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/*
6This file contains the code to check for useless function comparisons.
7A useless comparison is one like f == nil as opposed to f() == nil.
8*/
9
10package main
11
12import (
13	"go/ast"
14	"go/token"
15	"go/types"
16)
17
18func init() {
19	register("nilfunc",
20		"check for comparisons between functions and nil",
21		checkNilFuncComparison,
22		binaryExpr)
23}
24
25func checkNilFuncComparison(f *File, node ast.Node) {
26	e := node.(*ast.BinaryExpr)
27
28	// Only want == or != comparisons.
29	if e.Op != token.EQL && e.Op != token.NEQ {
30		return
31	}
32
33	// Only want comparisons with a nil identifier on one side.
34	var e2 ast.Expr
35	switch {
36	case f.isNil(e.X):
37		e2 = e.Y
38	case f.isNil(e.Y):
39		e2 = e.X
40	default:
41		return
42	}
43
44	// Only want identifiers or selector expressions.
45	var obj types.Object
46	switch v := e2.(type) {
47	case *ast.Ident:
48		obj = f.pkg.uses[v]
49	case *ast.SelectorExpr:
50		obj = f.pkg.uses[v.Sel]
51	default:
52		return
53	}
54
55	// Only want functions.
56	if _, ok := obj.(*types.Func); !ok {
57		return
58	}
59
60	f.Badf(e.Pos(), "comparison of function %v %v nil is always %v", obj.Name(), e.Op, e.Op == token.NEQ)
61}
62
63// isNil reports whether the provided expression is the built-in nil
64// identifier.
65func (f *File) isNil(e ast.Expr) bool {
66	return f.pkg.types[e].Type == types.Typ[types.UntypedNil]
67}
68