Source file src/cmd/vendor/golang.org/x/tools/go/analysis/passes/unreachable/unreachable.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 unreachable defines an Analyzer that checks for unreachable code.
     6	package unreachable
     7	
     8	// TODO(adonovan): use the new cfg package, which is more precise.
     9	
    10	import (
    11		"go/ast"
    12		"go/token"
    13		"log"
    14	
    15		"golang.org/x/tools/go/analysis"
    16		"golang.org/x/tools/go/analysis/passes/inspect"
    17		"golang.org/x/tools/go/ast/inspector"
    18	)
    19	
    20	const Doc = `check for unreachable code
    21	
    22	The unreachable analyzer finds statements that execution can never reach
    23	because they are preceded by an return statement, a call to panic, an
    24	infinite loop, or similar constructs.`
    25	
    26	var Analyzer = &analysis.Analyzer{
    27		Name:             "unreachable",
    28		Doc:              Doc,
    29		Requires:         []*analysis.Analyzer{inspect.Analyzer},
    30		RunDespiteErrors: true,
    31		Run:              run,
    32	}
    33	
    34	func run(pass *analysis.Pass) (interface{}, error) {
    35		inspect := pass.ResultOf[inspect.Analyzer].(*inspector.Inspector)
    36	
    37		nodeFilter := []ast.Node{
    38			(*ast.FuncDecl)(nil),
    39			(*ast.FuncLit)(nil),
    40		}
    41		inspect.Preorder(nodeFilter, func(n ast.Node) {
    42			var body *ast.BlockStmt
    43			switch n := n.(type) {
    44			case *ast.FuncDecl:
    45				body = n.Body
    46			case *ast.FuncLit:
    47				body = n.Body
    48			}
    49			if body == nil {
    50				return
    51			}
    52			d := &deadState{
    53				pass:     pass,
    54				hasBreak: make(map[ast.Stmt]bool),
    55				hasGoto:  make(map[string]bool),
    56				labels:   make(map[string]ast.Stmt),
    57			}
    58			d.findLabels(body)
    59			d.reachable = true
    60			d.findDead(body)
    61		})
    62		return nil, nil
    63	}
    64	
    65	type deadState struct {
    66		pass        *analysis.Pass
    67		hasBreak    map[ast.Stmt]bool
    68		hasGoto     map[string]bool
    69		labels      map[string]ast.Stmt
    70		breakTarget ast.Stmt
    71	
    72		reachable bool
    73	}
    74	
    75	// findLabels gathers information about the labels defined and used by stmt
    76	// and about which statements break, whether a label is involved or not.
    77	func (d *deadState) findLabels(stmt ast.Stmt) {
    78		switch x := stmt.(type) {
    79		default:
    80			log.Fatalf("%s: internal error in findLabels: unexpected statement %T", d.pass.Fset.Position(x.Pos()), x)
    81	
    82		case *ast.AssignStmt,
    83			*ast.BadStmt,
    84			*ast.DeclStmt,
    85			*ast.DeferStmt,
    86			*ast.EmptyStmt,
    87			*ast.ExprStmt,
    88			*ast.GoStmt,
    89			*ast.IncDecStmt,
    90			*ast.ReturnStmt,
    91			*ast.SendStmt:
    92			// no statements inside
    93	
    94		case *ast.BlockStmt:
    95			for _, stmt := range x.List {
    96				d.findLabels(stmt)
    97			}
    98	
    99		case *ast.BranchStmt:
   100			switch x.Tok {
   101			case token.GOTO:
   102				if x.Label != nil {
   103					d.hasGoto[x.Label.Name] = true
   104				}
   105	
   106			case token.BREAK:
   107				stmt := d.breakTarget
   108				if x.Label != nil {
   109					stmt = d.labels[x.Label.Name]
   110				}
   111				if stmt != nil {
   112					d.hasBreak[stmt] = true
   113				}
   114			}
   115	
   116		case *ast.IfStmt:
   117			d.findLabels(x.Body)
   118			if x.Else != nil {
   119				d.findLabels(x.Else)
   120			}
   121	
   122		case *ast.LabeledStmt:
   123			d.labels[x.Label.Name] = x.Stmt
   124			d.findLabels(x.Stmt)
   125	
   126		// These cases are all the same, but the x.Body only works
   127		// when the specific type of x is known, so the cases cannot
   128		// be merged.
   129		case *ast.ForStmt:
   130			outer := d.breakTarget
   131			d.breakTarget = x
   132			d.findLabels(x.Body)
   133			d.breakTarget = outer
   134	
   135		case *ast.RangeStmt:
   136			outer := d.breakTarget
   137			d.breakTarget = x
   138			d.findLabels(x.Body)
   139			d.breakTarget = outer
   140	
   141		case *ast.SelectStmt:
   142			outer := d.breakTarget
   143			d.breakTarget = x
   144			d.findLabels(x.Body)
   145			d.breakTarget = outer
   146	
   147		case *ast.SwitchStmt:
   148			outer := d.breakTarget
   149			d.breakTarget = x
   150			d.findLabels(x.Body)
   151			d.breakTarget = outer
   152	
   153		case *ast.TypeSwitchStmt:
   154			outer := d.breakTarget
   155			d.breakTarget = x
   156			d.findLabels(x.Body)
   157			d.breakTarget = outer
   158	
   159		case *ast.CommClause:
   160			for _, stmt := range x.Body {
   161				d.findLabels(stmt)
   162			}
   163	
   164		case *ast.CaseClause:
   165			for _, stmt := range x.Body {
   166				d.findLabels(stmt)
   167			}
   168		}
   169	}
   170	
   171	// findDead walks the statement looking for dead code.
