go - sync/mutex.go 中的 “_ = variable” 是什么意思

Question

同步/mutex.go:

func (m *Mutex) Unlock() {
    if race.Enabled {
         _ = m.state
         race.Release(unsafe.Pointer(m))
     }
...

_ = m.state是什么意思？

我知道 var _ interface = Object 的意思是检查 Object 是否实现了接口(interface)。

Answer 1

从提交日志来看，原因是“_ = m.state”确保m不为nil。

commit 5bb3a66a973ea87494b9197091e8c1f122080627
Author: Rémy Oudompheng <oudomphe@phare.normalesup.org>
Date:   Mon Apr 8 23:46:54 2013 +0200

    sync, sync/atomic: do not corrupt race detector after a nil dereference.

    The race detector uses a global lock to analyze atomic
    operations. A panic in the middle of the code leaves the
    lock acquired.

    Similarly, the sync package may leave the race detectro
    inconsistent when methods are called on nil pointers.

    R=golang-dev, r, minux.ma, dvyukov, rsc, adg
    CC=golang-dev
    https://golang.org/cl/7981043