cmd/cgo: use a named type to indicate syntactic context

We previously used bare strings, which made it difficult to see (and
to cross-reference) the set of allowed context values.

This change is purely cosmetic, but makes it easier for me to
understand how to address #21878.

updates #21878

Change-Id: I9027d94fd5997a0fe857c0055dea8719e1511f03
Reviewed-on: https://go-review.googlesource.com/63830
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/cgo/ast.go

@@ -126,7 +126,7 @@ func (f *File) ParseGo(name string, src []byte) {
 	if f.Ref == nil {
 		f.Ref = make([]*Ref, 0, 8)
 	}
-	f.walk(ast2, "prog", (*File).saveExprs)
+	f.walk(ast2, ctxProg, (*File).saveExprs)
 
 	// Accumulate exported functions.
 	// The comments are only on ast1 but we need to
@@ -134,8 +134,8 @@ func (f *File) ParseGo(name string, src []byte) {
 	// The first walk fills in ExpFunc, and the
 	// second walk changes the entries to
 	// refer to ast2 instead.
-	f.walk(ast1, "prog", (*File).saveExport)
-	f.walk(ast2, "prog", (*File).saveExport2)
+	f.walk(ast1, ctxProg, (*File).saveExport)
+	f.walk(ast2, ctxProg, (*File).saveExport2)
 
 	f.Comments = ast1.Comments
 	f.AST = ast2
@@ -166,7 +166,7 @@ func commentText(g *ast.CommentGroup) string {
 }
 
 // Save various references we are going to need later.
-func (f *File) saveExprs(x interface{}, context string) {
+func (f *File) saveExprs(x interface{}, context astContext) {
 	switch x := x.(type) {
 	case *ast.Expr:
 		switch (*x).(type) {
@@ -179,7 +179,7 @@ func (f *File) saveExprs(x interface{}, context string) {
 }
 
 // Save references to C.xxx for later processing.
-func (f *File) saveRef(n *ast.Expr, context string) {
+func (f *File) saveRef(n *ast.Expr, context astContext) {
 	sel := (*n).(*ast.SelectorExpr)
 	// For now, assume that the only instance of capital C is when
 	// used as the imported package identifier.
@@ -189,10 +189,10 @@ func (f *File) saveRef(n *ast.Expr, context string) {
 	if l, ok := sel.X.(*ast.Ident); !ok || l.Name != "C" {
 		return
 	}
-	if context == "as2" {
-		context = "expr"
+	if context == ctxAssign2 {
+		context = ctxExpr
 	}
-	if context == "embed-type" {
+	if context == ctxEmbedType {
 		error_(sel.Pos(), "cannot embed C type")
 	}
 	goname := sel.Sel.Name
@@ -223,7 +223,7 @@ func (f *File) saveRef(n *ast.Expr, context string) {
 }
 
 // Save calls to C.xxx for later processing.
-func (f *File) saveCall(call *ast.CallExpr, context string) {
+func (f *File) saveCall(call *ast.CallExpr, context astContext) {
 	sel, ok := call.Fun.(*ast.SelectorExpr)
 	if !ok {
 		return
@@ -231,12 +231,12 @@ func (f *File) saveCall(call *ast.CallExpr, context string) {
 	if l, ok := sel.X.(*ast.Ident); !ok || l.Name != "C" {
 		return
 	}
-	c := &Call{Call: call, Deferred: context == "defer"}
+	c := &Call{Call: call, Deferred: context == ctxDefer}
 	f.Calls = append(f.Calls, c)
 }
 
 // If a function should be exported add it to ExpFunc.
-func (f *File) saveExport(x interface{}, context string) {
+func (f *File) saveExport(x interface{}, context astContext) {
 	n, ok := x.(*ast.FuncDecl)
 	if !ok {
 		return
@@ -276,7 +276,7 @@ func (f *File) saveExport(x interface{}, context string) {
 }
 
 // Make f.ExpFunc[i] point at the Func from this AST instead of the other one.
-func (f *File) saveExport2(x interface{}, context string) {
+func (f *File) saveExport2(x interface{}, context astContext) {
 	n, ok := x.(*ast.FuncDecl)
 	if !ok {
 		return
@@ -290,8 +290,30 @@ func (f *File) saveExport2(x interface{}, context string) {
 	}
 }
 
