avoid breaking const declarations using iotas

With the -literals flag, we try to convert some const declarations to
vars, as long as that doesn't break typechecking. We really only do that
for typed constant strings, really.

There was a quirk: if a numerical constant had a type and used iota, we
would not obfuscate its value, but we would still convert the
declaration from const to var. Since iotas only work within const
declarations, that would break compilation:

	> garble -literals build
	[stderr]
	# test/main
	FeWE3zwi.go:19: undefined: iota
	exit status 2

To fix the problem, make the logic more conservative: only obfuscate
constant declarations where the values are typed strings, meaning that
any numerical constants are left entirely untouched.

This fixes the build of google.golang.org/protobuf/runtime/protoiface
with -literals turned on.

internal/literals/literals.go

@@ -51,14 +51,8 @@ func Obfuscate(file *ast.File, info *types.Info, fset *token.FileSet, ignoreObj
 				for _, name := range spec.Names {
 					obj := info.ObjectOf(name)
 
-					basic, ok := obj.Type().(*types.Basic)
-					if !ok {
-						// skip the block if it contains non basic types
-						return false
-					}
-
-					if basic.Info()&types.IsUntyped != 0 {
-						// skip the block if it contains untyped constants
+					// We only obfuscate const declarations with typed string values.
+					if obj.Type() != types.Typ[types.String] {
 						return false
 					}
 

testdata/scripts/literals.txt

@@ -88,6 +88,11 @@ const (
 	skip2 = "also skip this"
 )
 
+// We used to conver this to a var in an attempt of obfuscating the literal.
+// That would break the iota, which only works inside const declarations.
+// We only obfuscate constant declarations with string values, anyway.
+const fooTyped uint64 = 1 << iota
+
 const arrayLen = 4
 
 var array [arrayLen]byte