cmd/vet: allow large shifts of constants

Large shifts of constants are frequently
used for fancy 32/64 bit detection.

This removes 14 false positives from the
standard library.

Updates #11041

Change-Id: Ib39346e5c161da04c38a6a3067932ef43bf74f2d
Reviewed-on: https://go-review.googlesource.com/27155
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/vet/shift.go

@@ -41,6 +41,12 @@ func checkShift(f *File, node ast.Node) {
 // checkLongShift checks if shift or shift-assign operations shift by more than
 // the length of the underlying variable.
 func checkLongShift(f *File, node ast.Node, x, y ast.Expr) {
+	if f.pkg.types[x].Value != nil {
+		// Ignore shifts of constants.
+		// These are frequently used for bit-twiddling tricks
+		// like ^uint(0) >> 63 for 32/64 bit detection and compatibility.
+		return
+	}
 	v := f.pkg.types[y].Value
 	if v == nil {
 		return

src/cmd/vet/testdata/shift.go

@@ -75,4 +75,6 @@ func ShiftTest() {
 	_ = p >> 32 // ERROR "p might be too small for shift of 32"
 	p <<= 32    // ERROR "p might be too small for shift of 32"
 	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
 }