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// 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{}
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 keyType, eltType ast.Expr
switch typ := outer.Type.(type) {
case *ast.ArrayType:
eltType = typ.Elt
case *ast.MapType:
keyType = typ.Key
eltType = typ.Value
}
if eltType != nil {
var ktyp reflect.Value
if keyType != nil {
ktyp = reflect.ValueOf(keyType)
}
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 {
if keyType != nil {
s.simplifyLiteral(ktyp, keyType, t.Key, &t.Key)
}
x = t.Value
px = &t.Value
}
s.simplifyLiteral(typ, eltType, x, px)
}
// 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)
//
// Note: This may not be correct because len may have been redeclared in
// the same package. However, this is extremely unlikely and so far
// (April 2022, after years of supporting this rewrite feature)
// has never come up, so let's keep it working as is (see also #15153).
//
// Also note that this code used to use go/ast's object tracking,
// which was removed in exchange for go/parser.Mode.SkipObjectResolution.
// False positives are extremely unlikely as described above,
// and go/ast's object tracking is incomplete in any case.
if n.Max != nil {
// - 3-index slices always require the 2nd and 3rd index
break
}
if s, _ := n.X.(*ast.Ident); s != nil {
// the array/slice object is a single 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" {
// the function called is "len"
if arg, _ := call.Args[0].(*ast.Ident); arg != nil && arg.Name == s.Name {
// 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 (s simplifier) simplifyLiteral(typ reflect.Value, astType, x ast.Expr, px *ast.Expr) {
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 := astType.(*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 &
}
}
}
}
}
func isBlank(x ast.Expr) bool {
ident, ok := x.(*ast.Ident)
return ok && ident.Name == "_"
}
func simplify(f *ast.File) {
// remove empty declarations such as "const ()", etc
removeEmptyDeclGroups(f)
var s simplifier
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
}
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