cmd/vet: diagnose unreachable code

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/7493048

src/cmd/vet/deadcode.go

+// Copyright 2013 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Check for syntactically unreachable code.
+
+package main
+
+import (
+	"go/ast"
+	"go/token"
+)
+
+type deadState struct {
+	f           *File
+	hasBreak    map[ast.Stmt]bool
+	hasGoto     map[string]bool
+	labels      map[string]ast.Stmt
+	breakTarget ast.Stmt
+
+	reachable bool
+}
+
+// checkUnreachable checks a function body for dead code.
+func (f *File) checkUnreachable(body *ast.BlockStmt) {
+	if !vet("unreachable") || body == nil {
+		return
+	}
+
+	d := &deadState{
+		f:        f,
+		hasBreak: make(map[ast.Stmt]bool),
+		hasGoto:  make(map[string]bool),
+		labels:   make(map[string]ast.Stmt),
+	}
+
+	d.findLabels(body)
+
+	d.reachable = true
+	d.findDead(body)
+}
+
+// findLabels gathers information about the labels defined and used by stmt
+// and about which statements break, whether a label is involved or not.
+func (d *deadState) findLabels(stmt ast.Stmt) {
+	switch x := stmt.(type) {
+	default:
+		d.f.Warnf(x.Pos(), "internal error in findLabels: unexpected statement %T", x)
+
+	case *ast.AssignStmt,
+		*ast.BadStmt,
+		*ast.DeclStmt,
+		*ast.DeferStmt,
+		*ast.EmptyStmt,
+		*ast.ExprStmt,
+		*ast.GoStmt,
+		*ast.IncDecStmt,
+		*ast.ReturnStmt,
+		*ast.SendStmt:
+		// no statements inside
+
+	case *ast.BlockStmt:
+		for _, stmt := range x.List {
+			d.findLabels(stmt)
+		}
+
+	case *ast.BranchStmt:
+		switch x.Tok {
+		case token.GOTO:
+			d.hasGoto[x.Label.Name] = true
+
+		case token.BREAK:
+			stmt := d.breakTarget
+			if x.Label != nil {
+				stmt = d.labels[x.Label.Name]
+			}
+			if stmt != nil {
+				d.hasBreak[stmt] = true
+			}
+		}
+
+	case *ast.IfStmt:
+		d.findLabels(x.Body)
+		if x.Else != nil {
+			d.findLabels(x.Else)
+		}
+
+	case *ast.LabeledStmt:
+		d.labels[x.Label.Name] = x.Stmt
+		d.findLabels(x.Stmt)
+
+	// These cases are all the same, but the x.Body only works
+	// when the specific type of x is known, so the cases cannot
+	// be merged.
+	case *ast.ForStmt:
+		outer := d.breakTarget
+		d.breakTarget = x
+		d.findLabels(x.Body)
+		d.breakTarget = outer
+
+	case *ast.RangeStmt:
+		outer := d.breakTarget
+		d.breakTarget = x
+		d.findLabels(x.Body)
+		d.breakTarget = outer
+
+	case *ast.SelectStmt:
+		outer := d.breakTarget
+		d.breakTarget = x
+		d.findLabels(x.Body)
+		d.breakTarget = outer
+
+	case *ast.SwitchStmt:
+		outer := d.breakTarget
+		d.breakTarget = x
+		d.findLabels(x.Body)
+		d.breakTarget = outer
+
+	case *ast.TypeSwitchStmt:
+		outer := d.breakTarget
+		d.breakTarget = x
+		d.findLabels(x.Body)
+		d.breakTarget = outer
+
+	case *ast.CommClause:
+		for _, stmt := range x.Body {
+			d.findLabels(stmt)
+		}
+
+	case *ast.CaseClause:
+		for _, stmt := range x.Body {
+			d.findLabels(stmt)
+		}
+	}
+}
+
+// findDead walks the statement looking for dead code.
+// If d.reachable is false on entry, stmt itself is dead.
+// When findDead returns, d.reachable tells whether the
+// statement following stmt is reachable.
+func (d *deadState) findDead(stmt ast.Stmt) {
+	// Is this a labeled goto target?
+	// If so, assume it is reachable due to the goto.
+	// This is slightly conservative, in that we don't
+	// check that the goto is reachable, so
+	//	L: goto L
+	// will not provoke a warning.
+	// But it's good enough.
