new rule: unused-receiver (#119)

* new rule: unused-receiver

* unused-receiver: adds more test cases

README.md

@@ -296,6 +296,7 @@ List of all available rules. The rules ported from `golint` are left unchanged a
 | [`duplicated-imports`](./RULES_DESCRIPTIONS#duplicated-imports) | n/a  | Looks for packages that are imported two or more times   |    no    |  no   |
 | [`import-shadowing`](./RULES_DESCRIPTIONS.md#import-shadowing)   | n/a    | Spots identifiers that shadow an import    |    no    |  no   |
 | [`bare-return`](./RULES_DESCRIPTIONS#bare-return) | n/a  | Warns on bare returns   |    no    |  no   |
+| [`unused-receiver`](./RULES_DESCRIPTIONS.md#unused-receiver)   | n/a    | Suggests to rename or remove unused method receivers    |    no    |  no   |
 
 ## Configurable rules
 

RULES_DESCRIPTIONS.md

@@ -54,6 +54,7 @@ List of all available rules.
   - [unnecessary-stmt](#unnecessary-stmt)
   - [unreachable-code](#unreachable-code)
   - [unused-parameter](#unused-parameter)
+  - [unused-receiver](#unused-receiver)
   - [waitgroup-by-value](#waitgroup-by-value)
 
 ## add-constant
@@ -451,6 +452,12 @@ _Description_: This rule warns on unused parameters. Functions or methods with u
 
 _Configuration_: N/A
 
+## unused-receiver
+
+_Description_: This rule warns on unused method receivers. Methods with unused receivers can be a symptom of an unfinished refactoring or a bug.
+
+_Configuration_: N/A
+
 ## waitgroup-by-value
 
 _Description_: Function parameters that are passed by value, are in fact a copy of the original argument. Passing a copy of a `sync.WaitGroup` is usually not what the developer wants to do.

config.go

@@ -76,6 +76,7 @@ var allRules = append([]lint.Rule{
 	&rule.DuplicatedImportsRule{},
 	&rule.ImportShadowingRule{},
 	&rule.BareReturnRule{},
+	&rule.UnusedReceiverRule{},
 }, defaultRules...)
 
 var allFormatters = []lint.Formatter{

fixtures/unused-receiver.go

@@ -0,0 +1,43 @@
+package fixtures
+
+import (
+	"fmt"
+
+	"github.com/mgechev/revive/lint"
+)
+
+func (f *Unix) Name() string { // MATCH /method receiver 'f' is not referenced in method's body, consider removing or renaming it as _/
+	return "unix"
+}
+
+func (f *Unix) Format(failures <-chan lint.Failure, _ lint.RulesConfig) (string, error) { // MATCH /method receiver 'f' is not referenced in method's body, consider removing or renaming it as _/
+	for failure := range failures {
+		fmt.Printf("%v: [%s] %s\n", failure.Position.Start, failure.RuleName, failure.Failure)
+	}
+	return "", nil
+}
+
+func (u *Unix) Foo() (string, error) { // MATCH /method receiver 'u' is not referenced in method's body, consider removing or renaming it as _/
+	for failure := range failures {
+		u := 1 // shadowing the receiver
+		fmt.Printf("%v\n", u)
+	}
+	return "", nil
+}
+
+func (u *Unix) Foos() (string, error) {
+	for failure := range failures {
+		u := 1 // shadowing the receiver
+		fmt.Printf("%v\n", u)
+	}
+
+	return u, nil // use of the receiver
+}
+
+func (u *Unix) Bar() (string, error) {
+	for failure := range failures {
+		u.path = nil // modifies the receiver
+		fmt.Printf("%v\n", u)
+	}
+	return "", nil
+}

rule/unused-receiver.go

@@ -0,0 +1,77 @@
+package rule
+
+import (
+	"fmt"
+	"go/ast"
+
+	"github.com/mgechev/revive/lint"
+)
+
+// UnusedReceiverRule lints unused params in functions.
+type UnusedReceiverRule struct{}
+
+// Apply applies the rule to given file.
+func (_ *UnusedReceiverRule) Apply(file *lint.File, _ lint.Arguments) []lint.Failure {
+	var failures []lint.Failure
+
+	onFailure := func(failure lint.Failure) {
+		failures = append(failures, failure)
+	}
+
+	w := lintUnusedReceiverRule{onFailure: onFailure}
+
+	ast.Walk(w, file.AST)
+
+	return failures
+}
+
+// Name returns the rule name.
+func (_ *UnusedReceiverRule) Name() string {
+	return "unused-receiver"
+}
+
+type lintUnusedReceiverRule struct {
+	onFailure func(lint.Failure)
+}
+
+func (w lintUnusedReceiverRule) Visit(node ast.Node) ast.Visitor {
+	switch n := node.(type) {
+	case *ast.FuncDecl:
+		if n.Recv == nil {
+			return nil // skip this func decl, not a method
+		}
+
+		rec := n.Recv.List[0] // safe to access only the first (unique) element of the list
+		if len(rec.Names) < 1 {
+			return nil // the receiver is anonymous: func (aType) Foo(...) ...
+		}
+
+		recID := rec.Names[0]
+		if recID.Name == "_" {
+			return nil // the receiver is already named _
+		}
+
+		// inspect the func body looking for references to the receiver id
+		fselect := func(n ast.Node) bool {
+			ident, isAnID := n.(*ast.Ident)
+
+			return isAnID && ident.Obj == recID.Obj
+		}
+		refs2recID := pick(n.Body, fselect, nil)
+
+		if len(refs2recID) > 0 {
+			return nil // the receiver is referenced in the func body
+		}
+
+		w.onFailure(lint.Failure{
+			Confidence: 1,
+			Node:       recID,
+			Category:   "bad practice",
+			Failure:    fmt.Sprintf("method receiver '%s' is not referenced in method's body, consider removing or renaming it as _", recID.Name),
+		})
+
+		return nil // full method body already inspected
+	}
+
+	return w
+}

test/unused-receiver_test.go

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