fix issue 386: Incorrectly identifies channel draining as "empty code… (#415)

* fix issue 386: Incorrectly identifies channel draining as "empty code block"

* updates doc of empty-block rule

rule/empty-block.go

@@ -2,6 +2,7 @@ package rule
 
 import (
 	"go/ast"
+	"go/types"
 
 	"github.com/mgechev/revive/lint"
 )
@@ -17,7 +18,12 @@ func (r *EmptyBlockRule) Apply(file *lint.File, _ lint.Arguments) []lint.Failure
 		failures = append(failures, failure)
 	}
 
-	w := lintEmptyBlock{make([]*ast.BlockStmt, 0), onFailure}
+	err := file.Pkg.TypeCheck()
+	if err != nil {
+		panic("unable to type check " + file.Name + ":" + err.Error())
+	}
+
+	w := lintEmptyBlock{make(map[*ast.BlockStmt]bool, 0), file.Pkg, onFailure}
 	ast.Walk(w, file.AST)
 	return failures
 }
@@ -28,49 +34,35 @@ func (r *EmptyBlockRule) Name() string {
 }
 
 type lintEmptyBlock struct {
-	ignore    []*ast.BlockStmt
+	ignore    map[*ast.BlockStmt]bool
+	pkg       *lint.Package
 	onFailure func(lint.Failure)
 }
 
 func (w lintEmptyBlock) Visit(node ast.Node) ast.Visitor {
-	fd, ok := node.(*ast.FuncDecl)
-	if ok {
-		w.ignore = append(w.ignore, fd.Body)
+	switch n := node.(type) {
+	case *ast.FuncDecl:
+		w.ignore[n.Body] = true
 		return w
-	}
-
-	fl, ok := node.(*ast.FuncLit)
-	if ok {
-		w.ignore = append(w.ignore, fl.Body)
+	case *ast.FuncLit:
+		w.ignore[n.Body] = true
 		return w
-	}
+	case *ast.RangeStmt:
+		t := w.pkg.TypeOf(n.X)
 
-	block, ok := node.(*ast.BlockStmt)
-	if !ok {
-		return w
-	}
-
-	if mustIgnore(block, w.ignore) {
-		return w
-	}
-
-	if len(block.List) == 0 {
-		w.onFailure(lint.Failure{
-			Confidence: 1,
-			Node:       block,
-			Category:   "logic",
-			Failure:    "this block is empty, you can remove it",
-		})
+		if _, ok := t.(*types.Chan); ok {
+			w.ignore[n.Body] = true
+		}
+	case *ast.BlockStmt:
+		if !w.ignore[n] && len(n.List) == 0 {
+			w.onFailure(lint.Failure{
+				Confidence: 1,
+				Node:       n,
+				Category:   "logic",
+				Failure:    "this block is empty, you can remove it",
+			})
+		}
 	}
 
 	return w
 }
-
-func mustIgnore(block *ast.BlockStmt, blackList []*ast.BlockStmt) bool {
-	for _, b := range blackList {
-		if b == block {
-			return true
-		}
-	}
-	return false
-}