   172	// If d.reachable is false on entry, stmt itself is dead.
   173	// When findDead returns, d.reachable tells whether the
   174	// statement following stmt is reachable.
   175	func (d *deadState) findDead(stmt ast.Stmt) {
   176		// Is this a labeled goto target?
   177		// If so, assume it is reachable due to the goto.
   178		// This is slightly conservative, in that we don't
   179		// check that the goto is reachable, so
   180		//	L: goto L
   181		// will not provoke a warning.
   182		// But it's good enough.
   183		if x, isLabel := stmt.(*ast.LabeledStmt); isLabel && d.hasGoto[x.Label.Name] {
   184			d.reachable = true
   185		}
   186	
   187		if !d.reachable {
   188			switch stmt.(type) {
   189			case *ast.EmptyStmt:
   190				// do not warn about unreachable empty statements
   191			default:
   192				d.pass.Reportf(stmt.Pos(), "unreachable code")
   193				d.reachable = true // silence error about next statement
   194			}
   195		}
   196	
   197		switch x := stmt.(type) {
   198		default:
   199			log.Fatalf("%s: internal error in findDead: unexpected statement %T", d.pass.Fset.Position(x.Pos()), x)
   200	
   201		case *ast.AssignStmt,
   202			*ast.BadStmt,
   203			*ast.DeclStmt,
   204			*ast.DeferStmt,
   205			*ast.EmptyStmt,
   206			*ast.GoStmt,
   207			*ast.IncDecStmt,
   208			*ast.SendStmt:
   209			// no control flow
   210	
   211		case *ast.BlockStmt:
   212			for _, stmt := range x.List {
   213				d.findDead(stmt)
   214			}
   215	
   216		case *ast.BranchStmt:
   217			switch x.Tok {
   218			case token.BREAK, token.GOTO, token.FALLTHROUGH:
   219				d.reachable = false
   220			case token.CONTINUE:
   221				// NOTE: We accept "continue" statements as terminating.
   222				// They are not necessary in the spec definition of terminating,
   223				// because a continue statement cannot be the final statement
   224				// before a return. But for the more general problem of syntactically
   225				// identifying dead code, continue redirects control flow just
   226				// like the other terminating statements.
   227				d.reachable = false
   228			}
   229	
   230		case *ast.ExprStmt:
   231			// Call to panic?
   232			call, ok := x.X.(*ast.CallExpr)
   233			if ok {
   234				name, ok := call.Fun.(*ast.Ident)
   235				if ok && name.Name == "panic" && name.Obj == nil {
   236					d.reachable = false
   237				}
   238			}
   239	
   240		case *ast.ForStmt:
   241			d.findDead(x.Body)
   242			d.reachable = x.Cond != nil || d.hasBreak[x]
   243	
   244		case *ast.IfStmt:
   245			d.findDead(x.Body)
   246			if x.Else != nil {
   247				r := d.reachable
   248				d.reachable = true
   249				d.findDead(x.Else)
   250				d.reachable = d.reachable || r
   251			} else {
   252				// might not have executed if statement
   253				d.reachable = true
   254			}
   255	
   256		case *ast.LabeledStmt:
   257			d.findDead(x.Stmt)
   258	
   259		case *ast.RangeStmt:
   260			d.findDead(x.Body)
   261			d.reachable = true
   262	
   263		case *ast.ReturnStmt:
   264			d.reachable = false
   265	
   266		case *ast.SelectStmt:
   267			// NOTE: Unlike switch and type switch below, we don't care
   268			// whether a select has a default, because a select without a
   269			// default blocks until one of the cases can run. That's different
   270			// from a switch without a default, which behaves like it has
   271			// a default with an empty body.
   272			anyReachable := false
   273			for _, comm := range x.Body.List {
   274				d.reachable = true
   275				for _, stmt := range comm.(*ast.CommClause).Body {
   276					d.findDead(stmt)
   277				}
   278				anyReachable = anyReachable || d.reachable
   279			}
   280			d.reachable = anyReachable || d.hasBreak[x]
   281	
   282		case *ast.SwitchStmt:
   283			anyReachable := false
   284			hasDefault := false
   285			for _, cas := range x.Body.List {
   286				cc := cas.(*ast.CaseClause)
   287				if cc.List == nil {
   288					hasDefault = true
   289				}
   290				d.reachable = true
   291				for _, stmt := range cc.Body {
   292					d.findDead(stmt)
   293				}
   294				anyReachable = anyReachable || d.reachable
   295			}
   296			d.reachable = anyReachable || d.hasBreak[x] || !hasDefault
   297	
   298		case *ast.TypeSwitchStmt:
   299			anyReachable := false
   300			hasDefault := false
   301			for _, cas := range x.Body.List {
   302				cc := cas.(*ast.CaseClause)
   303				if cc.List == nil {
   304					hasDefault = true
   305				}
   306				d.reachable = true
   307				for _, stmt := range cc.Body {
   308					d.findDead(stmt)
   309				}
   310				anyReachable = anyReachable || d.reachable
   311			}
   312			d.reachable = anyReachable || d.hasBreak[x] || !hasDefault
   313		}
   314	}
   315	

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