cmd/vet: check shift calculations with "unsafe" package

vet should properly handle shift calculations via "unsafe"
package after the CL 37950.

Change-Id: I7737f2e656a5166337a17b92db46a0997f2a4e0e
Reviewed-on: https://go-review.googlesource.com/38064Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/vet/shift.go

@@ -48,28 +48,6 @@ func checkLongShift(f *File, node ast.Node, x, y ast.Expr) {
 		return
 	}
 
-	// Ignore shifts where the shift amount is calculated using unsafe.
-	// These are used for bit-twiddling tricks.
-	var hasUnsafe bool
-	ast.Inspect(y, func(n ast.Node) bool {
-		sel, ok := n.(*ast.SelectorExpr)
-		if !ok {
-			return true
-		}
-		pkg, ok := sel.X.(*ast.Ident)
-		if !ok {
-			return true
-		}
-		if pkg.Name == "unsafe" {
-			hasUnsafe = true
-			return false
-		}
-		return true
-	})
-	if hasUnsafe {
-		return
-	}
-
 	v := f.pkg.types[y].Value
 	if v == nil {
 		return

src/cmd/vet/testdata/shift.go

@@ -102,5 +102,8 @@ func ShiftTest() {
 	const oneIf64Bit = ^uint(0) >> 63 // allow large shifts of constants; they are used for 32/64 bit compatibility tricks
 
 	var h uintptr
-	h = h<<8 | (h >> (8 * (unsafe.Sizeof(h) - 1))) // shifts by unsafe amounts are safe
+	h = h<<8 | (h >> (8 * (unsafe.Sizeof(h) - 1)))
+	h <<= 8 * unsafe.Sizeof(h) // ERROR "too small for shift"
+	h >>= 7 * unsafe.Alignof(h)
+	h >>= 8 * unsafe.Alignof(h) // ERROR "too small for shift"
 }