labels.go

Documentation: go/types

     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 types
     6  
     7  import (
     8  	"go/ast"
     9  	"go/token"
    10  	. "internal/types/errors"
    11  )
    12  
    13  // labels checks correct label use in body.
    14  func (check *Checker) labels(body *ast.BlockStmt) {
    15  	// set of all labels in this body
    16  	all := NewScope(nil, body.Pos(), body.End(), "label")
    17  
    18  	fwdJumps := check.blockBranches(all, nil, nil, body.List)
    19  
    20  	// If there are any forward jumps left, no label was found for
    21  	// the corresponding goto statements. Either those labels were
    22  	// never defined, or they are inside blocks and not reachable
    23  	// for the respective gotos.
    24  	for _, jmp := range fwdJumps {
    25  		var msg string
    26  		var code Code
    27  		name := jmp.Label.Name
    28  		if alt := all.Lookup(name); alt != nil {
    29  			msg = "goto %s jumps into block"
    30  			alt.(*Label).used = true // avoid another error
    31  			code = JumpIntoBlock
    32  		} else {
    33  			msg = "label %s not declared"
    34  			code = UndeclaredLabel
    35  		}
    36  		check.errorf(jmp.Label, code, msg, name)
    37  	}
    38  
    39  	// spec: "It is illegal to define a label that is never used."
    40  	for name, obj := range all.elems {
    41  		obj = resolve(name, obj)
    42  		if lbl := obj.(*Label); !lbl.used {
    43  			check.softErrorf(lbl, UnusedLabel, "label %s declared and not used", lbl.name)
    44  		}
    45  	}
    46  }
    47  
    48  // A block tracks label declarations in a block and its enclosing blocks.
    49  type block struct {
    50  	parent *block                      // enclosing block
    51  	lstmt  *ast.LabeledStmt            // labeled statement to which this block belongs, or nil
    52  	labels map[string]*ast.LabeledStmt // allocated lazily
    53  }
    54  
    55  // insert records a new label declaration for the current block.
    56  // The label must not have been declared before in any block.
    57  func (b *block) insert(s *ast.LabeledStmt) {
    58  	name := s.Label.Name
    59  	if debug {
    60  		assert(b.gotoTarget(name) == nil)
    61  	}
    62  	labels := b.labels
    63  	if labels == nil {
    64  		labels = make(map[string]*ast.LabeledStmt)
    65  		b.labels = labels
    66  	}
    67  	labels[name] = s
    68  }
    69  
    70  // gotoTarget returns the labeled statement in the current
    71  // or an enclosing block with the given label name, or nil.
    72  func (b *block) gotoTarget(name string) *ast.LabeledStmt {
    73  	for s := b; s != nil; s = s.parent {
    74  		if t := s.labels[name]; t != nil {
    75  			return t
    76  		}
    77  	}
    78  	return nil
    79  }
    80  
    81  // enclosingTarget returns the innermost enclosing labeled
    82  // statement with the given label name, or nil.
    83  func (b *block) enclosingTarget(name string) *ast.LabeledStmt {
    84  	for s := b; s != nil; s = s.parent {
    85  		if t := s.lstmt; t != nil && t.Label.Name == name {
    86  			return t
    87  		}
    88  	}
    89  	return nil
    90  }
    91  
    92  // blockBranches processes a block's statement list and returns the set of outgoing forward jumps.
    93  // all is the scope of all declared labels, parent the set of labels declared in the immediately
    94  // enclosing block, and lstmt is the labeled statement this block is associated with (or nil).
    95  func (check *Checker) blockBranches(all *Scope, parent *block, lstmt *ast.LabeledStmt, list []ast.Stmt) []*ast.BranchStmt {
    96  	b := &block{parent: parent, lstmt: lstmt}
    97  
    98  	var (
    99  		varDeclPos         token.Pos
   100  		fwdJumps, badJumps []*ast.BranchStmt
   101  	)
   102  
   103  	// All forward jumps jumping over a variable declaration are possibly
   104  	// invalid (they may still jump out of the block and be ok).
   105  	// recordVarDecl records them for the given position.
   106  	recordVarDecl := func(pos token.Pos) {
   107  		varDeclPos = pos
   108  		badJumps = append(badJumps[:0], fwdJumps...) // copy fwdJumps to badJumps
   109  	}
   110  
   111  	jumpsOverVarDecl := func(jmp *ast.BranchStmt) bool {
   112  		if varDeclPos.IsValid() {
   113  			for _, bad := range badJumps {
   114  				if jmp == bad {
   115  					return true
   116  				}
   117  			}
   118  		}
   119  		return false
   120  	}
   121  
   122  	blockBranches := func(lstmt *ast.LabeledStmt, list []ast.Stmt) {
   123  		// Unresolved forward jumps inside the nested block
   124  		// become forward jumps in the current block.
   125  		fwdJumps = append(fwdJumps, check.blockBranches(all, b, lstmt, list)...)
