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// Copyright 2020 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 simplifyslice defines an Analyzer that simplifies slice statements.
// https://github.com/golang/go/blob/master/src/cmd/gofmt/simplify.go
// https://golang.org/cmd/gofmt/#hdr-The_simplify_command
package simplifyslice
import (
"bytes"
"fmt"
"go/ast"
"go/printer"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/ast/inspector"
)
const Doc = `check for slice simplifications
A slice expression of the form:
s[a:len(s)]
will be simplified to:
s[a:]
This is one of the simplifications that "gofmt -s" applies.`
var Analyzer = &analysis.Analyzer{
Name: "simplifyslice",
Doc: Doc,
Requires: []*analysis.Analyzer{inspect.Analyzer},
Run: run,
}
// 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:]
func run(pass *analysis.Pass) (interface{}, error) {
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
nodeFilter := []ast.Node{
(*ast.SliceExpr)(nil),
}
inspect.Preorder(nodeFilter, func(n ast.Node) {
expr := n.(*ast.SliceExpr)
// - 3-index slices always require the 2nd and 3rd index
if expr.Max != nil {
return
}
s, ok := expr.X.(*ast.Ident)
// the array/slice object is a single, resolved identifier
if !ok || s.Obj == nil {
return
}
call, ok := expr.High.(*ast.CallExpr)
// the high expression is a function call with a single argument
if !ok || len(call.Args) != 1 || call.Ellipsis.IsValid() {
return
}
fun, ok := call.Fun.(*ast.Ident)
// 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 !ok || fun.Name != "len" || fun.Obj != nil {
return
}
arg, ok := call.Args[0].(*ast.Ident)
// the len argument is the array/slice object
if !ok || arg.Obj != s.Obj {
return
}
var b bytes.Buffer
printer.Fprint(&b, pass.Fset, expr.High)
pass.Report(analysis.Diagnostic{
Pos: expr.High.Pos(),
End: expr.High.End(),
Message: fmt.Sprintf("unneeded: %s", b.String()),
SuggestedFixes: []analysis.SuggestedFix{{
Message: fmt.Sprintf("Remove '%s'", b.String()),
TextEdits: []analysis.TextEdit{{
Pos: expr.High.Pos(),
End: expr.High.End(),
NewText: []byte{},
}},
}},
})
})
return nil, nil
}