cmd/vet: do not report variable capture for loop variables with the new lifetime rules

[timothy-king]

This issue tracks support in x/tools/go/analysis/passes/loopclosure for the new loop variable lifetime rules of https://go.dev/issue/60078, available now behind GOEXPERIMENT=loopvar and likely on for language versions >= 1.22.

Marking as a release blocker as this will make cmd/vet would report a large number of false positives due to the new language feature.

[timothy-king]

Example of a false positive https://go.dev/play/p/WsTuyNr6YnK

package main

import (
	"fmt"
	"sync"
)

func main() {
	const N = 4
	var wg sync.WaitGroup
	for i := 0; i < N; i++ {
		wg.Add(1)
		go func() { defer wg.Done(); fmt.Println(i) }()
	}
	wg.Wait()
}

Playground output for Go 1.21:

./prog.go:13:44: loop variable i captured by func literal

Go vet failed.

4
4
4
4

Program exited.

Playground output for Go dev branch:

# [play]
./prog.go:13:44: loop variable i captured by func literal

Go vet failed.

3
2
0
1

Program exited.

The dev currently branch implements the https://go.dev/issue/60078 loop lifetimes. So go vet should not continue to report on this example after 1.22.

[timothy-king]

Note this will need to support packages with a mixture of lifetimes in different files or files without //go: version declared in the files (like the playground example). See #62605 for a potential solution via go/types.

[gopherbot]

Change https://go.dev/cl/539016 mentions this issue: go/analysis/passes/loopclosure: disable checker after go1.22.

[timothy-king]

Fixed by https://go.dev/cl/c/go/+/543975