sync/atomic: remove noCopy from Value

Values must not be copied after the first use.

Using noCopy makes vet complain about copies
even before the first use, which is incorrect
and very frustrating.

Drop it.

Fixes #21504.

Change-Id: Icd3a5ac3fe11e84525b998e848ed18a5d996f45a
Reviewed-on: https://go-review.googlesource.com/80836
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/vet/testdata/copylock.go

@@ -178,9 +178,11 @@ func AtomicTypesCheck() {
 	var vX atomic.Value
 	var vXX = atomic.Value{}
 	vX1 := new(atomic.Value)
-	vY := vX     // ERROR "assignment copies lock value to vY: sync/atomic.Value contains sync/atomic.noCopy"
-	vY = vX      // ERROR "assignment copies lock value to vY: sync/atomic.Value contains sync/atomic.noCopy"
-	var vYY = vX // ERROR "variable declaration copies lock value to vYY: sync/atomic.Value contains sync/atomic.noCopy"
+	// These are OK because the value has not been used yet.
+	// (And vet can't tell whether it has been used, so they're always OK.)
+	vY := vX
+	vY = vX
+	var vYY = vX
 	vP := &vX
 	vZ := &atomic.Value{}
 }

src/sync/atomic/value.go

@@ -14,8 +14,6 @@ import (
 //
 // A Value must not be copied after first use.
 type Value struct {
-	noCopy noCopy
-
 	v interface{}
 }
 
@@ -86,13 +84,3 @@ func (v *Value) Store(x interface{}) {
 // Disable/enable preemption, implemented in runtime.
 func runtime_procPin()
 func runtime_procUnpin()
-
-// noCopy may be embedded into structs which must not be copied
-// after the first use.
-//
-// See https://golang.org/issues/8005#issuecomment-190753527
-// for details.
-type noCopy struct{}
-
-// Lock is a no-op used by -copylocks checker from `go vet`.
-func (*noCopy) Lock() {}