*ssa.BinOp)
				if !ok {
					continue
				}
				if binop.Op != token.EQL && binop.Op != token.NEQ {
					continue
				}
				_, ok1 := binop.X.(*ssa.Slice)
				_, ok2 := binop.Y.(*ssa.Slice)
				if !ok1 && !ok2 {
					continue
				}
				r := c.funcDescs.Get(ssafn).Ranges
				r1, ok1 := r.Get(binop.X).(vrp.StringInterval)
				r2, ok2 := r.Get(binop.Y).(vrp.StringInterval)
				if !ok1 || !ok2 {
					continue
				}
				if r1.Length.Intersection(r2.Length).Empty() {
					j.Errorf(binop, "comparing strings of different sizes for equality will always return false")
				}
			}
		}
	}
}

func (c *Checker) CheckCanonicalHeaderKey(j *lint.Job) {
	fn := func(node ast.Node) bool {
		assign, ok := node.(*ast.AssignStmt)
		if ok {
			// TODO(dh): This risks missing some Header reads, for
			// example in `h1["foo"] = h2["foo"]` – these edge
			// cases are probably rare enough to ignore for now.
			for _, expr := range assign.Lhs {
				op, ok := expr.(*ast.IndexExpr)
				if !ok {
					continue
				}
				if hasType(j, op.X, "net/http.Header") {
					return false
				}
			}
			return true
		}
		op, ok := node.(*ast.IndexExpr)
		if !ok {
			return true
		}
		if !hasType(j, op.X, "net/http.Header") {
			return true
		}
		s, ok := j.ExprToString(op.Index)
		if !ok {
			return true
		}
		if s == http.CanonicalHeaderKey(s) {
			return true
		}
		j.Errorf(op, "keys in http.Header are canonicalized, %q is not canonical; fix the constant or use http.CanonicalHeaderKey", s)
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckBenchmarkN(j *lint.Job) {
	fn := func(node ast.Node) bool {
		assign, ok := node.(*ast.AssignStmt)
		if !ok {
			return true
		}
		if len(assign.Lhs) != 1 || len(assign.Rhs) != 1 {
			return true
		}
		sel, ok := assign.Lhs[0].(*ast.SelectorExpr)
		if !ok {
			return true
		}
		if sel.Sel.Name != "N" {
			return true
		}
		if !hasType(j, sel.X, "*testing.B") {
			return true
		}
		j.Errorf(assign, "should not assign to %s", j.Render(sel))
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckIneffecitiveFieldAssignments(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		// fset := j.Program.SSA.Fset
		// if fset.File(f.File.Pos()) != fset.File(ssafn.Pos()) {
		// 	continue
		// }
		if ssafn.Signature.Recv() == nil {
			continue
		}

		if len(ssafn.Blocks) == 0 {
			// External function
			continue
		}

		reads := map[*ssa.BasicBlock]map[ssa.Value]bool{}
		writes := map[*ssa.BasicBlock]map[ssa.Value]bool{}

		recv := ssafn.Params[0]
		if _, ok := recv.Type().Underlying().(*types.Struct); !ok {
			continue
		}
		recvPtrs := map[ssa.Value]bool{
			recv: true,
		}
		if len(ssafn.Locals) == 0 || ssafn.Locals[0].Heap {
			continue
		}
		blocks := ssafn.DomPreorder()
		for _, block := range blocks {
			if writes[block] == nil {
				writes[block] = map[ssa.Value]bool{}
			}
			if reads[block] == nil {
				reads[block] = map[ssa.Value]bool{}
			}

			for _, ins := range block.Instrs {
				switch ins := ins.(type) {
				case *ssa.Store:
					if recvPtrs[ins.Val] {
						recvPtrs[ins.Addr] = true
					}
					fa, ok := ins.Addr.(*ssa.FieldAddr)
					if !ok {
						continue
					}
					if !recvPtrs[fa.X] {
						continue
					}
					writes[block][fa] = true
				case *ssa.UnOp:
					if ins.Op != token.MUL {
						continue
					}
					if recvPtrs[ins.X] {
						reads[block][ins] = true
						continue
					}
					fa, ok := ins.X.(*ssa.FieldAddr)
					if !ok {
						continue
					}
					if !recvPtrs[fa.X] {
						continue
					}
					reads[block][fa] = true
				}
			}
		}

		for block, writes := range writes {
			seen := map[*ssa.BasicBlock]bool{}
			var hasRead func(block *ssa.BasicBlock, write *ssa.FieldAddr) bool
			hasRead = func(block *ssa.BasicBlock, write *ssa.FieldAddr) bool {
				seen[block] = true
				for read := range reads[block] {
					switch ins := read.(type) {
					case *ssa.FieldAddr:
						if ins.Field == write.Field && read.Pos() > write.Pos() {
							return true
						}
					case *ssa.UnOp:
						if ins.Pos() >= write.Pos() {
							return true
						}
					}
				}
				for _, succ := range block.Succs {
					if !seen[succ] {
						if hasRead(succ, write) {
							return true
						}
					}
				}
				return false
			}
			for write := range writes {
				fa := write.(*ssa.FieldAddr)
				if !hasRead(block, fa) {
					name := recv.Type().Underlying().(*types.Struct).Field(fa.Field).Name()
					j.Errorf(fa, "ineffective assignment to field %s", name)
				}
			}
		}
	}
}

