1// Copyright 2013 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5// Package nilfunc defines an Analyzer that checks for useless
6// comparisons against nil.
7package nilfunc
8
9import (
10	"go/ast"
11	"go/token"
12	"go/types"
13
14	"golang.org/x/tools/go/analysis"
15	"golang.org/x/tools/go/analysis/passes/inspect"
16	"golang.org/x/tools/go/ast/inspector"
17)
18
19const Doc = `check for useless comparisons between functions and nil
20
21A useless comparison is one like f == nil as opposed to f() == nil.`
22
23var Analyzer = &analysis.Analyzer{
24	Name:     "nilfunc",
25	Doc:      Doc,
26	Requires: []*analysis.Analyzer{inspect.Analyzer},
27	Run:      run,
28}
29
30func run(pass *analysis.Pass) (interface{}, error) {
31	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
32
33	nodeFilter := []ast.Node{
34		(*ast.BinaryExpr)(nil),
35	}
36	inspect.Preorder(nodeFilter, func(n ast.Node) {
37		e := n.(*ast.BinaryExpr)
38
39		// Only want == or != comparisons.
40		if e.Op != token.EQL && e.Op != token.NEQ {
41			return
42		}
43
44		// Only want comparisons with a nil identifier on one side.
45		var e2 ast.Expr
46		switch {
47		case pass.TypesInfo.Types[e.X].IsNil():
48			e2 = e.Y
49		case pass.TypesInfo.Types[e.Y].IsNil():
50			e2 = e.X
51		default:
52			return
53		}
54
55		// Only want identifiers or selector expressions.
56		var obj types.Object
57		switch v := e2.(type) {
58		case *ast.Ident:
59			obj = pass.TypesInfo.Uses[v]
60		case *ast.SelectorExpr:
61			obj = pass.TypesInfo.Uses[v.Sel]
62		default:
63			return
64		}
65
66		// Only want functions.
67		if _, ok := obj.(*types.Func); !ok {
68			return
69		}
70
71		pass.Reportf(e.Pos(), "comparison of function %v %v nil is always %v", obj.Name(), e.Op, e.Op == token.NEQ)
72	})
73	return nil, nil
74}
75