+type astContext int
+
+const (
+	ctxProg astContext = iota
+	ctxEmbedType
+	ctxType
+	ctxStmt
+	ctxExpr
+	ctxField
+	ctxParam
+	ctxAssign2 // assignment of a single expression to two variables
+	ctxSwitch
+	ctxTypeSwitch
+	ctxFile
+	ctxDecl
+	ctxSpec
+	ctxDefer
+	ctxCall  // any function call other than ctxCall2
+	ctxCall2 // function call whose result is assigned to two variables
+	ctxSelector
+)
+
 // walk walks the AST x, calling visit(f, x, context) for each node.
-func (f *File) walk(x interface{}, context string, visit func(*File, interface{}, string)) {
+func (f *File) walk(x interface{}, context astContext, visit func(*File, interface{}, astContext)) {
 	visit(f, x, context)
 	switch n := x.(type) {
 	case *ast.Expr:
@@ -306,10 +328,10 @@ func (f *File) walk(x interface{}, context string, visit func(*File, interface{}
 
 	// These are ordered and grouped to match ../../go/ast/ast.go
 	case *ast.Field:
-		if len(n.Names) == 0 && context == "field" {
-			f.walk(&n.Type, "embed-type", visit)
+		if len(n.Names) == 0 && context == ctxField {
+			f.walk(&n.Type, ctxEmbedType, visit)
 		} else {
-			f.walk(&n.Type, "type", visit)
+			f.walk(&n.Type, ctxType, visit)
 		}
 	case *ast.FieldList:
 		for _, field := range n.List {
@@ -320,163 +342,163 @@ func (f *File) walk(x interface{}, context string, visit func(*File, interface{}
 	case *ast.Ellipsis:
 	case *ast.BasicLit:
 	case *ast.FuncLit:
-		f.walk(n.Type, "type", visit)
-		f.walk(n.Body, "stmt", visit)
+		f.walk(n.Type, ctxType, visit)
+		f.walk(n.Body, ctxStmt, visit)
 	case *ast.CompositeLit:
-		f.walk(&n.Type, "type", visit)
-		f.walk(n.Elts, "expr", visit)
+		f.walk(&n.Type, ctxType, visit)
+		f.walk(n.Elts, ctxExpr, visit)
 	case *ast.ParenExpr:
 		f.walk(&n.X, context, visit)
 	case *ast.SelectorExpr:
-		f.walk(&n.X, "selector", visit)
+		f.walk(&n.X, ctxSelector, visit)
 	case *ast.IndexExpr:
-		f.walk(&n.X, "expr", visit)
-		f.walk(&n.Index, "expr", visit)
+		f.walk(&n.X, ctxExpr, visit)
+		f.walk(&n.Index, ctxExpr, visit)
 	case *ast.SliceExpr:
-		f.walk(&n.X, "expr", visit)
+		f.walk(&n.X, ctxExpr, visit)
 		if n.Low != nil {
-			f.walk(&n.Low, "expr", visit)
+			f.walk(&n.Low, ctxExpr, visit)
 		}
 		if n.High != nil {
-			f.walk(&n.High, "expr", visit)
+			f.walk(&n.High, ctxExpr, visit)
 		}
 		if n.Max != nil {
-			f.walk(&n.Max, "expr", visit)
+			f.walk(&n.Max, ctxExpr, visit)
 		}
 	case *ast.TypeAssertExpr:
-		f.walk(&n.X, "expr", visit)
-		f.walk(&n.Type, "type", visit)
+		f.walk(&n.X, ctxExpr, visit)
+		f.walk(&n.Type, ctxType, visit)
 	case *ast.CallExpr:
-		if context == "as2" {
-			f.walk(&n.Fun, "call2", visit)
+		if context == ctxAssign2 {
+			f.walk(&n.Fun, ctxCall2, visit)
 		} else {
-			f.walk(&n.Fun, "call", visit)
+			f.walk(&n.Fun, ctxCall, visit)
 		}
-		f.walk(n.Args, "expr", visit)
+		f.walk(n.Args, ctxExpr, visit)
 	case *ast.StarExpr:
 		f.walk(&n.X, context, visit)
 	case *ast.UnaryExpr:
-		f.walk(&n.X, "expr", visit)
+		f.walk(&n.X, ctxExpr, visit)
 	case *ast.BinaryExpr:
-		f.walk(&n.X, "expr", visit)
-		f.walk(&n.Y, "expr", visit)
+		f.walk(&n.X, ctxExpr, visit)
+		f.walk(&n.Y, ctxExpr, visit)
 	case *ast.KeyValueExpr:
-		f.walk(&n.Key, "expr", visit)
-		f.walk(&n.Value, "expr", visit)
+		f.walk(&n.Key, ctxExpr, visit)
+		f.walk(&n.Value, ctxExpr, visit)
 