func (c *Checker) CheckUnreadVariableValues(j *lint.Job) {
	fn := func(node ast.Node) bool {
		fn, ok := node.(*ast.FuncDecl)
		if !ok {
			return true
		}
		ssafn := c.nodeFns[fn]
		if ssafn == nil {
			return true
		}
		ast.Inspect(fn, func(node ast.Node) bool {
			assign, ok := node.(*ast.AssignStmt)
			if !ok {
				return true
			}
			if len(assign.Lhs) > 1 && len(assign.Rhs) == 1 {
				// Either a function call with multiple return values,
				// or a comma-ok assignment

				val, _ := ssafn.ValueForExpr(assign.Rhs[0])
				if val == nil {
					return true
				}
				refs := val.Referrers()
				if refs == nil {
					return true
				}
				for _, ref := range *refs {
					ex, ok := ref.(*ssa.Extract)
					if !ok {
						continue
					}
					exrefs := ex.Referrers()
					if exrefs == nil {
						continue
					}
					if len(lint.FilterDebug(*exrefs)) == 0 {
						lhs := assign.Lhs[ex.Index]
						if ident, ok := lhs.(*ast.Ident); !ok || ok && ident.Name == "_" {
							continue
						}
						j.Errorf(lhs, "this value of %s is never used", lhs)
					}
				}
				return true
			}
			for i, lhs := range assign.Lhs {
				rhs := assign.Rhs[i]
				if ident, ok := lhs.(*ast.Ident); !ok || ok && ident.Name == "_" {
					continue
				}
				val, _ := ssafn.ValueForExpr(rhs)
				if val == nil {
					continue
				}

				refs := val.Referrers()
				if refs == nil {
					// TODO investigate why refs can be nil
					return true
				}
				if len(lint.FilterDebug(*refs)) == 0 {
					j.Errorf(lhs, "this value of %s is never used", lhs)
				}
			}
			return true
		})
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckPredeterminedBooleanExprs(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				ssabinop, ok := ins.(*ssa.BinOp)
				if !ok {
					continue
				}
				switch ssabinop.Op {
				case token.GTR, token.LSS, token.EQL, token.NEQ, token.LEQ, token.GEQ:
				default:
					continue
				}

				xs, ok1 := consts(ssabinop.X, nil, nil)
				ys, ok2 := consts(ssabinop.Y, nil, nil)
				if !ok1 || !ok2 || len(xs) == 0 || len(ys) == 0 {
					continue
				}

				trues := 0
				for _, x := range xs {
					for _, y := range ys {
						if x.Value == nil {
							if y.Value == nil {
								trues++
							}
							continue
						}
						if constant.Compare(x.Value, ssabinop.Op, y.Value) {
							trues++
						}
					}
				}
				b := trues != 0
				if trues == 0 || trues == len(xs)*len(ys) {
					j.Errorf(ssabinop, "binary expression is always %t for all possible values (%s %s %s)",
						b, xs, ssabinop.Op, ys)
				}
			}
		}
	}
}

func (c *Checker) CheckNilMaps(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				mu, ok := ins.(*ssa.MapUpdate)
				if !ok {
					continue
				}
				c, ok := mu.Map.(*ssa.Const)
				if !ok {
					continue
				}
				if c.Value != nil {
					continue
				}
				j.Errorf(mu, "assignment to nil map")
			}
		}
	}
}

func (c *Checker) CheckUnsignedComparison(j *lint.Job) {
	fn := func(node ast.Node) bool {
		expr, ok := node.(*ast.BinaryExpr)
		if !ok {
			return true
		}
		tx := j.Program.Info.TypeOf(expr.X)
		basic, ok := tx.Underlying().(*types.Basic)
		if !ok {
			return true
		}
		if (basic.Info() & types.IsUnsigned) == 0 {
			return true
		}
		lit, ok := expr.Y.(*ast.BasicLit)
		if !ok || lit.Value != "0" {
			return true
		}
		switch expr.Op {
		case token.GEQ:
			j.Errorf(expr, "unsigned values are always >= 0")
		case token.LSS:
			j.Errorf(expr, "unsigned values are never < 0")
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func consts(val ssa.Value, out []*ssa.Const, visitedPhis map[string]bool) ([]*ssa.Const, bool) {
	if visitedPhis == nil {
		visitedPhis = map[string]bool{}
	}
	var ok bool
	switch val := val.(type) {
	case *ssa.Phi:
		if visitedPhis[val.Name()] {
			break
		}
		visitedPhis[val.Name()] = true
		vals := val.Operands(nil)
		for _, phival := range vals {
			out, ok = consts(*phival, out, visitedPhis)
			if !ok {
				return nil, false
			}
		}
	case *ssa.Const:
		out = append(out, val)
	case *ssa.Convert:
		out, ok = consts(val.X, out, visitedPhis)
		if !ok {
			return nil, false
		}
	default:
		return nil, false
	}
	if len(out) < 2 {
		return out, true
	}
	uniq := []*ssa.Const{out[0]}
	for _, val := range out[1:] {
		if val.Value == uniq[len(uniq)-1].Value {
			continue
		}
		uniq = append(uniq, val)
	}
	return uniq, true
}