+	if x, isLabel := stmt.(*ast.LabeledStmt); isLabel && d.hasGoto[x.Label.Name] {
+		d.reachable = true
+	}
+
+	if !d.reachable {
+		switch stmt.(type) {
+		case *ast.EmptyStmt:
+			// do not warn about unreachable empty statements
+		default:
+			d.f.Warnf(stmt.Pos(), "unreachable code")
+			d.reachable = true // silence error about next statement
+		}
+	}
+
+	switch x := stmt.(type) {
+	default:
+		d.f.Warnf(x.Pos(), "internal error in findDead: unexpected statement %T", x)
+
+	case *ast.AssignStmt,
+		*ast.BadStmt,
+		*ast.DeclStmt,
+		*ast.DeferStmt,
+		*ast.EmptyStmt,
+		*ast.GoStmt,
+		*ast.IncDecStmt,
+		*ast.SendStmt:
+		// no control flow
+
+	case *ast.BlockStmt:
+		for _, stmt := range x.List {
+			d.findDead(stmt)
+		}
+
+	case *ast.BranchStmt:
+		switch x.Tok {
+		case token.BREAK, token.GOTO, token.FALLTHROUGH:
+			d.reachable = false
+		case token.CONTINUE:
+			// NOTE: We accept "continue" statements as terminating.
+			// They are not necessary in the spec definition of terminating,
+			// because a continue statement cannot be the final statement
+			// before a return. But for the more general problem of syntactically
+			// identifying dead code, continue redirects control flow just
+			// like the other terminating statements.
+			d.reachable = false
+		}
+
+	case *ast.ExprStmt:
+		// Call to panic?
+		call, ok := x.X.(*ast.CallExpr)
+		if ok {
+			name, ok := call.Fun.(*ast.Ident)
+			if ok && name.Name == "panic" && name.Obj == nil {
+				d.reachable = false
+			}
+		}
+
+	case *ast.ForStmt:
+		d.findDead(x.Body)
+		d.reachable = x.Cond != nil || d.hasBreak[x]
+
+	case *ast.IfStmt:
+		d.findDead(x.Body)
+		if x.Else != nil {
+			r := d.reachable
+			d.reachable = true
+			d.findDead(x.Else)
+			d.reachable = d.reachable || r
+		} else {
+			// might not have executed if statement
+			d.reachable = true
+		}
+
+	case *ast.LabeledStmt:
+		d.findDead(x.Stmt)
+
+	case *ast.RangeStmt:
+		d.findDead(x.Body)
+		d.reachable = true
+
+	case *ast.ReturnStmt:
+		d.reachable = false
+
+	case *ast.SelectStmt:
+		// NOTE: Unlike switch and type switch below, we don't care
+		// whether a select has a default, because a select without a
+		// default blocks until one of the cases can run. That's different
+		// from a switch without a default, which behaves like it has
+		// a default with an empty body.
+		anyReachable := false
+		for _, comm := range x.Body.List {
+			d.reachable = true
+			for _, stmt := range comm.(*ast.CommClause).Body {
+				d.findDead(stmt)
+			}
+			anyReachable = anyReachable || d.reachable
+		}
+		d.reachable = anyReachable || d.hasBreak[x]
+
+	case *ast.SwitchStmt:
+		anyReachable := false
+		hasDefault := false
+		for _, cas := range x.Body.List {
+			cc := cas.(*ast.CaseClause)
+			if cc.List == nil {
+				hasDefault = true
+			}
+			d.reachable = true
+			for _, stmt := range cc.Body {
+				d.findDead(stmt)
+			}
+			anyReachable = anyReachable || d.reachable
+		}
+		d.reachable = anyReachable || d.hasBreak[x] || !hasDefault
+
+	case *ast.TypeSwitchStmt:
+		anyReachable := false
+		hasDefault := false
+		for _, cas := range x.Body.List {
+			cc := cas.(*ast.CaseClause)
+			if cc.List == nil {
+				hasDefault = true
+			}
+			d.reachable = true
+			for _, stmt := range cc.Body {
+				d.findDead(stmt)
+			}
+			anyReachable = anyReachable || d.reachable
+		}
+		d.reachable = anyReachable || d.hasBreak[x] || !hasDefault
+	}
+}

