cmd/vet: allow shifts by amounts calculated using unsafe

The real world code that inspired this fix,
from runtime/pprof/map.go:

	// Compute hash of (stk, tag).
	h := uintptr(0)
	for _, x := range stk {
		h = h<<8 | (h >> (8 * (unsafe.Sizeof(h) - 1)))
		h += uintptr(x) * 41
	}
	h = h<<8 | (h >> (8 * (unsafe.Sizeof(h) - 1)))
	h += uintptr(tag) * 41

Change-Id: I99a95b97cba73811faedb0b9a1b9b54e9a1784a3
Reviewed-on: https://go-review.googlesource.com/37574
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/vet/shift.go

@@ -47,6 +47,29 @@ func checkLongShift(f *File, node ast.Node, x, y ast.Expr) {
 		// like ^uint(0) >> 63 for 32/64 bit detection and compatibility.
 		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

@@ -6,6 +6,8 @@
 
 package testdata
 
+import "unsafe"
+
 func ShiftTest() {
 	var i8 int8
 	_ = i8 << 7
@@ -77,4 +79,7 @@ func ShiftTest() {
 	p >>= 32    // ERROR "p might be too small for shift of 32"
 
 	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
 }