cmd/vet: allow ^& uintptr arithmetic

The unsafe.Pointer check allows adding to
and subtracting from uintptrs in order to do
arithmetic.

Some code needs to round uintptrs.
Allow &^ for that purpose.

Updates #11041

Change-Id: Ib90dd2954bb6c78427058271e13f2ce4c4af38fb
Reviewed-on: https://go-review.googlesource.com/27156
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/cmd/vet/testdata/unsafeptr.go

@@ -15,13 +15,15 @@ func f() {
 	x = unsafe.Pointer(y) // ERROR "possible misuse of unsafe.Pointer"
 	y = uintptr(x)
 
-	// only allowed pointer arithmetic is ptr +/- num.
+	// only allowed pointer arithmetic is ptr +/-/&^ num.
 	// num+ptr is technically okay but still flagged: write ptr+num instead.
 	x = unsafe.Pointer(uintptr(x) + 1)
 	x = unsafe.Pointer(1 + uintptr(x))          // ERROR "possible misuse of unsafe.Pointer"
 	x = unsafe.Pointer(uintptr(x) + uintptr(x)) // ERROR "possible misuse of unsafe.Pointer"
 	x = unsafe.Pointer(uintptr(x) - 1)
 	x = unsafe.Pointer(1 - uintptr(x)) // ERROR "possible misuse of unsafe.Pointer"
+	x = unsafe.Pointer(uintptr(x) &^ 3)
+	x = unsafe.Pointer(1 &^ uintptr(x)) // ERROR "possible misuse of unsafe.Pointer"
 
 	// certain uses of reflect are okay
 	var v reflect.Value

src/cmd/vet/unsafeptr.go

@@ -89,7 +89,7 @@ func (f *File) isSafeUintptr(x ast.Expr) bool {
 
 	case *ast.BinaryExpr:
 		switch x.Op {
-		case token.ADD, token.SUB:
+		case token.ADD, token.SUB, token.AND_NOT:
 			return f.isSafeUintptr(x.X) && !f.isSafeUintptr(x.Y)
 		}
 	}