func (c *Checker) CheckLoopCondition(j *lint.Job) {
	fn := func(node ast.Node) bool {
		loop, ok := node.(*ast.ForStmt)
		if !ok {
			return true
		}
		if loop.Init == nil || loop.Cond == nil || loop.Post == nil {
			return true
		}
		init, ok := loop.Init.(*ast.AssignStmt)
		if !ok || len(init.Lhs) != 1 || len(init.Rhs) != 1 {
			return true
		}
		cond, ok := loop.Cond.(*ast.BinaryExpr)
		if !ok {
			return true
		}
		x, ok := cond.X.(*ast.Ident)
		if !ok {
			return true
		}
		lhs, ok := init.Lhs[0].(*ast.Ident)
		if !ok {
			return true
		}
		if x.Obj != lhs.Obj {
			return true
		}
		if _, ok := loop.Post.(*ast.IncDecStmt); !ok {
			return true
		}

		ssafn := j.EnclosingSSAFunction(cond)
		if ssafn == nil {
			return true
		}
		v, isAddr := ssafn.ValueForExpr(cond.X)
		if v == nil || isAddr {
			return true
		}
		switch v := v.(type) {
		case *ssa.Phi:
			ops := v.Operands(nil)
			if len(ops) != 2 {
				return true
			}
			_, ok := (*ops[0]).(*ssa.Const)
			if !ok {
				return true
			}
			sigma, ok := (*ops[1]).(*ssa.Sigma)
			if !ok {
				return true
			}
			if sigma.X != v {
				return true
			}
		case *ssa.UnOp:
			return true
		}
		j.Errorf(cond, "variable in loop condition never changes")

		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckArgOverwritten(j *lint.Job) {
	fn := func(node ast.Node) bool {
		fn, ok := node.(*ast.FuncDecl)
		if !ok {
			return true
		}
		if fn.Body == nil {
			return true
		}
		ssafn := c.nodeFns[fn]
		if ssafn == nil {
			return true
		}
		if len(fn.Type.Params.List) == 0 {
			return true
		}
		for _, field := range fn.Type.Params.List {
			for _, arg := range field.Names {
				obj := j.Program.Info.ObjectOf(arg)
				var ssaobj *ssa.Parameter
				for _, param := range ssafn.Params {
					if param.Object() == obj {
						ssaobj = param
						break
					}
				}
				if ssaobj == nil {
					continue
				}
				refs := ssaobj.Referrers()
				if refs == nil {
					continue
				}
				if len(lint.FilterDebug(*refs)) != 0 {
					continue
				}

				assigned := false
				ast.Inspect(fn.Body, func(node ast.Node) bool {
					assign, ok := node.(*ast.AssignStmt)
					if !ok {
						return true
					}
					for _, lhs := range assign.Lhs {
						ident, ok := lhs.(*ast.Ident)
						if !ok {
							continue
						}
						if j.Program.Info.ObjectOf(ident) == obj {
							assigned = true
							return false
						}
					}
					return true
				})
				if assigned {
					j.Errorf(arg, "argument %s is overwritten before first use", arg)
				}
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckIneffectiveLoop(j *lint.Job) {
	// This check detects some, but not all unconditional loop exits.
	// We give up in the following cases:
	//
	// - a goto anywhere in the loop. The goto might skip over our
	// return, and we don't check that it doesn't.
	//
	// - any nested, unlabelled continue, even if it is in another
	// loop or closure.
	fn := func(node ast.Node) bool {
		fn, ok := node.(*ast.FuncDecl)
		if !ok {
			return true
		}
		if fn.Body == nil {
			return true
		}
		labels := map[*ast.Object]ast.Stmt{}
		ast.Inspect(fn.Body, func(node ast.Node) bool {
			label, ok := node.(*ast.LabeledStmt)
			if !ok {
				return true
			}
			labels[label.Label.Obj] = label.Stmt
			return true
		})

