facts: fix purity detection for allocations

Neither make nor new should have ever been considered pure. They
allocate memory and different invocations return different addresses.

The ir.UnOp case has never handled allocations correctly (there was no
'&' UnOp), and loads were broken when we introduced the Load
instruction.

facts/purity.go

@@ -1,7 +1,6 @@
 package facts
 
 import (
-	"go/token"
 	"go/types"
 	"reflect"
 
@@ -100,11 +99,14 @@ func purity(pass *analysis.Pass) (interface{}, error) {
 		}
 
 		for _, param := range fn.Params {
+			// TODO(dh): this may not be strictly correct. pure code
+			// can, to an extent, operate on non-basic types.
 			if _, ok := param.Type().Underlying().(*types.Basic); !ok {
 				return false
 			}
 		}
 
+		// Don't consider external functions pure.
 		if fn.Blocks == nil {
 			return false
 		}
@@ -124,7 +126,7 @@ func purity(pass *analysis.Pass) (interface{}, error) {
 				}
 			} else {
 				switch builtin.Name() {
-				case "len", "cap", "make", "new":
+				case "len", "cap":
 				default:
 					return false
 				}
@@ -154,10 +156,10 @@ func purity(pass *analysis.Pass) (interface{}, error) {
 					return false
 				case *ir.FieldAddr:
 					return false
-				case *ir.UnOp:
-					if ins.Op == token.MUL || ins.Op == token.AND {
-						return false
-					}
+				case *ir.Alloc:
+					return false
+				case *ir.Load:
+					return false
 				}
 			}
 		}

facts/testdata/src/Purity/CheckPureFunctions.go

@@ -15,3 +15,12 @@ func fn3() {
 	stubPointer()
 	stubInt()
 }
+
+func ptr1() *int { return new(int) }
+func ptr2() *int { var x int; return &x }
+func lit() []int { return []int{} }
+
+var X int
+
+func load() int        { _ = X; return 0 }
+func assign(x int) int { _ = x; return 0 } // want assign:"is pure"