cmd/vet: do not treat declaration as asignment in atomic check

Fixes #15118

Change-Id: Iad56ed412535c8ac0a01c4bd7769fd3d37688ac9
Reviewed-on: https://go-review.googlesource.com/21526
Run-TryBot: Rob Pike <r@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/atomic.go

@@ -23,6 +23,9 @@ func checkAtomicAssignment(f *File, node ast.Node) {
 	if len(n.Lhs) != len(n.Rhs) {
 		return
 	}
+	if len(n.Lhs) == 1 && n.Tok == token.DEFINE {
+		return
+	}
 
 	for i, right := range n.Rhs {
 		call, ok := right.(*ast.CallExpr)

src/cmd/vet/testdata/atomic.go

@@ -40,4 +40,13 @@ func AtomicTests() {
 	*ap[1] = atomic.AddUint64(ap[0], 1)
 
 	x = atomic.AddUint64() // Used to make vet crash; now silently ignored.
+
+	{
+		// A variable declaration creates a new variable in the current scope.
+		x := atomic.AddUint64(&x, 1) // ERROR "declaration of .x. shadows declaration at testdata/atomic.go:16"
+
+		// Re-declaration assigns a new value.
+		x, w := atomic.AddUint64(&x, 1), 10 // ERROR "direct assignment to atomic value"
+		_ = w
+	}
 }