   126  	}
   127  
   128  	var stmtBranches func(ast.Stmt)
   129  	stmtBranches = func(s ast.Stmt) {
   130  		switch s := s.(type) {
   131  		case *ast.DeclStmt:
   132  			if d, _ := s.Decl.(*ast.GenDecl); d != nil && d.Tok == token.VAR {
   133  				recordVarDecl(d.Pos())
   134  			}
   135  
   136  		case *ast.LabeledStmt:
   137  			// declare non-blank label
   138  			if name := s.Label.Name; name != "_" {
   139  				lbl := NewLabel(s.Label.Pos(), check.pkg, name)
   140  				if alt := all.Insert(lbl); alt != nil {
   141  					check.softErrorf(lbl, DuplicateLabel, "label %s already declared", name)
   142  					check.reportAltDecl(alt)
   143  					// ok to continue
   144  				} else {
   145  					b.insert(s)
   146  					check.recordDef(s.Label, lbl)
   147  				}
   148  				// resolve matching forward jumps and remove them from fwdJumps
   149  				i := 0
   150  				for _, jmp := range fwdJumps {
   151  					if jmp.Label.Name == name {
   152  						// match
   153  						lbl.used = true
   154  						check.recordUse(jmp.Label, lbl)
   155  						if jumpsOverVarDecl(jmp) {
   156  							check.softErrorf(
   157  								jmp.Label,
   158  								JumpOverDecl,
   159  								"goto %s jumps over variable declaration at line %d",
   160  								name,
   161  								check.fset.Position(varDeclPos).Line,
   162  							)
   163  							// ok to continue
   164  						}
   165  					} else {
   166  						// no match - record new forward jump
   167  						fwdJumps[i] = jmp
   168  						i++
   169  					}
   170  				}
   171  				fwdJumps = fwdJumps[:i]
   172  				lstmt = s
   173  			}
   174  			stmtBranches(s.Stmt)
   175  
   176  		case *ast.BranchStmt:
   177  			if s.Label == nil {
   178  				return // checked in 1st pass (check.stmt)
   179  			}
   180  
   181  			// determine and validate target
   182  			name := s.Label.Name
   183  			switch s.Tok {
   184  			case token.BREAK:
   185  				// spec: "If there is a label, it must be that of an enclosing
   186  				// "for", "switch", or "select" statement, and that is the one
   187  				// whose execution terminates."
   188  				valid := false
   189  				if t := b.enclosingTarget(name); t != nil {
   190  					switch t.Stmt.(type) {
   191  					case *ast.SwitchStmt, *ast.TypeSwitchStmt, *ast.SelectStmt, *ast.ForStmt, *ast.RangeStmt:
   192  						valid = true
   193  					}
   194  				}
   195  				if !valid {
   196  					check.errorf(s.Label, MisplacedLabel, "invalid break label %s", name)
   197  					return
   198  				}
   199  
   200  			case token.CONTINUE:
   201  				// spec: "If there is a label, it must be that of an enclosing
   202  				// "for" statement, and that is the one whose execution advances."
   203  				valid := false
   204  				if t := b.enclosingTarget(name); t != nil {
   205  					switch t.Stmt.(type) {
   206  					case *ast.ForStmt, *ast.RangeStmt:
   207  						valid = true
   208  					}
   209  				}
   210  				if !valid {
   211  					check.errorf(s.Label, MisplacedLabel, "invalid continue label %s", name)
   212  					return
   213  				}
   214  
   215  			case token.GOTO:
   216  				if b.gotoTarget(name) == nil {
   217  					// label may be declared later - add branch to forward jumps
   218  					fwdJumps = append(fwdJumps, s)
   219  					return
   220  				}
   221  
   222  			default:
   223  				check.errorf(s, InvalidSyntaxTree, "branch statement: %s %s", s.Tok, name)
   224  				return
   225  			}
   226  
   227  			// record label use
   228  			obj := all.Lookup(name)
   229  			obj.(*Label).used = true
   230  			check.recordUse(s.Label, obj)
   231  
   232  		case *ast.AssignStmt:
   233  			if s.Tok == token.DEFINE {
   234  				recordVarDecl(s.Pos())
   235  			}
   236  
   237  		case *ast.BlockStmt:
   238  			blockBranches(lstmt, s.List)
   239  
   240  		case *ast.IfStmt:
   241  			stmtBranches(s.Body)
   242  			if s.Else != nil {
   243  				stmtBranches(s.Else)
   244  			}
   245  
   246  		case *ast.CaseClause:
   247  			blockBranches(nil, s.Body)
   248  
   249  		case *ast.SwitchStmt:
   250  			stmtBranches(s.Body)
   251  
   252  		case *ast.TypeSwitchStmt:
   253  			stmtBranches(s.Body)
   254  
   255  		case *ast.CommClause:
   256  			blockBranches(nil, s.Body)
   257  
   258  		case *ast.SelectStmt:
   259  			stmtBranches(s.Body)
   260  
   261  		case *ast.ForStmt:
   262  			stmtBranches(s.Body)
   263  
   264  		case *ast.RangeStmt:
   265  			stmtBranches(s.Body)
   266  		}
   267  	}
   268  
   269  	for _, s := range list {
   270  		stmtBranches(s)
   271  	}
   272  
   273  	return fwdJumps
   274  }
   275
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