assign.go

Documentation: golang.org/x/tools/go/analysis/passes/assign

     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  	"go/types"
    16  	"reflect"
    17  
    18  	"golang.org/x/tools/go/analysis"
    19  	"golang.org/x/tools/go/analysis/passes/inspect"
    20  	"golang.org/x/tools/go/analysis/passes/internal/analysisutil"
    21  	"golang.org/x/tools/go/ast/inspector"
    22  )
    23  
    24  const Doc = `check for useless assignments
    25  
    26  This checker reports assignments of the form x = x or a[i] = a[i].
    27  These are almost always useless, and even when they aren't they are
    28  usually a mistake.`
    29  
    30  var Analyzer = &analysis.Analyzer{
    31  	Name:     "assign",
    32  	Doc:      Doc,
    33  	Requires: []*analysis.Analyzer{inspect.Analyzer},
    34  	Run:      run,
    35  }
    36  
    37  func run(pass *analysis.Pass) (interface{}, error) {
    38  	inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
    39  
    40  	nodeFilter := []ast.Node{
    41  		(*ast.AssignStmt)(nil),
    42  	}
    43  	inspect.Preorder(nodeFilter, func(n ast.Node) {
    44  		stmt := n.(*ast.AssignStmt)
    45  		if stmt.Tok != token.ASSIGN {
    46  			return // ignore :=
    47  		}
    48  		if len(stmt.Lhs) != len(stmt.Rhs) {
    49  			// If LHS and RHS have different cardinality, they can't be the same.
    50  			return
    51  		}
    52  		for i, lhs := range stmt.Lhs {
    53  			rhs := stmt.Rhs[i]
    54  			if analysisutil.HasSideEffects(pass.TypesInfo, lhs) ||
    55  				analysisutil.HasSideEffects(pass.TypesInfo, rhs) ||
    56  				isMapIndex(pass.TypesInfo, lhs) {
    57  				continue // expressions may not be equal
    58  			}
    59  			if reflect.TypeOf(lhs) != reflect.TypeOf(rhs) {
    60  				continue // short-circuit the heavy-weight gofmt check
    61  			}
    62  			le := analysisutil.Format(pass.Fset, lhs)
    63  			re := analysisutil.Format(pass.Fset, rhs)
    64  			if le == re {
    65  				pass.Report(analysis.Diagnostic{
    66  					Pos: stmt.Pos(), Message: fmt.Sprintf("self-assignment of %s to %s", re, le),
    67  					SuggestedFixes: []analysis.SuggestedFix{
    68  						{Message: "Remove", TextEdits: []analysis.TextEdit{
    69  							{Pos: stmt.Pos(), End: stmt.End(), NewText: []byte{}},
    70  						}},
    71  					},
    72  				})
    73  			}
    74  		}
    75  	})
    76  
    77  	return nil, nil
    78  }
    79  
    80  // isMapIndex returns true if e is a map index expression.
    81  func isMapIndex(info *types.Info, e ast.Expr) bool {
    82  	if idx, ok := analysisutil.Unparen(e).(*ast.IndexExpr); ok {
    83  		if typ := info.Types[idx.X].Type; typ != nil {
    84  			_, ok := typ.Underlying().(*types.Map)
    85  			return ok
    86  		}
    87  	}
    88  	return false
    89  }
    90
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