gofix: add mapdelete

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5286043

src/cmd/gofix/Makefile

@@ -16,6 +16,7 @@ GOFILES=\
 	imagenew.go\
 	iocopyn.go\
 	main.go\
+	mapdelete.go\
 	math.go\
 	netdial.go\
 	netudpgroup.go\

src/cmd/gofix/fix.go

@@ -53,6 +53,7 @@ var fixes = fixlist{
 	mathFix,
 	ioCopyNFix,
 	imagecolorFix,
+	mapdeleteFix,
 }
 
 // walk traverses the AST x, calling visit(y) for each node y in the tree but

src/cmd/gofix/mapdelete.go

+// Copyright 2011 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.
+
+package main
+
+import "go/ast"
+
+var mapdeleteFix = fix{
+	"mapdelete",
+	mapdelete,
+	`Use delete(m, k) instead of m[k] = 0, false.
+
+http://codereview.appspot.com/5272045
+`,
+}
+
+func mapdelete(f *ast.File) bool {
+	fixed := false
+	walk(f, func(n interface{}) {
+		stmt, ok := n.(*ast.Stmt)
+		if !ok {
+			return
+		}
+		as, ok := (*stmt).(*ast.AssignStmt)
+		if !ok || len(as.Lhs) != 1 || len(as.Rhs) != 2 {
+			return
+		}
+		ix, ok := as.Lhs[0].(*ast.IndexExpr)
+		if !ok {
+			return
+		}
+		if !isTopName(as.Rhs[1], "false") {
+			warn(as.Pos(), "two-element map assignment with non-false second value")
+			return
+		}
+		if !canDrop(as.Rhs[0]) {
+			warn(as.Pos(), "two-element map assignment with non-trivial first value")
+			return
+		}
+		*stmt = &ast.ExprStmt{
+			X: &ast.CallExpr{
+				Fun: &ast.Ident{
+					NamePos: as.Pos(),
+					Name:    "delete",
+				},
+				Args: []ast.Expr{ix.X, ix.Index},
+			},
+		}
+		fixed = true
+	})
+	return fixed
+}
+
+// canDrop reports whether it is safe to drop the
+// evaluation of n from the program.
+// It is very conservative.
+func canDrop(n ast.Expr) bool {
+	switch n := n.(type) {
+	case *ast.Ident, *ast.BasicLit:
+		return true
+	case *ast.ParenExpr:
+		return canDrop(n.X)
+	case *ast.SelectorExpr:
+		return canDrop(n.X)
+	case *ast.CompositeLit:
+		if !canDrop(n.Type) {
+			return false
+		}
+		for _, e := range n.Elts {
+			if !canDrop(e) {
+				return false
+			}
+		}
+		return true
+	case *ast.StarExpr:
+		// Dropping *x is questionable,
+		// but we have to be able to drop (*T)(nil).
+		return canDrop(n.X)
+	case *ast.ArrayType, *ast.ChanType, *ast.FuncType, *ast.InterfaceType, *ast.MapType, *ast.StructType:
+		return true
+	}
+	return false
+}

src/cmd/gofix/mapdelete_test.go

+// Copyright 2011 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.
+
+package main
+
+func init() {
+	addTestCases(mapdeleteTests, mapdelete)
+}
+
+var mapdeleteTests = []testCase{
+	{
+		Name: "mapdelete.0",
+		In: `package main
+
+func f() {
+	m[x] = 0, false
+	m[x] = g(), false
+	m[x] = 1
+	delete(m, x)
+	m[x] = 0, b
+}
+
+func g(false bool) {
+	m[x] = 0, false
+}
+`,
+		Out: `package main
+
+func f() {
+	delete(m, x)
+	m[x] = g(), false
+	m[x] = 1
+	delete(m, x)
+	m[x] = 0, b
+}
+
+func g(false bool) {
+	m[x] = 0, false
+}
+`,
+	},
+}