New rule: range-loop-var

README.md

@@ -271,6 +271,7 @@ List of all available rules. The rules ported from `golint` are left unchanged a
 | `redefines-builtin-id`|  n/a   | Warns on redefinitions of builtin identifiers                    |    no    |  no   |
 | `function-result-limit` |  int | Specifies the maximum number of results a function can return    |    no    |  no   |
 | `imports-blacklist`   | []string | Disallows importing the specified packages                     |    no    |  no   |
+| `range-loop-var`      | n/a    | Disallows incorrect uses of range loop variables in closures     |    no    |  no   |
 
 ## Configurable rules
 

config.go

@@ -67,6 +67,7 @@ var allRules = append([]lint.Rule{
 	&rule.ImportsBlacklistRule{},
 	&rule.FunctionResultsLimitRule{},
 	&rule.MaxPublicStructsRule{},
+	&rule.RangeLoopVarRule{},
 }, defaultRules...)
 
 var allFormatters = []lint.Formatter{

fixtures/range-loop-var.go

@@ -0,0 +1,24 @@
+package fixtures
+
+import "fmt"
+
+func foo() {
+	mySlice := []string{"A", "B", "C"}
+	for index, value := range mySlice {
+		go func() {
+			fmt.Printf("Index: %d\n", index) // MATCH /loop variable index captured by func literal/
+			fmt.Printf("Value: %s\n", value) // MATCH /loop variable value captured by func literal/
+		}()
+	}
+
+	myDict := make(map[string]int)
+	myDict["A"] = 1
+	myDict["B"] = 2
+	myDict["C"] = 3
+	for key, value := range myDict {
+		defer func() {
+			fmt.Printf("Index: %d\n", key)   // MATCH /loop variable key captured by func literal/
+			fmt.Printf("Value: %s\n", value) // MATCH /loop variable value captured by func literal/
+		}()
+	}
+}

rule/range-loop-var.go

@@ -0,0 +1,111 @@
+package rule
+
+import (
+	"fmt"
+	"go/ast"
+
+	"github.com/mgechev/revive/lint"
+)
+
+// RangeLoopVarRule lints given else constructs.
+type RangeLoopVarRule struct{}
+
+// Apply applies the rule to given file.
+func (r *RangeLoopVarRule) Apply(file *lint.File, arguments lint.Arguments) []lint.Failure {
+	var failures []lint.Failure
+
+	walker := RangeLoopVar{
+		onFailure: func(failure lint.Failure) {
+			failures = append(failures, failure)
+		},
+	}
+
+	ast.Walk(walker, file.AST)
+
+	return failures
+}
+
+// Name returns the rule name.
+func (r *RangeLoopVarRule) Name() string {
+	return "range-loop-var"
+}
+
+type RangeLoopVar struct {
+	onFailure func(lint.Failure)
+}
+
+func (w RangeLoopVar) Visit(node ast.Node) ast.Visitor {
+
+	// Find the variables updated by the loop statement.
+	var vars []*ast.Ident
+	addVar := func(expr ast.Expr) {
+		if id, ok := expr.(*ast.Ident); ok {
+			vars = append(vars, id)
+		}
+	}
+	var body *ast.BlockStmt
+	switch n := node.(type) {
+	case *ast.RangeStmt:
+		body = n.Body
+		addVar(n.Key)
+		addVar(n.Value)
+	case *ast.ForStmt:
+		body = n.Body
+		switch post := n.Post.(type) {
+		case *ast.AssignStmt:
+			// e.g. for p = head; p != nil; p = p.next
+			for _, lhs := range post.Lhs {
+				addVar(lhs)
+			}
+		case *ast.IncDecStmt:
+			// e.g. for i := 0; i < n; i++
+			addVar(post.X)
+		}
+	}
+	if vars == nil {
+		return w
+	}
+
+	// Inspect a go or defer statement
+	// if it's the last one in the loop body.
+	// (We give up if there are following statements,
+	// because it's hard to prove go isn't followed by wait,
+	// or defer by return.)
+	if len(body.List) == 0 {
+		return w
+	}
+	var last *ast.CallExpr
+	switch s := body.List[len(body.List)-1].(type) {
+	case *ast.GoStmt:
+		last = s.Call
+	case *ast.DeferStmt:
+		last = s.Call
+	default:
+		return w
+	}
+	lit, ok := last.Fun.(*ast.FuncLit)
+	if !ok {
+		return w
+	}
+	ast.Inspect(lit.Body, func(n ast.Node) bool {
+		id, ok := n.(*ast.Ident)
+		if !ok || id.Obj == nil {
+			return true
+		}
+		if lit.Type == nil {
+			// Not referring to a variable (e.g. struct field name)
+			return true
+		}
+		for _, v := range vars {
+			if v.Obj == id.Obj {
+				w.onFailure(lint.Failure{
+					Confidence: 1,
+					Failure:    fmt.Sprintf("loop variable %v captured by func literal", id.Name),
+					Node:       n,
+				})
+			}
+		}
+		return true
+	})
+	return w
+}

test/range-loop-var_test.go

@@ -0,0 +1,12 @@
+package test
+
+import (
+	"testing"
+
+	"github.com/mgechev/revive/lint"
+	"github.com/mgechev/revive/rule"
+)
+
+func TestRangeLoopVar(t *testing.T) {
+	testRule(t, "range-loop-var", &rule.RangeLoopVarRule{}, &lint.RuleConfig{})
+}

untyped.toml

@@ -13,3 +13,4 @@
 [rule.receiver-naming]
 [rule.indent-error-flow]
 [rule.empty-block]
+[rule.range-loop-var]