1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
|
// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package main
import (
"go/ast"
"go/token"
"reflect"
)
type simplifier struct {
hasDotImport bool // package file contains: import . "some/import/path"
}
func (s *simplifier) Visit(node ast.Node) ast.Visitor {
switch n := node.(type) {
case *ast.CompositeLit:
// array, slice, and map composite literals may be simplified
outer := n
var eltType ast.Expr
switch typ := outer.Type.(type) {
case *ast.ArrayType:
eltType = typ.Elt
case *ast.MapType:
eltType = typ.Value
}
if eltType != nil {
typ := reflect.ValueOf(eltType)
for i, x := range outer.Elts {
px := &outer.Elts[i]
// look at value of indexed/named elements
if t, ok := x.(*ast.KeyValueExpr); ok {
x = t.Value
px = &t.Value
}
ast.Walk(s, x) // simplify x
// if the element is a composite literal and its literal type
// matches the outer literal's element type exactly, the inner
// literal type may be omitted
if inner, ok := x.(*ast.CompositeLit); ok {
if match(nil, typ, reflect.ValueOf(inner.Type)) {
inner.Type = nil
}
}
// if the outer literal's element type is a pointer type *T
// and the element is & of a composite literal of type T,
// the inner &T may be omitted.
if ptr, ok := eltType.(*ast.StarExpr); ok {
if addr, ok := x.(*ast.UnaryExpr); ok && addr.Op == token.AND {
if inner, ok := addr.X.(*ast.CompositeLit); ok {
if match(nil, reflect.ValueOf(ptr.X), reflect.ValueOf(inner.Type)) {
inner.Type = nil // drop T
*px = inner // drop &
}
}
}
}
}
// node was simplified - stop walk (there are no subnodes to simplify)
return nil
}
case *ast.SliceExpr:
// a slice expression of the form: s[a:len(s)]
// can be simplified to: s[a:]
// if s is "simple enough" (for now we only accept identifiers)
if n.Max != nil || s.hasDotImport {
// - 3-index slices always require the 2nd and 3rd index
// - if dot imports are present, we cannot be certain that an
// unresolved "len" identifier refers to the predefined len()
break
}
if s, _ := n.X.(*ast.Ident); s != nil && s.Obj != nil {
// the array/slice object is a single, resolved identifier
if call, _ := n.High.(*ast.CallExpr); call != nil && len(call.Args) == 1 && !call.Ellipsis.IsValid() {
// the high expression is a function call with a single argument
if fun, _ := call.Fun.(*ast.Ident); fun != nil && fun.Name == "len" && fun.Obj == nil {
// the function called is "len" and it is not locally defined; and
// because we don't have dot imports, it must be the predefined len()
if arg, _ := call.Args[0].(*ast.Ident); arg != nil && arg.Obj == s.Obj {
// the len argument is the array/slice object
n.High = nil
}
}
}
}
// Note: We could also simplify slice expressions of the form s[0:b] to s[:b]
// but we leave them as is since sometimes we want to be very explicit
// about the lower bound.
// An example where the 0 helps:
// x, y, z := b[0:2], b[2:4], b[4:6]
// An example where it does not:
// x, y := b[:n], b[n:]
case *ast.RangeStmt:
// - a range of the form: for x, _ = range v {...}
// can be simplified to: for x = range v {...}
// - a range of the form: for _ = range v {...}
// can be simplified to: for range v {...}
if isBlank(n.Value) {
n.Value = nil
}
if isBlank(n.Key) && n.Value == nil {
n.Key = nil
}
}
return s
}
func isBlank(x ast.Expr) bool {
ident, ok := x.(*ast.Ident)
return ok && ident.Name == "_"
}
func simplify(f *ast.File) {
var s simplifier
// determine if f contains dot imports
for _, imp := range f.Imports {
if imp.Name != nil && imp.Name.Name == "." {
s.hasDotImport = true
break
}
}
// remove empty declarations such as "const ()", etc
removeEmptyDeclGroups(f)
ast.Walk(&s, f)
}
func removeEmptyDeclGroups(f *ast.File) {
i := 0
for _, d := range f.Decls {
if g, ok := d.(*ast.GenDecl); !ok || !isEmpty(f, g) {
f.Decls[i] = d
i++
}
}
f.Decls = f.Decls[:i]
}
func isEmpty(f *ast.File, g *ast.GenDecl) bool {
if g.Doc != nil || g.Specs != nil {
return false
}
for _, c := range f.Comments {
// if there is a comment in the declaration, it is not considered empty
if g.Pos() <= c.Pos() && c.End() <= g.End() {
return false
}
}
return true
}
|