		ast.Inspect(fn.Body, func(node ast.Node) bool {
			var loop ast.Node
			var body *ast.BlockStmt
			switch node := node.(type) {
			case *ast.ForStmt:
				body = node.Body
				loop = node
			case *ast.RangeStmt:
				typ := j.Program.Info.TypeOf(node.X)
				if _, ok := typ.Underlying().(*types.Map); ok {
					// looping once over a map is a valid pattern for
					// getting an arbitrary element.
					return true
				}
				body = node.Body
				loop = node
			default:
				return true
			}
			if len(body.List) < 2 {
				// avoid flagging the somewhat common pattern of using
				// a range loop to get the first element in a slice,
				// or the first rune in a string.
				return true
			}
			var unconditionalExit ast.Node
			hasBranching := false
			for _, stmt := range body.List {
				switch stmt := stmt.(type) {
				case *ast.BranchStmt:
					switch stmt.Tok {
					case token.BREAK:
						if stmt.Label == nil || labels[stmt.Label.Obj] == loop {
							unconditionalExit = stmt
						}
					case token.CONTINUE:
						if stmt.Label == nil || labels[stmt.Label.Obj] == loop {
							unconditionalExit = nil
							return false
						}
					}
				case *ast.ReturnStmt:
					unconditionalExit = stmt
				case *ast.IfStmt, *ast.ForStmt, *ast.RangeStmt, *ast.SwitchStmt, *ast.SelectStmt:
					hasBranching = true
				}
			}
			if unconditionalExit == nil || !hasBranching {
				return false
			}
			ast.Inspect(body, func(node ast.Node) bool {
				if branch, ok := node.(*ast.BranchStmt); ok {

					switch branch.Tok {
					case token.GOTO:
						unconditionalExit = nil
						return false
					case token.CONTINUE:
						if branch.Label != nil && labels[branch.Label.Obj] != loop {
							return true
						}
						unconditionalExit = nil
						return false
					}
				}
				return true
			})
			if unconditionalExit != nil {
				j.Errorf(unconditionalExit, "the surrounding loop is unconditionally terminated")
			}
			return true
		})
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckNilContext(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		if len(call.Args) == 0 {
			return true
		}
		if typ, ok := j.Program.Info.TypeOf(call.Args[0]).(*types.Basic); !ok || typ.Kind() != types.UntypedNil {
			return true
		}
		sig, ok := j.Program.Info.TypeOf(call.Fun).(*types.Signature)
		if !ok {
			return true
		}
		if sig.Params().Len() == 0 {
			return true
		}
		if types.TypeString(sig.Params().At(0).Type(), nil) != "context.Context" {
			return true
		}
		j.Errorf(call.Args[0],
			"do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use")
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckSeeker(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		sel, ok := call.Fun.(*ast.SelectorExpr)
		if !ok {
			return true
		}
		if sel.Sel.Name != "Seek" {
			return true
		}
		if len(call.Args) != 2 {
			return true
		}
		arg0, ok := call.Args[0].(*ast.SelectorExpr)
		if !ok {
			return true
		}
		switch arg0.Sel.Name {
		case "SeekStart", "SeekCurrent", "SeekEnd":
		default:
			return true
		}
		pkg, ok := arg0.X.(*ast.Ident)
		if !ok {
			return true
		}
		if pkg.Name != "io" {
			return true
		}
		j.Errorf(call, "the first argument of io.Seeker is the offset, but an io.Seek* constant is being used instead")
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckConcurrentTesting(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				gostmt, ok := ins.(*ssa.Go)
				if !ok {
					continue
				}
				var fn *ssa.Function
				switch val := gostmt.Call.Value.(type) {
				case *ssa.Function:
					fn = val
				case *ssa.MakeClosure:
					fn = val.Fn.(*ssa.Function)
				default:
					continue
				}
				if fn.Blocks == nil {
					continue
				}
				for _, block := range fn.Blocks {
					for _, ins := range block.Instrs {
						call, ok := ins.(*ssa.Call)
						if !ok {
							continue
						}
						if call.Call.IsInvoke() {
							continue
						}
						callee := call.Call.StaticCallee()
						if callee == nil {
							continue
						}
						recv := callee.Signature.Recv()
						if recv == nil {
							continue
						}
						if types.TypeString(recv.Type(), nil) != "*testing.common" {
							continue
						}
						fn, ok := call.Call.StaticCallee().Object().(*types.Func)
						if !ok {
							continue
						}
						name := fn.Name()
						switch name {
						case "FailNow", "Fatal", "Fatalf", "SkipNow", "Skip", "Skipf":
						default:
							continue
						}
						j.Errorf(gostmt, "the goroutine calls T.%s, which must be called in the same goroutine as the test", name)
					}
				}
			}
		}
	}
}