 	case *ast.ArrayType:
-		f.walk(&n.Len, "expr", visit)
-		f.walk(&n.Elt, "type", visit)
+		f.walk(&n.Len, ctxExpr, visit)
+		f.walk(&n.Elt, ctxType, visit)
 	case *ast.StructType:
-		f.walk(n.Fields, "field", visit)
+		f.walk(n.Fields, ctxField, visit)
 	case *ast.FuncType:
-		f.walk(n.Params, "param", visit)
+		f.walk(n.Params, ctxParam, visit)
 		if n.Results != nil {
-			f.walk(n.Results, "param", visit)
+			f.walk(n.Results, ctxParam, visit)
 		}
 	case *ast.InterfaceType:
-		f.walk(n.Methods, "field", visit)
+		f.walk(n.Methods, ctxField, visit)
 	case *ast.MapType:
-		f.walk(&n.Key, "type", visit)
-		f.walk(&n.Value, "type", visit)
+		f.walk(&n.Key, ctxType, visit)
+		f.walk(&n.Value, ctxType, visit)
 	case *ast.ChanType:
-		f.walk(&n.Value, "type", visit)
+		f.walk(&n.Value, ctxType, visit)
 
 	case *ast.BadStmt:
 	case *ast.DeclStmt:
-		f.walk(n.Decl, "decl", visit)
+		f.walk(n.Decl, ctxDecl, visit)
 	case *ast.EmptyStmt:
 	case *ast.LabeledStmt:
-		f.walk(n.Stmt, "stmt", visit)
+		f.walk(n.Stmt, ctxStmt, visit)
 	case *ast.ExprStmt:
-		f.walk(&n.X, "expr", visit)
+		f.walk(&n.X, ctxExpr, visit)
 	case *ast.SendStmt:
-		f.walk(&n.Chan, "expr", visit)
-		f.walk(&n.Value, "expr", visit)
+		f.walk(&n.Chan, ctxExpr, visit)
+		f.walk(&n.Value, ctxExpr, visit)
 	case *ast.IncDecStmt:
-		f.walk(&n.X, "expr", visit)
+		f.walk(&n.X, ctxExpr, visit)
 	case *ast.AssignStmt:
-		f.walk(n.Lhs, "expr", visit)
+		f.walk(n.Lhs, ctxExpr, visit)
 		if len(n.Lhs) == 2 && len(n.Rhs) == 1 {
-			f.walk(n.Rhs, "as2", visit)
+			f.walk(n.Rhs, ctxAssign2, visit)
 		} else {
-			f.walk(n.Rhs, "expr", visit)
+			f.walk(n.Rhs, ctxExpr, visit)
 		}
 	case *ast.GoStmt:
-		f.walk(n.Call, "expr", visit)
+		f.walk(n.Call, ctxExpr, visit)
 	case *ast.DeferStmt:
-		f.walk(n.Call, "defer", visit)
+		f.walk(n.Call, ctxDefer, visit)
 	case *ast.ReturnStmt:
-		f.walk(n.Results, "expr", visit)
+		f.walk(n.Results, ctxExpr, visit)
 	case *ast.BranchStmt:
 	case *ast.BlockStmt:
 		f.walk(n.List, context, visit)
 	case *ast.IfStmt:
-		f.walk(n.Init, "stmt", visit)
-		f.walk(&n.Cond, "expr", visit)
-		f.walk(n.Body, "stmt", visit)
-		f.walk(n.Else, "stmt", visit)
+		f.walk(n.Init, ctxStmt, visit)
+		f.walk(&n.Cond, ctxExpr, visit)
+		f.walk(n.Body, ctxStmt, visit)
+		f.walk(n.Else, ctxStmt, visit)
 	case *ast.CaseClause:
-		if context == "typeswitch" {
