cmd/vet: teach vet that putting a large value in an interface makes a copy

[josharian]

package main

import "sync"

type T struct {
    sync.Mutex
    _ [4]byte
}

func main() {
    var t T

    var s T
    s = t

    var e interface{}
    e = t
}

$ go vet x.go
x.go:14: assignment copies lock value to s: main.T
exit status 1

vet should also complain about the assignment e = t, since it also copies t's value.

[rogpeppe]

This would have caught some bugs for us.

[josharian]

cc @valyala :)

[valyala]

It looks like this is already fixed in #14664 (go tip):

go vet 1.go
1.go:14: assignment copies lock value to s: main.T
1.go:17: assignment copies lock value to e: main.T

[valyala]

Other cases must be checked such as passing lock as function argument accepting interface{} or returning lock from function returning interface{}

[valyala]

Prepared a CL, which expands copylock check to function calls and return values - see https://go-review.googlesource.com/20832 . This catches passing lock into interface{} in these cases.

[gopherbot]

CL https://golang.org/cl/20832 mentions this issue.