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// Copyright 2013 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 assign defines an Analyzer that detects useless assignments.
package assign
// TODO(adonovan): check also for assignments to struct fields inside
// methods that are on T instead of *T.
import (
"fmt"
"go/ast"
"go/token"
"go/types"
"reflect"
"golang.org/x/tools/go/analysis"
"golang.org/x/tools/go/analysis/passes/inspect"
"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
"golang.org/x/tools/go/ast/inspector"
)
const Doc = `check for useless assignments
This checker reports assignments of the form x = x or a[i] = a[i].
These are almost always useless, and even when they aren't they are
usually a mistake.`
var Analyzer = &analysis.Analyzer{
Name: "assign",
Doc: Doc,
Requires: []*analysis.Analyzer{inspect.Analyzer},
Run: run,
}
func run(pass *analysis.Pass) (interface{}, error) {
inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
nodeFilter := []ast.Node{
(*ast.AssignStmt)(nil),
}
inspect.Preorder(nodeFilter, func(n ast.Node) {
stmt := n.(*ast.AssignStmt)
if stmt.Tok != token.ASSIGN {
return // ignore :=
}
if len(stmt.Lhs) != len(stmt.Rhs) {
// If LHS and RHS have different cardinality, they can't be the same.
return
}
for i, lhs := range stmt.Lhs {
rhs := stmt.Rhs[i]
if analysisutil.HasSideEffects(pass.TypesInfo, lhs) ||
analysisutil.HasSideEffects(pass.TypesInfo, rhs) ||
isMapIndex(pass.TypesInfo, lhs) {
continue // expressions may not be equal
}
if reflect.TypeOf(lhs) != reflect.TypeOf(rhs) {
continue // short-circuit the heavy-weight gofmt check
}
le := analysisutil.Format(pass.Fset, lhs)
re := analysisutil.Format(pass.Fset, rhs)
if le == re {
pass.Report(analysis.Diagnostic{
Pos: stmt.Pos(), Message: fmt.Sprintf("self-assignment of %s to %s", re, le),
SuggestedFixes: []analysis.SuggestedFix{
{Message: "Remove", TextEdits: []analysis.TextEdit{
{Pos: stmt.Pos(), End: stmt.End(), NewText: []byte{}},
}},
},
})
}
}
})
return nil, nil
}
// isMapIndex returns true if e is a map index expression.
func isMapIndex(info *types.Info, e ast.Expr) bool {
if idx, ok := analysisutil.Unparen(e).(*ast.IndexExpr); ok {
if typ := info.Types[idx.X].Type; typ != nil {
_, ok := typ.Underlying().(*types.Map)
return ok
}
}
return false
}
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