modifies-value-receiver (new rule) (#51)

README.md

@@ -262,6 +262,7 @@ List of all available rules. The rules ported from `golint` are left unchanged a
 | `flag-parameter`      |  n/a   | Warns on boolean parameters that create a control coupling       |    no    |  no   |
 | `unnecessary-stmt`    |  n/a   | Suggests removing or simplifying unnecessary statements          |    no    |  no   |
 | `struct-tag`          |  n/a   | Checks common struct tags like `json`,`xml`,`yaml`               |    no    |  no   |
+| `modifies-value-receiver` |  n/a   | Warns on assignments to value-passed method receivers        |    no    |  yes  |
 
 ## Available Formatters
 

config.go

@@ -60,6 +60,7 @@ var allRules = append([]lint.Rule{
 	&rule.FlagParamRule{},
 	&rule.UnnecessaryStmtRule{},
 	&rule.StructTagRule{},
+	&rule.ModifiesValRecRule{},
 }, defaultRules...)
 
 var allFormatters = []lint.Formatter{

fixtures/modifies-value-receiver.go

@@ -0,0 +1,13 @@
+package fixtures
+
+type data struct {
+	num   int
+	key   *string
+	items map[string]bool
+}
+
+func (this data) vmethod() {
+	this.num = 8 // MATCH /suspicious assignment to a by-value method receiver/
+	*this.key = "v.key"
+	this.items["vmethod"] = true
+}

rule/modifies-value-receiver.go

@@ -0,0 +1,134 @@
+package rule
+
+import (
+	"go/ast"
+	"strings"
+
+	"github.com/mgechev/revive/lint"
+)
+
+// ModifiesValRecRule lints assignments to value method-receivers.
+type ModifiesValRecRule struct{}
+
+// Apply applies the rule to given file.
+func (r *ModifiesValRecRule) Apply(file *lint.File, _ lint.Arguments) []lint.Failure {
+	var failures []lint.Failure
+
+	onFailure := func(failure lint.Failure) {
+		failures = append(failures, failure)
+	}
+
+	w := lintModifiesValRecRule{file: file, onFailure: onFailure}
+	file.Pkg.TypeCheck()
+	ast.Walk(w, file.AST)
+
+	return failures
+}
+
+// Name returns the rule name.
+func (r *ModifiesValRecRule) Name() string {
+	return "modifies-value-receiver"
+}
+
+type lintModifiesValRecRule struct {
+	file      *lint.File
+	onFailure func(lint.Failure)
+}
+
+func (w lintModifiesValRecRule) Visit(node ast.Node) ast.Visitor {
+	switch n := node.(type) {
+	case *ast.FuncDecl:
+		if n.Recv == nil {
+			return nil // skip, not a method
+		}
+
+		receiver := n.Recv.List[0]
+		if _, ok := receiver.Type.(*ast.StarExpr); ok {
+			return nil // skip, method with pointer receiver
+		}
+
+		if w.skipType(receiver.Type) {
+			return nil // skip, receiver is a map or array
+		}
+
+		if len(receiver.Names) < 1 {
+			return nil // skip, anonymous receiver
+		}
+
+		receiverName := receiver.Names[0].Name
+		if receiverName == "_" {
+			return nil // skip, anonymous receiver
+		}
+
+		fselect := func(n ast.Node) bool {
+			// look for assignments with the receiver in the right hand
+			asgmt, ok := n.(*ast.AssignStmt)
+			if !ok {
+				return false
+			}
+
+			for _, exp := range asgmt.Lhs {
+				switch e := exp.(type) {
+				case *ast.IndexExpr: // receiver...[] = ...
+					continue
+				case *ast.StarExpr: // *receiver = ...
+					continue
+				case *ast.SelectorExpr: // receiver.field = ...
+					name := w.getNameFromExpr(e.X)
+					if name == "" || name != receiverName {
+						continue
+					}
+
+					if w.skipType(ast.Expr(e.Sel)) {
+						continue
+					}
+
+				case *ast.Ident: // receiver := ...
+					if e.Name != receiverName {
+						continue
+					}
+				default:
+					continue
+				}
+
+				return true
+			}
+
+			return false
+		}
+
+		assignmentsToReceiver := pick(n.Body, fselect, nil)
+
+		for _, assignment := range assignmentsToReceiver {
+			w.onFailure(lint.Failure{
+				Node:       assignment,
+				Confidence: 1,
+				Failure:    "suspicious assignment to a by-value method receiver",
+			})
+		}
+	}
+
+	return w
+}
+
+func (w lintModifiesValRecRule) skipType(t ast.Expr) bool {
+	rt := w.file.Pkg.TypeOf(t)
+	if rt == nil {
+		return false
+	}
+
+	rt = rt.Underlying()
+	rtName := rt.String()
+
+	// skip when receiver is a map or array
+	return strings.HasPrefix(rtName, "[]") || strings.HasPrefix(rtName, "map[")
+}
+
+func (_ lintModifiesValRecRule) getNameFromExpr(ie ast.Expr) string {
+	ident, ok := ie.(*ast.Ident)
+	if !ok {
+		return ""
+	}
+
+	return ident.Name
+}

test/modifies-value-receiver_test.go

@@ -0,0 +1,11 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/mgechev/revive/rule"
+)
+
+func TestModifiesValRec(t *testing.T) {
+	testRule(t, "modifies-value-receiver", &rule.ModifiesValRecRule{})
+}