func (c *Checker) CheckCyclicFinalizer(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		node := c.funcDescs.CallGraph.CreateNode(ssafn)
		for _, edge := range node.Out {
			if edge.Callee.Func.RelString(nil) != "runtime.SetFinalizer" {
				continue
			}
			arg0 := edge.Site.Common().Args[0]
			if iface, ok := arg0.(*ssa.MakeInterface); ok {
				arg0 = iface.X
			}
			unop, ok := arg0.(*ssa.UnOp)
			if !ok {
				continue
			}
			v, ok := unop.X.(*ssa.Alloc)
			if !ok {
				continue
			}
			arg1 := edge.Site.Common().Args[1]
			if iface, ok := arg1.(*ssa.MakeInterface); ok {
				arg1 = iface.X
			}
			mc, ok := arg1.(*ssa.MakeClosure)
			if !ok {
				continue
			}
			for _, b := range mc.Bindings {
				if b == v {
					pos := j.Program.SSA.Fset.Position(mc.Fn.Pos())
					j.Errorf(edge.Site, "the finalizer closes over the object, preventing the finalizer from ever running (at %s)", pos)
				}
			}
		}
	}
}

func (c *Checker) CheckSliceOutOfBounds(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				ia, ok := ins.(*ssa.IndexAddr)
				if !ok {
					continue
				}
				if _, ok := ia.X.Type().Underlying().(*types.Slice); !ok {
					continue
				}
				sr, ok1 := c.funcDescs.Get(ssafn).Ranges[ia.X].(vrp.SliceInterval)
				idxr, ok2 := c.funcDescs.Get(ssafn).Ranges[ia.Index].(vrp.IntInterval)
				if !ok1 || !ok2 || !sr.IsKnown() || !idxr.IsKnown() || sr.Length.Empty() || idxr.Empty() {
					continue
				}
				if idxr.Lower.Cmp(sr.Length.Upper) >= 0 {
					j.Errorf(ia, "index out of bounds")
				}
			}
		}
	}
}

func (c *Checker) CheckDeferLock(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			if len(block.Instrs) < 2 {
				continue
			}
			instrs := lint.FilterDebug(block.Instrs)
			for i, ins := range instrs[:len(instrs)-1] {
				call, ok := ins.(*ssa.Call)
				if !ok {
					continue
				}
				if !lint.IsCallTo(call.Common(), "(*sync.Mutex).Lock") && !lint.IsCallTo(call.Common(), "(*sync.RWMutex).RLock") {
					continue
				}
				nins, ok := instrs[i+1].(*ssa.Defer)
				if !ok {
					continue
				}
				if !lint.IsCallTo(&nins.Call, "(*sync.Mutex).Lock") && !lint.IsCallTo(&nins.Call, "(*sync.RWMutex).RLock") {
					continue
				}
				if call.Common().Args[0] != nins.Call.Args[0] {
					continue
				}
				name := shortCallName(call.Common())
				alt := ""
				switch name {
				case "Lock":
					alt = "Unlock"
				case "RLock":
					alt = "RUnlock"
				}
				j.Errorf(nins, "deferring %s right after having locked already; did you mean to defer %s?", name, alt)
			}
		}
	}
}

func (c *Checker) CheckNaNComparison(j *lint.Job) {
	isNaN := func(v ssa.Value) bool {
		call, ok := v.(*ssa.Call)
		if !ok {
			return false
		}
		return lint.IsCallTo(call.Common(), "math.NaN")
	}
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				ins, ok := ins.(*ssa.BinOp)
				if !ok {
					continue
				}
				if isNaN(ins.X) || isNaN(ins.Y) {
					j.Errorf(ins, "no value is equal to NaN, not even NaN itself")
				}
			}
		}
	}
}

func (c *Checker) CheckInfiniteRecursion(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		node := c.funcDescs.CallGraph.CreateNode(ssafn)
		for _, edge := range node.Out {
			if edge.Callee != node {
				continue
			}

			block := edge.Site.Block()
			canReturn := false
			for _, b := range ssafn.Blocks {
				if block.Dominates(b) {
					continue
				}
				if len(b.Instrs) == 0 {
					continue
				}
				if _, ok := b.Instrs[len(b.Instrs)-1].(*ssa.Return); ok {
					canReturn = true
					break
				}
			}
			if canReturn {
				continue
			}
			j.Errorf(edge.Site, "infinite recursive call")
		}
	}
}

func objectName(obj types.Object) string {
	if obj == nil {
		return "<nil>"
	}
	var name string
	if obj.Pkg() != nil && obj.Pkg().Scope().Lookup(obj.Name()) == obj {
		var s string
		s = obj.Pkg().Path()
		if s != "" {
			name += s + "."
		}
	}
	name += obj.Name()
	return name
}

func isName(j *lint.Job, expr ast.Expr, name string) bool {
	var obj types.Object
	switch expr := expr.(type) {
	case *ast.Ident:
		obj = j.Program.Info.ObjectOf(expr)
	case *ast.SelectorExpr:
		obj = j.Program.Info.ObjectOf(expr.Sel)
	}
	return objectName(obj) == name
}

func (c *Checker) CheckLeakyTimeTick(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		if j.IsInMain(ssafn) || j.IsInTest(ssafn) {
			continue
		}
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				call, ok := ins.(*ssa.Call)
				if !ok || !lint.IsCallTo(call.Common(), "time.Tick") {
					continue
				}
				if c.funcDescs.Get(call.Parent()).Infinite {
					continue
				}
				j.Errorf(call, "using time.Tick leaks the underlying ticker, consider using it only in endless functions, tests and the main package, and use time.NewTicker here")
			}
		}
	}
}

