fix: arithmetic virus isn't applied to expressions involing string li…

…terals anymore (#18)

arithmetic virus isn't applied to strings anymore

Note that this is only a partial fix (#17),
as it only works if the binary expression involves a string literal. Two string variables
will still cause an unwanted mutation.

viruses/arithmetic/arithmetic.go

@@ -33,6 +33,12 @@ func (v *Arithmetic) Incubate(node ast.Node, _ *types.Info) []*viruses.Infection
 		return nil
 	}
 
+	// Go overloaded the + operator, meaning is not just used for
+	// arithmetic operations, so it needs to be ignored for strings.
+	if isString(expression.X) || isString(expression.Y) {
+		return nil
+	}
+
 	originalOperation := expression.Op
 
 	mutatedOperation, matches := v.mutations[expression.Op]
@@ -48,3 +54,30 @@ func (v *Arithmetic) Incubate(node ast.Node, _ *types.Info) []*viruses.Infection
 		),
 	}
 }
+
+func isString(expr ast.Expr) bool {
+	switch expr := expr.(type) {
+	case *ast.Ident:
+		// We need to get the type here, however, the `types` parameter
+		// is explicitly set to `nil` upon function invocation.
+	case *ast.BasicLit:
+		// Strings only support concatenation (+)
+		if expr.Kind == token.STRING {
+			return true
+		}
+	case *ast.CallExpr:
+		if expr, isIdentifier := expr.Fun.(*ast.Ident); isIdentifier {
+			// Make sure we have a builtin.
+			// Technically, we could encounter something like this:
+			//
+			// func string(b []byte) int {
+			// 	return len(b)
+			// }
+			if expr.Name == "string" && expr.Obj == nil {
+				return true
+			}
+		}
+	}
+
+	return false
+}

viruses/arithmetic/arithmetic_test.go

@@ -12,7 +12,8 @@ func TestArithmetic(t *testing.T) {
 		"Arithmetic",
 		arithmetic.New(),
 		oozetesting.Mutations{
-			"no mutations": {"source.0.go", []string{}},
+			"no mutations":                        {"source.0.go", []string{}},
+			"no mutation of string concatenation": {"source.7.go", []string{}},
 			"one mutation + to -": {"source.1.go", []string{
 				"source.2.go",
 			}},

viruses/arithmetic/testdata/source.7.go

@@ -0,0 +1,37 @@
+//!go:build testdata
+
+package source
+
+func hello0() string {
+	x := "hello"
+	return "/" + x
+}
+
+func hello1() string {
+	x := "hello"
+	return x + "/"
+}
+
+func hello3() string {
+	x := "hello"
+	y := []byte("/")
+	return x + string(y)
+}
+
+// FIXME This still fails
+// func hello2() string {
+// 	x := "hello"
+// 	y := "/"
+// 	return x + y
+// }
+
+// FIXME This still fails
+// func toString(b []byte) string {
+// 	return string(x)
+// }
+
+// func hello4() string {
+// 	x := "hello"
+// 	y := []byte{"/"}
+// 	return hello + toString(y)
+// }