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main.go
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main.go
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package main
import (
"fmt"
"go/ast"
"go/importer"
"go/parser"
"go/token"
"go/types"
"log"
)
//!+input
const input = `package main
import "bytes"
func main() {
var buf bytes.Buffer
if buf.Bytes == nil && bytes.Repeat != nil && main == nil {
// ...
}
}
`
//!-input
var fset = token.NewFileSet()
func main() {
f, err := parser.ParseFile(fset, "input.go", input, 0)
if err != nil {
log.Fatal(err) // parse error
}
conf := types.Config{Importer: importer.Default()}
info := &types.Info{
Defs: make(map[*ast.Ident]types.Object),
Uses: make(map[*ast.Ident]types.Object),
Types: make(map[ast.Expr]types.TypeAndValue),
}
if _, err = conf.Check("cmd/hello", fset, []*ast.File{f}, info); err != nil {
log.Fatal(err) // type error
}
ast.Inspect(f, func(n ast.Node) bool {
if n != nil {
CheckNilFuncComparison(info, n)
}
return true
})
}
//!+
// CheckNilFuncComparison reports unintended comparisons
// of functions against nil, e.g., "if x.Method == nil {".
func CheckNilFuncComparison(info *types.Info, n ast.Node) {
e, ok := n.(*ast.BinaryExpr)
if !ok {
return // not a binary operation
}
if e.Op != token.EQL && e.Op != token.NEQ {
return // not a comparison
}
// If this is a comparison against nil, find the other operand.
var other ast.Expr
if info.Types[e.X].IsNil() {
other = e.Y
} else if info.Types[e.Y].IsNil() {
other = e.X
} else {
return // not a comparison against nil
}
// Find the object.
var obj types.Object
switch v := other.(type) {
case *ast.Ident:
obj = info.Uses[v]
case *ast.SelectorExpr:
obj = info.Uses[v.Sel]
default:
return // not an identifier or selection
}
if _, ok := obj.(*types.Func); !ok {
return // not a function or method
}
fmt.Printf("%s: comparison of function %v %v nil is always %v\n",
fset.Position(e.Pos()), obj.Name(), e.Op, e.Op == token.NEQ)
}
//!-
/*
//!+output
$ go build github.com/golang/example/gotypes/nilfunc
$ ./nilfunc
input.go:7:5: comparison of function Bytes == nil is always false
input.go:7:25: comparison of function Repeat != nil is always true
input.go:7:48: comparison of function main == nil is always false
//!-output
*/