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package rule
import (
// DeferRule lints unused params in functions.
type DeferRule struct{}
// Apply applies the rule to given file.
func (r *DeferRule) Apply(file *lint.File, arguments lint.Arguments) []lint.Failure {
allow := r.allowFromArgs(arguments)
var failures []lint.Failure
onFailure := func(failure lint.Failure) {
failures = append(failures, failure)
w := lintDeferRule{onFailure: onFailure, allow: allow}
ast.Walk(w, file.AST)
return failures
// Name returns the rule name.
func (r *DeferRule) Name() string {
return "defer"
func (r *DeferRule) allowFromArgs(args lint.Arguments) map[string]bool {
if len(args) < 1 {
allow := map[string]bool{
"loop": true,
"call-chain": true,
"method-call": true,
"return": true,
"recover": true,
return allow
aa, ok := args[0].([]interface{})
if !ok {
panic(fmt.Sprintf("Invalid argument '%v' for 'defer' rule. Expecting []string, got %T", args[0], args[0]))
allow := make(map[string]bool, len(aa))
for _, subcase := range aa {
sc, ok := subcase.(string)
if !ok {
panic(fmt.Sprintf("Invalid argument '%v' for 'defer' rule. Expecting string, got %T", subcase, subcase))
allow[sc] = true
return allow
type lintDeferRule struct {
onFailure func(lint.Failure)
inALoop bool
inADefer bool
inAFuncLit bool
allow map[string]bool
func (w lintDeferRule) Visit(node ast.Node) ast.Visitor {
switch n := node.(type) {
case *ast.ForStmt:
w.visitSubtree(n.Body, w.inADefer, true, w.inAFuncLit)
return nil
case *ast.RangeStmt:
w.visitSubtree(n.Body, w.inADefer, true, w.inAFuncLit)
return nil
case *ast.FuncLit:
w.visitSubtree(n.Body, w.inADefer, false, true)
return nil
case *ast.ReturnStmt:
if len(n.Results) != 0 && w.inADefer && w.inAFuncLit {
w.newFailure("return in a defer function has no effect", n, 1.0, "logic", "return")
case *ast.CallExpr:
if isIdent(n.Fun, "recover") && !w.inADefer {
// confidence is not 1 because recover can be in a function that is deferred elsewhere
w.newFailure("recover must be called inside a deferred function", n, 0.8, "logic", "recover")
case *ast.DeferStmt:
w.visitSubtree(n.Call.Fun, true, false, false)
if w.inALoop {
w.newFailure("prefer not to defer inside loops", n, 1.0, "bad practice", "loop")
switch fn := n.Call.Fun.(type) {
case *ast.CallExpr:
w.newFailure("prefer not to defer chains of function calls", fn, 1.0, "bad practice", "call-chain")
case *ast.SelectorExpr:
if id, ok := fn.X.(*ast.Ident); ok {
isMethodCall := id != nil && id.Obj != nil && id.Obj.Kind == ast.Typ
if isMethodCall {
w.newFailure("be careful when deferring calls to methods without pointer receiver", fn, 0.8, "bad practice", "method-call")
return nil
return w
func (w lintDeferRule) visitSubtree(n ast.Node, inADefer, inALoop, inAFuncLit bool) {
nw := &lintDeferRule{
onFailure: w.onFailure,
inADefer: inADefer,
inALoop: inALoop,
inAFuncLit: inAFuncLit,
allow: w.allow}
ast.Walk(nw, n)
func (w lintDeferRule) newFailure(msg string, node ast.Node, confidence float64, cat string, subcase string) {
if !w.allow[subcase] {
Confidence: confidence,
Node: node,
Category: cat,
Failure: msg,