assign.go

     1  // Copyright 2013 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // Package assign defines an Analyzer that detects useless assignments.
     6  package assign
     7  
     8  // TODO(adonovan): check also for assignments to struct fields inside
     9  // methods that are on T instead of *T.
    10  
    11  import (
    12  	"fmt"
    13  	"go/ast"
    14  	"go/token"
    15  	"reflect"
    16  
    17  	"golang.org/x/tools/go/analysis"
    18  	"golang.org/x/tools/go/analysis/passes/inspect"
    19  	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
    20  	"golang.org/x/tools/go/ast/inspector"
    21  )
    22  
    23  const Doc = `check for useless assignments
    24  
    25  This checker reports assignments of the form x = x or a[i] = a[i].
    26  These are almost always useless, and even when they aren't they are
    27  usually a mistake.`
    28  
    29  var Analyzer = &analysis.Analyzer{
    30  	Name:     "assign",
    31  	Doc:      Doc,
    32  	Requires: []*analysis.Analyzer{inspect.Analyzer},
    33  	Run:      run,
    34  }
    35  
    36  func run(pass *analysis.Pass) (interface{}, error) {
    37  	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
    38  
    39  	nodeFilter := []ast.Node{
    40  		(*ast.AssignStmt)(nil),
    41  	}
    42  	inspect.Preorder(nodeFilter, func(n ast.Node) {
    43  		stmt := n.(*ast.AssignStmt)
    44  		if stmt.Tok != token.ASSIGN {
    45  			return // ignore :=
    46  		}
    47  		if len(stmt.Lhs) != len(stmt.Rhs) {
    48  			// If LHS and RHS have different cardinality, they can't be the same.
    49  			return
    50  		}
    51  		for i, lhs := range stmt.Lhs {
    52  			rhs := stmt.Rhs[i]
    53  			if analysisutil.HasSideEffects(pass.TypesInfo, lhs) ||
    54  				analysisutil.HasSideEffects(pass.TypesInfo, rhs) {
    55  				continue // expressions may not be equal
    56  			}
    57  			if reflect.TypeOf(lhs) != reflect.TypeOf(rhs) {
    58  				continue // short-circuit the heavy-weight gofmt check
    59  			}
    60  			le := analysisutil.Format(pass.Fset, lhs)
    61  			re := analysisutil.Format(pass.Fset, rhs)
    62  			if le == re {
    63  				pass.Report(analysis.Diagnostic{
    64  					Pos: stmt.Pos(), Message: fmt.Sprintf("self-assignment of %s to %s", re, le),
    65  					SuggestedFixes: []analysis.SuggestedFix{
    66  						{Message: "Remove", TextEdits: []analysis.TextEdit{
    67  							{Pos: stmt.Pos(), End: stmt.End(), NewText: []byte{}},
    68  						}},
    69  					},
    70  				})
    71  			}
    72  		}
    73  	})
    74  
    75  	return nil, nil
    76  }
    77
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