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// Copyright 2023 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.
//
// # Analyzer loopclosure
//
// loopclosure: check references to loop variables from within nested functions
//
// This analyzer reports places where a function literal references the
// iteration variable of an enclosing loop, and the loop calls the function
// in such a way (e.g. with go or defer) that it may outlive the loop
// iteration and possibly observe the wrong value of the variable.
//
// In this example, all the deferred functions run after the loop has
// completed, so all observe the final value of v.
//
// for _, v := range list {
// defer func() {
// use(v) // incorrect
// }()
// }
//
// One fix is to create a new variable for each iteration of the loop:
//
// for _, v := range list {
// v := v // new var per iteration
// defer func() {
// use(v) // ok
// }()
// }
//
// The next example uses a go statement and has a similar problem.
// In addition, it has a data race because the loop updates v
// concurrent with the goroutines accessing it.
//
// for _, v := range elem {
// go func() {
// use(v) // incorrect, and a data race
// }()
// }
//
// A fix is the same as before. The checker also reports problems
// in goroutines started by golang.org/x/sync/errgroup.Group.
// A hard-to-spot variant of this form is common in parallel tests:
//
// func Test(t *testing.T) {
// for _, test := range tests {
// t.Run(test.name, func(t *testing.T) {
// t.Parallel()
// use(test) // incorrect, and a data race
// })
// }
// }
//
// The t.Parallel() call causes the rest of the function to execute
// concurrent with the loop.
//
// The analyzer reports references only in the last statement,
// as it is not deep enough to understand the effects of subsequent
// statements that might render the reference benign.
// ("Last statement" is defined recursively in compound
// statements such as if, switch, and select.)
//
// See: https://golang.org/doc/go_faq.html#closures_and_goroutines
package loopclosure