From fc4e8ec9dee07f955278814b6b84389fd46816dd Mon Sep 17 00:00:00 2001
From: mgechev <mgechev@gmail.com>
Date: Sun, 26 Nov 2017 20:07:53 -0800
Subject: [PATCH] Add rule

---
 defaultrule/else-error.go | 116 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 116 insertions(+)
 create mode 100644 defaultrule/else-error.go

diff --git a/defaultrule/else-error.go b/defaultrule/else-error.go
new file mode 100644
index 0000000..0a44300
--- /dev/null
+++ b/defaultrule/else-error.go
@@ -0,0 +1,116 @@
+package defaultrule
+
+import (
+	"go/ast"
+	"go/token"
+	"strings"
+
+	"github.com/mgechev/revive/file"
+	"github.com/mgechev/revive/rule"
+)
+
+// LintElseRule lints given else constructs.
+type LintElseRule struct{}
+
+// Apply applies the rule to given file.
+func (r *LintElseRule) Apply(file *file.File, arguments rule.Arguments) []rule.Failure {
+	var failures []rule.Failure
+
+	onFailure := func(failure rule.Failure) {
+		failures = append(failures, failure)
+	}
+
+	astFile := file.GetAST()
+	w := &lintElseError{astFile, onFailure}
+	ast.Walk(w, astFile)
+	return failures
+}
+
+// Name returns the rule name.
+func (r *LintElseRule) Name() string {
+	return "no-else-return"
+}
+
+type lintElseError struct {
+	file      *ast.File
+	onFailure func(rule.Failure)
+}
+
+func (w *lintElseError) Visit(node ast.Node) ast.Visitor {
+	switch v := node.(type) {
+	case *ast.BlockStmt:
+		for i := 0; i < len(v.List)-1; i++ {
+			// if var := whatever; var != nil { return var }
+			s, ok := v.List[i].(*ast.IfStmt)
+			if !ok || s.Body == nil || len(s.Body.List) != 1 || s.Else != nil {
+				continue
+			}
+			assign, ok := s.Init.(*ast.AssignStmt)
+			if !ok || len(assign.Lhs) != 1 || !(assign.Tok == token.DEFINE || assign.Tok == token.ASSIGN) {
+				continue
+			}
+			id, ok := assign.Lhs[0].(*ast.Ident)
+			if !ok {
+				continue
+			}
+			expr, ok := s.Cond.(*ast.BinaryExpr)
+			if !ok || expr.Op != token.NEQ {
+				continue
+			}
+			if lhs, ok := expr.X.(*ast.Ident); !ok || lhs.Name != id.Name {
+				continue
+			}
+			if rhs, ok := expr.Y.(*ast.Ident); !ok || rhs.Name != "nil" {
+				continue
+			}
+			r, ok := s.Body.List[0].(*ast.ReturnStmt)
+			if !ok || len(r.Results) != 1 {
+				continue
+			}
+			if r, ok := r.Results[0].(*ast.Ident); !ok || r.Name != id.Name {
+				continue
+			}
+
+			// return nil
+			r, ok = v.List[i+1].(*ast.ReturnStmt)
+			if !ok || len(r.Results) != 1 {
+				continue
+			}
+			if r, ok := r.Results[0].(*ast.Ident); !ok || r.Name != "nil" {
+				continue
+			}
+
+			// check if there are any comments explaining the construct, don't emit an error if there are some.
+			if containsComments(w.file, s.Pos(), r.Pos()) {
+				continue
+			}
+
+			w.onFailure(rule.Failure{
+				Confidence: 0.9,
+				Failure:    "redundant if ...; err != nil check, just return error instead.",
+				Node:       v.List[i],
+			})
+		}
+	}
+	return w
+}
+
+func containsComments(f *ast.File, start, end token.Pos) bool {
+	for _, cgroup := range f.Comments {
+		comments := cgroup.List
+		if comments[0].Slash >= end {
+			// All comments starting with this group are after end pos.
+			return false
+		}
+		if comments[len(comments)-1].Slash < start {
+			// Comments group ends before start pos.
+			continue
+		}
+		for _, c := range comments {
+			if start <= c.Slash && c.Slash < end && !strings.HasPrefix(c.Text, "// MATCH ") {
+				return true
+			}
+		}
+	}
+	return false
+}