-			context = "type"
+		if context == ctxTypeSwitch {
+			context = ctxType
 		} else {
-			context = "expr"
+			context = ctxExpr
 		}
 		f.walk(n.List, context, visit)
-		f.walk(n.Body, "stmt", visit)
+		f.walk(n.Body, ctxStmt, visit)
 	case *ast.SwitchStmt:
-		f.walk(n.Init, "stmt", visit)
-		f.walk(&n.Tag, "expr", visit)
-		f.walk(n.Body, "switch", visit)
+		f.walk(n.Init, ctxStmt, visit)
+		f.walk(&n.Tag, ctxExpr, visit)
+		f.walk(n.Body, ctxSwitch, visit)
 	case *ast.TypeSwitchStmt:
-		f.walk(n.Init, "stmt", visit)
-		f.walk(n.Assign, "stmt", visit)
-		f.walk(n.Body, "typeswitch", visit)
+		f.walk(n.Init, ctxStmt, visit)
+		f.walk(n.Assign, ctxStmt, visit)
+		f.walk(n.Body, ctxTypeSwitch, visit)
 	case *ast.CommClause:
-		f.walk(n.Comm, "stmt", visit)
-		f.walk(n.Body, "stmt", visit)
+		f.walk(n.Comm, ctxStmt, visit)
+		f.walk(n.Body, ctxStmt, visit)
 	case *ast.SelectStmt:
-		f.walk(n.Body, "stmt", visit)
+		f.walk(n.Body, ctxStmt, visit)
 	case *ast.ForStmt:
-		f.walk(n.Init, "stmt", visit)
-		f.walk(&n.Cond, "expr", visit)
-		f.walk(n.Post, "stmt", visit)
-		f.walk(n.Body, "stmt", visit)
+		f.walk(n.Init, ctxStmt, visit)
+		f.walk(&n.Cond, ctxExpr, visit)
+		f.walk(n.Post, ctxStmt, visit)
+		f.walk(n.Body, ctxStmt, visit)
 	case *ast.RangeStmt:
-		f.walk(&n.Key, "expr", visit)
-		f.walk(&n.Value, "expr", visit)
-		f.walk(&n.X, "expr", visit)
-		f.walk(n.Body, "stmt", visit)
+		f.walk(&n.Key, ctxExpr, visit)
+		f.walk(&n.Value, ctxExpr, visit)
+		f.walk(&n.X, ctxExpr, visit)
+		f.walk(n.Body, ctxStmt, visit)
 
 	case *ast.ImportSpec:
 	case *ast.ValueSpec:
-		f.walk(&n.Type, "type", visit)
+		f.walk(&n.Type, ctxType, visit)
 		if len(n.Names) == 2 && len(n.Values) == 1 {
-			f.walk(&n.Values[0], "as2", visit)
+			f.walk(&n.Values[0], ctxAssign2, visit)
 		} else {
-			f.walk(n.Values, "expr", visit)
+			f.walk(n.Values, ctxExpr, visit)
 		}
 	case *ast.TypeSpec:
-		f.walk(&n.Type, "type", visit)
+		f.walk(&n.Type, ctxType, visit)
 
 	case *ast.BadDecl:
 	case *ast.GenDecl:
-		f.walk(n.Specs, "spec", visit)
+		f.walk(n.Specs, ctxSpec, visit)
 	case *ast.FuncDecl:
 		if n.Recv != nil {
-			f.walk(n.Recv, "param", visit)
+			f.walk(n.Recv, ctxParam, visit)
 		}
-		f.walk(n.Type, "type", visit)
+		f.walk(n.Type, ctxType, visit)
 		if n.Body != nil {
-			f.walk(n.Body, "stmt", visit)
+			f.walk(n.Body, ctxStmt, visit)
 		}
 