func (c *Checker) CheckDoubleNegation(j *lint.Job) {
	fn := func(node ast.Node) bool {
		unary1, ok := node.(*ast.UnaryExpr)
		if !ok {
			return true
		}
		unary2, ok := unary1.X.(*ast.UnaryExpr)
		if !ok {
			return true
		}
		if unary1.Op != token.NOT || unary2.Op != token.NOT {
			return true
		}
		j.Errorf(unary1, "negating a boolean twice has no effect; is this a typo?")
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckRepeatedIfElse(j *lint.Job) {
	seen := map[ast.Node]bool{}

	var collectConds func(ifstmt *ast.IfStmt, inits []ast.Stmt, conds []ast.Expr) ([]ast.Stmt, []ast.Expr)
	collectConds = func(ifstmt *ast.IfStmt, inits []ast.Stmt, conds []ast.Expr) ([]ast.Stmt, []ast.Expr) {
		seen[ifstmt] = true
		if ifstmt.Init != nil {
			inits = append(inits, ifstmt.Init)
		}
		conds = append(conds, ifstmt.Cond)
		if elsestmt, ok := ifstmt.Else.(*ast.IfStmt); ok {
			return collectConds(elsestmt, inits, conds)
		}
		return inits, conds
	}
	isDynamic := func(node ast.Node) bool {
		dynamic := false
		ast.Inspect(node, func(node ast.Node) bool {
			switch node := node.(type) {
			case *ast.CallExpr:
				dynamic = true
				return false
			case *ast.UnaryExpr:
				if node.Op == token.ARROW {
					dynamic = true
					return false
				}
			}
			return true
		})
		return dynamic
	}
	fn := func(node ast.Node) bool {
		ifstmt, ok := node.(*ast.IfStmt)
		if !ok {
			return true
		}
		if seen[ifstmt] {
			return true
		}
		inits, conds := collectConds(ifstmt, nil, nil)
		if len(inits) > 0 {
			return true
		}
		for _, cond := range conds {
			if isDynamic(cond) {
				return true
			}
		}
		counts := map[string]int{}
		for _, cond := range conds {
			s := j.Render(cond)
			counts[s]++
			if counts[s] == 2 {
				j.Errorf(cond, "this condition occurs multiple times in this if/else if chain")
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckSillyBitwiseOps(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				ins, ok := ins.(*ssa.BinOp)
				if !ok {
					continue
				}

				if c, ok := ins.Y.(*ssa.Const); !ok || c.Value == nil || c.Value.Kind() != constant.Int || c.Uint64() != 0 {
					continue
				}
				switch ins.Op {
				case token.AND, token.OR, token.XOR:
				default:
					// we do not flag shifts because too often, x<<0 is part
					// of a pattern, x<<0, x<<8, x<<16, ...
					continue
				}
				path, _ := astutil.PathEnclosingInterval(j.File(ins), ins.Pos(), ins.Pos())
				if len(path) == 0 {
					continue
				}
				if node, ok := path[0].(*ast.BinaryExpr); !ok || !lint.IsZero(node.Y) {
					continue
				}

				switch ins.Op {
				case token.AND:
					j.Errorf(ins, "x & 0 always equals 0")
				case token.OR, token.XOR:
					j.Errorf(ins, "x %s 0 always equals x", ins.Op)
				}
			}
		}
	}
}

func (c *Checker) CheckNonOctalFileMode(j *lint.Job) {
	fn := func(node ast.Node) bool {
		call, ok := node.(*ast.CallExpr)
		if !ok {
			return true
		}
		sig, ok := j.Program.Info.TypeOf(call.Fun).(*types.Signature)
		if !ok {
			return true
		}
		n := sig.Params().Len()
		var args []int
		for i := 0; i < n; i++ {
			typ := sig.Params().At(i).Type()
			if types.TypeString(typ, nil) == "os.FileMode" {
				args = append(args, i)
			}
		}
		for _, i := range args {
			lit, ok := call.Args[i].(*ast.BasicLit)
			if !ok {
				continue
			}
			if len(lit.Value) == 3 &&
				lit.Value[0] != '0' &&
				lit.Value[0] >= '0' && lit.Value[0] <= '7' &&
				lit.Value[1] >= '0' && lit.Value[1] <= '7' &&
				lit.Value[2] >= '0' && lit.Value[2] <= '7' {