src/cmd/vet/main.go

@@ -28,16 +28,17 @@ var exitCode = 0
 // Flags to control which checks to perform. "all" is set to true here, and disabled later if
 // a flag is set explicitly.
 var report = map[string]*bool{
-	"all":        flag.Bool("all", true, "check everything; disabled if any explicit check is requested"),
-	"asmdecl":    flag.Bool("asmdecl", false, "check assembly against Go declarations"),
-	"assign":     flag.Bool("assign", false, "check for useless assignments"),
-	"atomic":     flag.Bool("atomic", false, "check for common mistaken usages of the sync/atomic package"),
-	"buildtags":  flag.Bool("buildtags", false, "check that +build tags are valid"),
-	"composites": flag.Bool("composites", false, "check that composite literals used type-tagged elements"),
-	"methods":    flag.Bool("methods", false, "check that canonically named methods are canonically defined"),
-	"printf":     flag.Bool("printf", false, "check printf-like invocations"),
-	"structtags": flag.Bool("structtags", false, "check that struct field tags have canonical format"),
-	"rangeloops": flag.Bool("rangeloops", false, "check that range loop variables are used correctly"),
+	"all":         flag.Bool("all", true, "check everything; disabled if any explicit check is requested"),
+	"asmdecl":     flag.Bool("asmdecl", false, "check assembly against Go declarations"),
+	"assign":      flag.Bool("assign", false, "check for useless assignments"),
+	"atomic":      flag.Bool("atomic", false, "check for common mistaken usages of the sync/atomic package"),
+	"buildtags":   flag.Bool("buildtags", false, "check that +build tags are valid"),
+	"composites":  flag.Bool("composites", false, "check that composite literals used type-tagged elements"),
+	"methods":     flag.Bool("methods", false, "check that canonically named methods are canonically defined"),
+	"printf":      flag.Bool("printf", false, "check printf-like invocations"),
+	"rangeloops":  flag.Bool("rangeloops", false, "check that range loop variables are used correctly"),
+	"structtags":  flag.Bool("structtags", false, "check that struct field tags have canonical format"),
+	"unreachable": flag.Bool("unreachable", false, "check for unreachable code"),
 }
 
 // vet tells whether to report errors for the named check, a flag name.
@@ -336,7 +337,9 @@ func (f *File) Visit(node ast.Node) ast.Visitor {
 	case *ast.Field:
 		f.walkFieldTag(n)
 	case *ast.FuncDecl:
-		f.walkMethodDecl(n)
+		f.walkFuncDecl(n)
+	case *ast.FuncLit:
+		f.walkFuncLit(n)
 	case *ast.InterfaceType:
 		f.walkInterfaceType(n)
 	case *ast.RangeStmt:
@@ -379,18 +382,22 @@ func (f *File) walkFieldTag(field *ast.Field) {
 	f.checkCanonicalFieldTag(field)
 }
 
-// walkMethodDecl walks the method's signature.
+// walkMethod walks the method's signature.
 func (f *File) walkMethod(id *ast.Ident, t *ast.FuncType) {
 	f.checkCanonicalMethod(id, t)
 }
 
-// walkMethodDecl walks the method signature in the declaration.
-func (f *File) walkMethodDecl(d *ast.FuncDecl) {
-	if d.Recv == nil {
-		// not a method
-		return
+// walkFuncDecl walks a function declaration.
+func (f *File) walkFuncDecl(d *ast.FuncDecl) {
+	f.checkUnreachable(d.Body)
+	if d.Recv != nil {
+		f.walkMethod(d.Name, d.Type)
 	}
-	f.walkMethod(d.Name, d.Type)
+}
+
+// walkFuncLit walks a function literal.
+func (f *File) walkFuncLit(x *ast.FuncLit) {
+	f.checkUnreachable(x.Body)
 }
 
 // walkInterfaceType walks the method signatures of an interface.