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// Copyright 2012 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 loopclosure defines an Analyzer that checks for references to
// enclosing loop variables from within nested functions.
package loopclosure
import (
"go/ast"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/ast/inspector"
)
// TODO(adonovan): also report an error for the following structure,
// which is often used to ensure that deferred calls do not accumulate
// in a loop:
//
// for i, x := range c {
// func() {
// ...reference to i or x...
// }()
// }
const Doc = `check references to loop variables from within nested functions
This analyzer checks for references to loop variables from within a
function literal inside the loop body. It checks only instances where
the function literal is called in a defer or go statement that is the
last statement in the loop body, as otherwise we would need whole
program analysis.
For example:
for i, v := range s {
go func() {
println(i, v) // not what you might expect
}()
}
See: https://golang.org/doc/go_faq.html#closures_and_goroutines`
var Analyzer = &analysis.Analyzer{
Name: "loopclosure",
Doc: Doc,
Requires: []*analysis.Analyzer{inspect.Analyzer},
Run: run,
}
func run(pass *analysis.Pass) (interface{}, error) {
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
nodeFilter := []ast.Node{
(*ast.RangeStmt)(nil),
(*ast.ForStmt)(nil),
}
inspect.Preorder(nodeFilter, func(n ast.Node) {
// Find the variables updated by the loop statement.
var vars []*ast.Ident
addVar := func(expr ast.Expr) {
if id, ok := expr.(*ast.Ident); ok {
vars = append(vars, id)
}
}
var body *ast.BlockStmt
switch n := n.(type) {
case *ast.RangeStmt:
body = n.Body
addVar(n.Key)
addVar(n.Value)
case *ast.ForStmt:
body = n.Body
switch post := n.Post.(type) {
case *ast.AssignStmt:
// e.g. for p = head; p != nil; p = p.next
for _, lhs := range post.Lhs {
addVar(lhs)
}
case *ast.IncDecStmt:
// e.g. for i := 0; i < n; i++
addVar(post.X)
}
}
if vars == nil {
return
}
// Inspect a go or defer statement
// if it's the last one in the loop body.
// (We give up if there are following statements,
// because it's hard to prove go isn't followed by wait,
// or defer by return.)
if len(body.List) == 0 {
return
}
var last *ast.CallExpr
switch s := body.List[len(body.List)-1].(type) {
case *ast.GoStmt:
last = s.Call
case *ast.DeferStmt:
last = s.Call
default:
return
}
lit, ok := last.Fun.(*ast.FuncLit)
if !ok {
return
}
ast.Inspect(lit.Body, func(n ast.Node) bool {
id, ok := n.(*ast.Ident)
if !ok || id.Obj == nil {
return true
}
if pass.TypesInfo.Types[id].Type == nil {
// Not referring to a variable (e.g. struct field name)
return true
}
for _, v := range vars {
if v.Obj == id.Obj {
pass.ReportRangef(id, "loop variable %s captured by func literal",
id.Name)
}
}
return true
})
})
return nil, nil
}
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