				v, err := strconv.ParseInt(lit.Value, 10, 64)
				if err != nil {
					continue
				}
				j.Errorf(call.Args[i], "file mode '%s' evaluates to %#o; did you mean '0%s'?", lit.Value, v, lit.Value)
			}
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) CheckPureFunctions(j *lint.Job) {
fnLoop:
	for _, ssafn := range j.Program.InitialFunctions {
		if j.IsInTest(ssafn) {
			params := ssafn.Signature.Params()
			for i := 0; i < params.Len(); i++ {
				param := params.At(i)
				if types.TypeString(param.Type(), nil) == "*testing.B" {
					// Ignore discarded pure functions in code related
					// to benchmarks. Instead of matching BenchmarkFoo
					// functions, we match any function accepting a
					// *testing.B. Benchmarks sometimes call generic
					// functions for doing the actual work, and
					// checking for the parameter is a lot easier and
					// faster than analyzing call trees.
					continue fnLoop
				}
			}
		}

		for _, b := range ssafn.Blocks {
			for _, ins := range b.Instrs {
				ins, ok := ins.(*ssa.Call)
				if !ok {
					continue
				}
				refs := ins.Referrers()
				if refs == nil || len(lint.FilterDebug(*refs)) > 0 {
					continue
				}
				callee := ins.Common().StaticCallee()
				if callee == nil {
					continue
				}
				if c.funcDescs.Get(callee).Pure {
					j.Errorf(ins, "%s is a pure function but its return value is ignored", callee.Name())
					continue
				}
			}
		}
	}
}

func enclosingFunction(j *lint.Job, node ast.Node) *ast.FuncDecl {
	f := j.File(node)
	path, _ := astutil.PathEnclosingInterval(f, node.Pos(), node.Pos())
	for _, e := range path {
		fn, ok := e.(*ast.FuncDecl)
		if !ok {
			continue
		}
		if fn.Name == nil {
			continue
		}
		return fn
	}
	return nil
}

func (c *Checker) isDeprecated(j *lint.Job, ident *ast.Ident) (bool, string) {
	obj := j.Program.Info.ObjectOf(ident)
	if obj.Pkg() == nil {
		return false, ""
	}
	alt := c.deprecatedObjs[obj]
	return alt != "", alt
}

func (c *Checker) CheckDeprecated(j *lint.Job) {
	fn := func(node ast.Node) bool {
		sel, ok := node.(*ast.SelectorExpr)
		if !ok {
			return true
		}
		if fn := enclosingFunction(j, sel); fn != nil {
			if ok, _ := c.isDeprecated(j, fn.Name); ok {
				// functions that are deprecated may use deprecated
				// symbols
				return true
			}
		}

		obj := j.Program.Info.ObjectOf(sel.Sel)
		if obj.Pkg() == nil {
			return true
		}
		nodePkg := j.NodePackage(node).Pkg
		if nodePkg == obj.Pkg() || obj.Pkg().Path()+"_test" == nodePkg.Path() {
			// Don't flag stuff in our own package
			return true
		}
		if ok, alt := c.isDeprecated(j, sel.Sel); ok {
			j.Errorf(sel, "%s is deprecated: %s", j.Render(sel), alt)
			return true
		}
		return true
	}
	for _, f := range j.Program.Files {
		ast.Inspect(f, fn)
	}
}

func (c *Checker) callChecker(rules map[string]CallCheck) func(j *lint.Job) {
	return func(j *lint.Job) {
		c.checkCalls(j, rules)
	}
}

func (c *Checker) checkCalls(j *lint.Job, rules map[string]CallCheck) {
	for _, ssafn := range j.Program.InitialFunctions {
		node := c.funcDescs.CallGraph.CreateNode(ssafn)
		for _, edge := range node.Out {
			callee := edge.Callee.Func
			obj, ok := callee.Object().(*types.Func)
			if !ok {
				continue
			}

			r, ok := rules[obj.FullName()]
			if !ok {
				continue
			}
			var args []*Argument
			ssaargs := edge.Site.Common().Args
			if callee.Signature.Recv() != nil {
				ssaargs = ssaargs[1:]
			}
			for _, arg := range ssaargs {
				if iarg, ok := arg.(*ssa.MakeInterface); ok {
					arg = iarg.X
				}
				vr := c.funcDescs.Get(edge.Site.Parent()).Ranges[arg]
				args = append(args, &Argument{Value: Value{arg, vr}})
			}
			call := &Call{
				Job:     j,
				Instr:   edge.Site,
				Args:    args,
				Checker: c,
				Parent:  edge.Site.Parent(),
			}
			r(call)
			for idx, arg := range call.Args {
				_ = idx
				for _, e := range arg.invalids {
					// path, _ := astutil.PathEnclosingInterval(f.File, edge.Site.Pos(), edge.Site.Pos())
					// if len(path) < 2 {
					// 	continue
					// }
					// astcall, ok := path[0].(*ast.CallExpr)
					// if !ok {
					// 	continue
					// }
					// j.Errorf(astcall.Args[idx], "%s", e)

					j.Errorf(edge.Site, "%s", e)
				}
			}
			for _, e := range call.invalids {
				j.Errorf(call.Instr.Common(), "%s", e)
			}
		}
	}
}

