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
     6  
     7  // TODO(adonovan): check also for assignments to struct fields inside
     8  // methods that are on T instead of *T.
     9  
    10  import (
    11  	_ "embed"
    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  	"golang.org/x/tools/internal/analysisinternal"
    23  )
    24  
    25  //go:embed doc.go
    26  var doc string
    27  
    28  var Analyzer = &analysis.Analyzer{
    29  	Name:     "assign",
    30  	Doc:      analysisutil.MustExtractDoc(doc, "assign"),
    31  	URL:      "https://pkg.go.dev/golang.org/x/tools/go/analysis/passes/assign",
    32  	Requires: []*analysis.Analyzer{inspect.Analyzer},
    33  	Run:      run,
    34  }
    35  
    36  func run(pass *analysis.Pass) (any, 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  				isMapIndex(pass.TypesInfo, lhs) {
    56  				continue // expressions may not be equal
    57  			}
    58  			if reflect.TypeOf(lhs) != reflect.TypeOf(rhs) {
    59  				continue // short-circuit the heavy-weight gofmt check
    60  			}
    61  			le := analysisinternal.Format(pass.Fset, lhs)
    62  			re := analysisinternal.Format(pass.Fset, rhs)
    63  			if le == re {
    64  				pass.Report(analysis.Diagnostic{
    65  					Pos: stmt.Pos(), Message: fmt.Sprintf("self-assignment of %s to %s", re, le),
    66  					SuggestedFixes: []analysis.SuggestedFix{{
    67  						Message: "Remove self-assignment",
    68  						TextEdits: []analysis.TextEdit{{
    69  							Pos: stmt.Pos(),
    70  							End: stmt.End(),
    71  						}}},
    72  					},
    73  				})
    74  			}
    75  		}
    76  	})
    77  
    78  	return nil, nil
    79  }
    80  
    81  // isMapIndex returns true if e is a map index expression.
    82  func isMapIndex(info *types.Info, e ast.Expr) bool {
    83  	if idx, ok := ast.Unparen(e).(*ast.IndexExpr); ok {
    84  		if typ := info.Types[idx.X].Type; typ != nil {
    85  			_, ok := typ.Underlying().(*types.Map)
    86  			return ok
    87  		}
    88  	}
    89  	return false
    90  }
    91
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