 	case *ast.File:
-		f.walk(n.Decls, "decl", visit)
+		f.walk(n.Decls, ctxDecl, visit)
 
 	case *ast.Package:
 		for _, file := range n.Files {
-			f.walk(file, "file", visit)
+			f.walk(file, ctxFile, visit)
 		}
 
 	case []ast.Decl:

src/cmd/cgo/gcc.go

@@ -748,7 +748,7 @@ func (p *Package) rewriteCall(f *File, call *Call, name *Name) bool {
 
 			// If this call expects two results, we have to
 			// adjust the results of the function we generated.
-			if ref.Context == "call2" {
+			if ref.Context == ctxCall2 {
 				if ftype.Results == nil {
 					// An explicit void argument
 					// looks odd but it seems to
@@ -940,8 +940,8 @@ func (p *Package) checkAddrArgs(f *File, args []ast.Expr, x ast.Expr) []ast.Expr
 // effect is a function call.
 func (p *Package) hasSideEffects(f *File, x ast.Expr) bool {
 	found := false
-	f.walk(x, "expr",
-		func(f *File, x interface{}, context string) {
+	f.walk(x, ctxExpr,
+		func(f *File, x interface{}, context astContext) {
 			switch x.(type) {
 			case *ast.CallExpr:
 				found = true
@@ -1080,10 +1080,10 @@ func (p *Package) rewriteRef(f *File) {
 		}
 		var expr ast.Expr = ast.NewIdent(r.Name.Mangle) // default
 		switch r.Context {
-		case "call", "call2":
+		case ctxCall, ctxCall2:
 			if r.Name.Kind != "func" {
 				if r.Name.Kind == "type" {
-					r.Context = "type"
+					r.Context = ctxType
 					if r.Name.Type == nil {
 						error_(r.Pos(), "invalid conversion to C.%s: undefined C type '%s'", fixGo(r.Name.Go), r.Name.C)
 						break
@@ -1095,7 +1095,7 @@ func (p *Package) rewriteRef(f *File) {
 				break
 			}
 			functions[r.Name.Go] = true
-			if r.Context == "call2" {
+			if r.Context == ctxCall2 {
 				if r.Name.Go == "_CMalloc" {
 					error_(r.Pos(), "no two-result form for C.malloc")
 					break
@@ -1113,7 +1113,7 @@ func (p *Package) rewriteRef(f *File) {
 				r.Name = n
 				break
 			}
-		case "expr":
+		case ctxExpr:
 			switch r.Name.Kind {
 			case "func":
 				if builtinDefs[r.Name.C] != "" {
@@ -1154,13 +1154,13 @@ func (p *Package) rewriteRef(f *File) {
 			case "macro":
 				expr = &ast.CallExpr{Fun: expr}
 			}
-		case "selector":
+		case ctxSelector:
 			if r.Name.Kind == "var" {
 				expr = &ast.StarExpr{Star: (*r.Expr).Pos(), X: expr}
 			} else {
 				error_(r.Pos(), "only C variables allowed in selector expression %s", fixGo(r.Name.Go))
 			}
-		case "type":
+		case ctxType:
 			if r.Name.Kind != "type" {
 				error_(r.Pos(), "expression C.%s used as type", fixGo(r.Name.Go))
 			} else if r.Name.Type == nil {

src/cmd/cgo/main.go

@@ -76,7 +76,7 @@ type Call struct {
 type Ref struct {
 	Name    *Name
 	Expr    *ast.Expr
-	Context string // "type", "expr", "call", or "call2"
+	Context astContext
 }
 
 func (r *Ref) Pos() token.Pos {
@@ -301,7 +301,7 @@ func main() {
 		p.Translate(f)
 		for _, cref := range f.Ref {
 			switch cref.Context {
-			case "call", "call2":
+			case ctxCall, ctxCall2:
 				if cref.Name.Kind != "type" {
 					break
 				}