func (c *Checker) CheckFlagUsage(j *lint.Job) {
	for _, ssafn := range j.Program.InitialFunctions {
		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				store, ok := ins.(*ssa.Store)
				if !ok {
					continue
				}
				switch addr := store.Addr.(type) {
				case *ssa.FieldAddr:
					typ := addr.X.Type()
					st := deref(typ).Underlying().(*types.Struct)
					if types.TypeString(typ, nil) != "*flag.FlagSet" {
						continue
					}
					if st.Field(addr.Field).Name() != "Usage" {
						continue
					}
				case *ssa.Global:
					if addr.Pkg.Pkg.Path() != "flag" || addr.Name() != "Usage" {
						continue
					}
				default:
					continue
				}
				fn := unwrapFunction(store.Val)
				if fn == nil {
					continue
				}
				for _, oblock := range fn.Blocks {
					if hasCallTo(oblock, "os.Exit") {
						j.Errorf(store, "the function assigned to Usage shouldn't call os.Exit, but it does")
						break
					}
				}
			}
		}
	}
}

func unwrapFunction(val ssa.Value) *ssa.Function {
	switch val := val.(type) {
	case *ssa.Function:
		return val
	case *ssa.MakeClosure:
		return val.Fn.(*ssa.Function)
	default:
		return nil
	}
}

func shortCallName(call *ssa.CallCommon) string {
	if call.IsInvoke() {
		return ""
	}
	switch v := call.Value.(type) {
	case *ssa.Function:
		fn, ok := v.Object().(*types.Func)
		if !ok {
			return ""
		}
		return fn.Name()
	case *ssa.Builtin:
		return v.Name()
	}
	return ""
}

func hasCallTo(block *ssa.BasicBlock, name string) bool {
	for _, ins := range block.Instrs {
		call, ok := ins.(*ssa.Call)
		if !ok {
			continue
		}
		if lint.IsCallTo(call.Common(), name) {
			return true
		}
	}
	return false
}

// deref returns a pointer's element type; otherwise it returns typ.
func deref(typ types.Type) types.Type {
	if p, ok := typ.Underlying().(*types.Pointer); ok {
		return p.Elem()
	}
	return typ
}

func (c *Checker) CheckWriterBufferModified(j *lint.Job) {
	// TODO(dh): this might be a good candidate for taint analysis.
	// Taint the argument as MUST_NOT_MODIFY, then propagate that
	// through functions like bytes.Split

	for _, ssafn := range j.Program.InitialFunctions {
		sig := ssafn.Signature
		if ssafn.Name() != "Write" || sig.Recv() == nil || sig.Params().Len() != 1 || sig.Results().Len() != 2 {
			continue
		}
		tArg, ok := sig.Params().At(0).Type().(*types.Slice)
		if !ok {
			continue
		}
		if basic, ok := tArg.Elem().(*types.Basic); !ok || basic.Kind() != types.Byte {
			continue
		}
		if basic, ok := sig.Results().At(0).Type().(*types.Basic); !ok || basic.Kind() != types.Int {
			continue
		}
		if named, ok := sig.Results().At(1).Type().(*types.Named); !ok || types.TypeString(named, nil) != "error" {
			continue
		}

		for _, block := range ssafn.Blocks {
			for _, ins := range block.Instrs {
				switch ins := ins.(type) {
				case *ssa.Store:
					addr, ok := ins.Addr.(*ssa.IndexAddr)
					if !ok {
						continue
					}
					if addr.X != ssafn.Params[1] {
						continue
					}
					j.Errorf(ins, "io.Writer.Write must not modify the provided buffer, not even temporarily")
				case *ssa.Call:
					if !lint.IsCallTo(ins.Common(), "append") {
						continue
					}
					if ins.Common().Args[0] != ssafn.Params[1] {
						continue
					}
					j.Errorf(ins, "io.Writer.Write must not modify the provided buffer, not even temporarily")
				}
			}
		}
	}
}

func loopedRegexp(name string) CallCheck {
	return func(call *Call) {
		if len(extractConsts(call.Args[0].Value.Value)) == 0 {
			return
		}
		if !call.Checker.isInLoop(call.Instr.Block()) {
			return
		}
		call.Invalid(fmt.Sprintf("calling %s in a loop has poor performance, consider using regexp.Compile", name))
	}
}

func (c *Checker) CheckEmptyBranch(j *lint.Job) {
	fn := func(node ast.Node) bool {
		ifstmt, ok := node.(*ast.IfStmt)
		if !ok {
			return true
		}
		if ifstmt.Else != nil {
			b, ok := ifstmt.Else.(*ast.BlockStmt)
			if !ok || len(b.List) != 0 {
				return true
			}
			j.Errorf(ifstmt.Else, "empty branch")
		}
		if len(ifstmt.Body.List) != 0 {
			return true
		}
		j.Errorf(ifstmt, "empty branch")
		return true
	}
	for _, f := range c.filterGenerated(j.Program.Files) {
		ast.Inspect(f, fn)
	}
}